Erwan Le Grognec

Rinaldo Poli
email: rinaldo.poli@u-bourgogne.fr

Philippe Richard

Prof Rinaldo Poli

Dialkyl(butadiene)cyclopentadienylmolybdenum(III) Complexes. Synthesis, Characterization, and Reactivity

Treatment of CpMo(η 4 -diene)Cl2 (diene = 1,3-butadiene, C4H6, 1; isoprene, C5H8, 1'; 2,3dimethyl-1,3-butadiene, C6H10, 1'') in diethyl ether at low temperature with 2 equiv of alkylmagnesium RMgX reagents affords the corresponding dialkyl complexes CpMo(η 4 -1,3diene)R2 (2, 2', 2", R = CH3, a; CH2Ph, b; CH2SiMe3, c). These species are isolable in moderate yields and have been fully characterized by EPR, elemental analyses, and cyclic voltammetry. They all show a reversible reduction process at relatively low potentials and an irreversible oxidation. The structure of 2"a was confirmed by single-crystal X-ray diffraction. The mixed complex CpMo(η 4 -C4H6)Cl(CH3), 3, has also been obtained by selective monomethylation of 1.

A slow ligand redistribution process occurs between equivalent amounts of 1 and 2a to afford 3 quantitatively. Compound 3 slowly decomposes by elimination of a CH3 group, yielding compound [CpMo(η 4 -C4H6)Cl]2, 4, which has been structurally characterized by X-ray crystallography. Arylation with PhMgBr or (C6H2Me3-2,4,6)MgBr provides EPR evidence for formation of arylated Mo(III) products. However, the isolation and crystallographic characterization of [CpMo(η 4 -C4H4Me2-2,3)Br0.77Cl0.23]2, 4", for the mesityl reaction indicates that halide exchange and electron transfer processes also take place competitively. Complex 2a does not react with Lewis bases such as phosphines or carbon monoxide nor with weak Brønsted acids (MeOH, H2O, CH3COOH, H3PO4, H3O + ). Protonolysis of the Mo-R bond could only be observed by interaction with HCl, HBF4, and CF3COOD, with formation of CH4 or CH3D.

Compounds CpMo(η 4 -C6H10)(CH3)(OCOCF3), 5, and CpMo(η 4 -C6H10)(OCOCF3)2, 6, have been isolated from the reaction between 2"a and 1 or 2 equiv of CF3COOD, respectively. Compound 6 has been structurally characterized by X-ray crystallography.

Introduction

There has long been an interest in free radicals in organometallic chemistry, because the presence of unpaired electrons may open up new reactivity pathways that are unavailable to the related and more frequently studied diamagnetic analogues. [1][2][3][4][5] In addition, fundamental reactions that are well established for diamagnetic compounds are often found to be accelerated upon transformation (e.g., by oxidation or reduction) to radical species. 6 A particularly interesting class of radical compounds are those containing alkyl ligands, since these are involved in most industrially relevant catalytic processes. The conditions for the inertness of transition metal alkyl compounds toward decomposition processes (i.e., by αor β-H elimination processes, reductive elimination, etc.) are usually stricter in radical species. A large variety of radical systems containing alkyl ligands has, however, been described. Selected examples by class are Cp2MR2 (M = Nb, Ta), [7][8][9] [Cp2WR2] + , [10][11][12] and CpCr(NO)(L)R. 13,14 As part of a systematic study of 17-electron monocyclopentadienyl complexes of the general formula CpMoL2X2 as starting materials or reactive intermediates for organometallic chemistry and catalysis, we reported a few years ago 15 the formation of a transient paramagnetic dimethylmolybdenum(III) complex, CpMo(CH3)2(PMe3)2, by the low-temperature alkylation of the dichloride precursor. This product was characterized in situ by EPR spectroscopy but could not be isolated because decomposition takes place at temperatures greater than ca. -20 °C, with homolytic cleavage of one Mo-CH3 bond. More recently, we have observed a similar instability for the methylation product of CpMoCl2(MeSCH2CH2PPh2), 16 and similar difficulties were encountered by Fryzuk et al. in the alkylation of [η 5 :P,P'-C5H3-1,3-(SiMe2CH2P i Pr2)2]MoCl2. 17 In parallel studies, we have found a novel high-yield synthesis of the previously known 18 CpMo(η 4 -butadiene)Cl2 and have started to investigate its reactivity. 19 We now report that the reaction of this compound and similar diene derivatives with several alkylating reagents affords the first isolable mononuclear alkyl complexes of Mo(III). Somewhat unexpectedly, we find these products to be chemically inert under a variety of conditions. A portion of this work has been previously communicated. 20 Results and Discussion 1. Preparation of the Diene-Containing Dichloride Precursors. The butadiene and isoprene compounds 1 and 1' were first reported by Davidson et al. 18 as the products of diene addition to a Mo(II) precursor, which limits the yield to 50% at best. The diene complex 1 has been more recently synthesized in 90% yields by addition of butadiene to CpMoCl2. 21 By the same procedure, we have now obtained 1' and the previously unreported 2,3-dimethylbutadiene analogue, 1", in isolated yields greater than 70% (eq 1). No reaction takes place between CpMoCl2 and cycloocta-1,3-diene or cycloocta-1,5-diene under the same conditions. These results may be rationalized by the steric repulsion between the diene ligand and the Cp ring, because the diene ligand preferentially adopts the supine (or endo) conformation in this system. 18 In this respect, it can be noted that Davidson and co-workers have also reported derivatives with 1,3-pentadiene but could not obtain analogues with 1,3-cyclohexadiene. 18 Compound 1" shows spectroscopic and electrochemical properties analogous to those previously reported for 1 and 1'.

The cyclic voltammogram of 1" in THF shows a reversible reduction wave at -1.24 V vs ferrocene (cf. -1.08 and -1.20 V for 1 and 1') and an irreversible oxidation at 0.66 V (cf. 0.75 V for 1'), while the EPR spectrum of 1" in THF shows a broad singlet at room temperature at g = 1.994. Upon cooling, a hyperfine splitting to the external diene H atoms becomes visible, but unlike compounds 1 and 1', no distinction between the anti and syn protons could be made.

CpMoCl2 + diene  CpMoCl2(η 4 -diene) (1)
(diene = 1,3-butadiene, 1; isoprene, 1'; 2,3-dimethyl-1,3-butadiene, 1") 2. Alkylations of CpMoCl2(η 4 -diene). Treatment of 1, 1', or 1" with 2.2 equiv of alkyl Grignard reagents in a diethyl ether or THF suspension at -78 °C leads to the formation of the corresponding dialkyl complexes (see Scheme 1), which have been isolated as air-and lightsensitive green solids. All these products, however, are stable at room temperature as solids and in solution under an inert atmosphere and in the dark. In addition, compound 2a has been tested

for thermal stability in toluene and shows, remarkably, less than 50% decomposition after 22 h heating at 70 °C, as monitored by EPR spectroscopy. The thermal stability both as solids and in solution critically depends on the absence of the magnesium salt byproducts, which must therefore be thoroughly eliminated. All these products are extremely soluble in all common organic solvents. Any attempt to crystallize compounds 2a, 2'a, 2b, and 2"b from cold concentrated solutions failed; however, they have been recovered as analytically pure powders by evaporation to dryness. In addition, the formulation of 2"a was confirmed by a single-crystal Xray analysis (vide infra).

Scheme 1

It is interesting to remark that the alkylation processes occur cleanly, with no competitive nucleophilic attack at the diene ligand nor SET processes. In comparison, the Mo(III) complex

[CpMo(η 4 -C4H6)(η 3 -C3H5)] + undergoes competitive nucleophilic attack and one-electron reduction, 22 while the Mo(II) complexes [CpMo(η 4 -C4H6)2] + and [CpMo(η 4 -diene)(CO)2] + undergo facile addition of the carbanion at a diene terminal position. [23][24][25] The arylation of 1 and

1" also provides evidence of competitive metathesis and SET processes (vide infra).

All dialkyl products exhibit a room-temperature EPR signal around g = 2.01 with molybdenum satelites (aMo ca. 33 G) due to 95 Mo (15.9%) and 97 Mo (9.5%), both having I = 5/2.

The spectra additionally show diagnostic hyperfine couplings to the diene terminal H atoms and to the alkyl R-H atoms. Like the previously described compounds 1, 1¢,18 and [CpMo(η 4 -C4H6)(η 3 -C3H5)] + , 21 the CpMo(III) diene complexes reported here do not show hyperfine couplings to the diene internal H atoms nor to the Cp H atoms in the EPR spectrum. The H couplings may be observed in some cases for dilute solutions at room temperature (2b, 2"b, 2c, As previously discussed for compound 2a, 20 the spectra of 2'a and 2"a qualitatively exhibit an apparent binomial undecet pattern, due to the unresolved coupling (aH ca. 5.5 G) to 10 H atoms of three different kinds: the six methyl hydrogens and the four (two syn and two anti) external diene hydrogens. In accord with this interpretation, the EPR resonance of the CD3 homologue 2a-d6 is reduced to a broad quintet with identical g, aMo, and aH values as 2a under similar conditions. The individual coupling constants reported in the Experimental Section derive from a standard simulation and fitting of the experimental spectra on the basis of the proposed spin system. The spectra of the CH2Ph derivatives (2b and 2"b) and CH2SiMe3 derivatives (2c and 2"c) analogously show couplings to the diene terminal H atoms and alkyl R-H atoms. In these cases, the spectra are more complex because the coupling constants to the different kinds of H atoms are sufficiently distinct (see Figure 1).

The structure of compound 2"a has been determined by a single-crystal X-ray analysis.

Two independent, noninteracting, and essentially identical molecules are contained in the asymmetric unit. Selected bond distances and angles for both molecules are collected and compared in Table 1. A view of one of them is shown in Figure 2. The geometry parallels that reported by Davidson and co-workers for compound 1, 18 including the supine conformation adopted by the diene ligand. The diene ligand has longer lateral C-C bonds with respect to the internal one (averages are 1.426(4) and 1.394(4) Å over the two independent molecules), indicating significant back-bonding. These values are not too different than those found for 1

(1.410(6) and 1.364(5) Å, respectively). 18 The values of d (average difference between Mo-Clateral and Mo-Cinternal distances) and θ (diene fold angle as defined by Yasuda and Nakamura) 26 Since dialkylation reactions are known to occasionally afford byproducts of monoalkylation, especially with the bulkier alkylating agents, 41 we deliberately synthesized and investigated the product of monomethylation for substrate 1, namely, compound 3; see Scheme 1.

Treatment of 1 with only 1 equiv of CH3MgBr in a THF or toluene solution affords the expected monomethyl derivative 3 as an analytically and spectroscopically pure dark green powder. The EPR spectrum (shown in the figure of the preliminary communication) has the qualitative appearance of a binomial octet because of unresolved coupling to the butadiene syn and anti external H atoms. Both the g value and the aMo and aH coupling constants are similar for 2a and 3. This feature, therefore, does not allow us to rule out the presence of minor amounts of each compound as a contaminant in the other. The purity of both products, however, is unambiguously shown by the electrochemical studies; see next section. Other selective monoalkylation processes of dichloride precursors have been previously reported. [42][43][44] 3. Cyclic Voltammetric Studies of the Alkylmolybdenum(III) Derivatives. All alkylmolybdenum(III) derivatives have been investigated by cyclic voltammetry, since halfsandwich molybdenum(III) complexes usually display an interesting redox behavior. 45 All compounds display a reversible reduction wave and an irreversible oxidation process. The latter one was investigated in greater details for 2a, for which no return wave could be observed even at scan rates up to 1000 V s -1 or at low temperatures (-30 °C) in different solvents (THF, MeCN, or CH2Cl2). The important results are collected in Table 3. It is immediately apparent that the reduction potentials are much more negative for the dialkyl complexes than for the dihalide species. As could be predicted, the reversible reduction wave for the mixed complex 3 is located halfway between the potentials measured for the dichloro and dimethyl species 1 and 2a. A similar situation was reported for compounds Cp*M(NO)XY (M = Mo, W; X,Y = Cl, CH2CMe3). 43 Therefore, cyclic voltammetry is the best method to evaluate the presence of monoalkylated species as a contaminant in the dialkylated products, and vice versa. The observed negative shift is approximately 0.5 V per Cl/R substitution. Related systems where a process interconverting 18-electron and 17-electron species has been measured include [Cp2WX2] n+ (n = 0, 1) 12 and [CpCr(NO)(L)X] n-(n = 0, 1), 14,46 where the observed negative shifts per Cl/R substitution are ca. 350 mV (R = Me) and 610 mV (R = CH2SiMe3), respectively. The irreversible oxidation process is also significantly shifted toward negative potentials on going from the dichloro to the dialkyl systems. Related systems where the conversion between 17-and 16-electron species has been observed, namely, [Cp2MX2] n+ (n = 0, 1; M = Ti, Zr) [47][48][49] and 43,50,51 show again similar negative shifts. -1.00 d a All potentials are relative to the ferrocene standard. b Measured by using a scan rate of 200 mV s -1 for all compounds. c This compound was also previously investigated by Davidson et al. 18 (reversible reduction at -1.09 V in THF and -1.21 V in CH2Cl2; irreversible oxidation at Ep,a = +0.62 V in CH2Cl2). d An oxidation wave for this compound was not observed up to the discharge of the solvent (ca. 1.5 V). e (ia/ic) < 1 (see text).

[(C5R5)M(NO)X2] n-(n = 0, 1; R = H, Me; M = Mo, W),
A comparison between members of homologous series shows that greater substitution on the diene ligand results in a small negative shift on both processes. For compounds with the same diene and different alkyl groups, the reduction potentials become less negative in the order CH3 < CH2SiMe3 < CH2Ph, in agreement with trends of the R-substituent electron-withdrawing properties (H < SiMe3 < Ph). On the other hand, the oxidation peak potential shifts in the opposite direction, in contrast with the above effect. However, since these oxidation processes are irreversible, the different observed potentials must also depend on the relative rates of the follow-up chemical processes.

Conproportionation Reaction of 1 and 2a.

The selective formation of the monoalkylated complex 3 by methylation of 1 with 1 equiv of CH3MgBr could be rationalized in two possible ways. The first alkylation could proceed much more rapidly than the second one, and/or compound 3 could be thermodynamically favored relative to a 50:50 mixture of 1 and 2a.

Note that the greater solubility in THF of the monoalkylation product 3 with respect to the precursor 1 is a factor in favor of a competitive second alkylation. The rationalization based on thermodynamics was easily tested by monitoring a solution obtained by mixing equivalent amounts of the dichloro and dimethyl complexes; see eq 2.

CpMo(η 4 -C4H6)Cl2 + CpMo(η 4 -C4H6)(CH3)2 2 CpMo(η 4 -C4H6)(CH3)Cl (2) 1 2a 3
As discussed above, the EPR signal of 3 has a g value identical with that of 2a. Thus, the EPR monitoring showed only the decrease of compound 1 and the increase of a signal corresponding to the sum of 2a and 3. On the other hand, cyclic voltammetry permitted us to follow the evolution of all three compounds independently; see Figure 3. It can be noted that the reaction is extremely slow and quantitative, a complete conversion requiring several weeks at room temperature. The slow rate of this ligand redistribution process also indicates that the thermodynamic preference of 3 relative to a mixture of 1 and 2a is not a sufficient criterion to rationalize the selectivity of the monoalkylation process. A much slower second alkylation step relative to the first one must also be invoked. Reaction 2 is an example of the general ligand redistribution process schematically described in eq 3, for which many examples are known. [52][53][54] Of particular relevance here, because the metal and electronic configuration are the same, is the equilibrium between CpMo(PMe3)2Cl2

and CpMo(PMe3)2I2 on one side with the mixed system CpMo(PMe3)2(Cl)(I) on the other side. 54 This equilibrium was found to lie in slight favor of the mixed chloroiodo system with respect to the statistical distribution (K = 12-15 in CH2Cl2 and 20-28 in THF in the temperature range 22-35 °C, corresponding to the experimental H° values of -0.7 and -1.0 kcal/mol, respectively). 54 The essential completeness of reaction 2 means that this must be characterized by an even greater exothermicity. For a reaction of this kind, it can be safely assumed that the entropic and (PV) contributions are negligible, thus G ~ H ~ E. Total energy calculations for geometry optimized 1, 2a, and 3 by DFT/B3LYP methods afford an E value of -7.4 kcal/mol for the isodesmic reaction 2. From this value and the above assumptions, the equilibrium constant for reaction 2 is estimated as K = 2.7 x 10 5 , in agreement with the observed complete conversion. A similar redistribution process, also thermodynamically favoring the mixed chloroalkyl species, is that involving Cp*2MR2 and Cp*2MCl2 (M = Th, U; R = Me, CH2SiMe3). 42

X-M-X + Y-M-Y 2 X-M-Y (3)
The ligand redistribution process of eq 2 is much slower than the Cl/I redistribution process mentioned above, for which the equilibrium position was attained within 48 h at room temperature. A possible reason for this slower rate is the reduced ability of the methyl groups to bridge two metals in a likely dinuclear intermediate.

Decomposition of 3 and Arylations of CpMoCl2(η 4 -diene).

The di-µchlorodimolybdenum(II) complex 4 crystallizes from a pentane solution of 3 upon prolonged standing in laboratory roomlight at room temperature. We believe that this conversion involves the expulsion of methyl radicals (eq 4), by analogy with the analogous process previously reported for CpMo(PMe3)2(CH3)2. 15 2 CpMo(η 4

-C4H6)(CH3)Cl  [CpMo(η 4 -C4H6)(µ-Cl)]2 + 2 CH3 • (4) 3 4
The structure of compound 4 is shown in Figure 4, and selected bond distances and angles are collected in The isolation of the reduced product 4 from a solution originating from the monoalkylation of the dichloride complex 1 might lead to the proposal of an alternative formation mechanism, namely, direct reduction of 1 by the methyl Grignard reagent (a well-known phenomenon).

Indeed, compound 1 was reported to undergo a reductive process by interaction with allylmagnesium bromide. 21 However, the product of this reduction was found to be the above- derivatives. These compounds, however, proved to be too unstable to isolate as pure products.

The EPR spectra of these derivatives show as expected only an unresolved binomial quintet pattern because of coupling with the four terminal diene H atoms, since these derivatives lack any R-H atom on the R 3 substituent. The lack of any signature in the EPR spectrum by the aryl substituents does not unequivocally establish whether the observed products are diaryl or mixed chloro-aryl species. From the mesityl Grignard reaction, however, crystals of compound [CpMo(η 4 -C4H4Me2-2,3)(µ-Br)0.77(µ-Cl)0.23]2, 4", were recovered, documenting that a reduction process takes place in this case as previously observed for the allyl Grignard reaction. 21 The structure of 4", shown in Figure 5, is very similar to that of 4 and to that previously reported for compound [CpMo(η 4 -C4H6)(µ-Br)0.89(µ-Cl)0.11]2 (see relevant parameters in Table 4). One minor difference, however, concerns the intimate geometry of the Mo(η 4 spectroscopy when the photodecomposition was carried out in the presence of ligand L. As a matter of fact, the photodecomposition of 2a was retarded by the presence of L. For instance, the half-life of 2a was 28 h in the presence of a 3-fold excess of PMe3 at room temperature, as compared with 10 h in the absence of the phosphine. The only tentative rationalization that we are able to offer is that the additional ligand somehow acts as a quencher of the photodecomposition process.

No increased loss of starting material (with respect to control experiments carried out without the Lewis basic reagent and in the absence of light) occurs upon interaction of 2a with PEt3 at the reflux temperature of THF or with CO at 65 °C and 600 psi. This behavior is quite surprising because 17-electron organometallic species are believed to establish rapid (electronically favored, sterically disfavored) equilibria with formally 19-electron products of ligand addition, 3,55,56 and because both alkyl-alkyl reductive elimination processes [57][58][59][60][61] and the insertion of CO into M-R bonds 58,[62][63][64][65][66][67][68][69][70][71] are known to be favored by an increase in metal oxidation state. Since half-sandwich derivatives of Mo(II) have been shown to undergo CO insertion processes, 72,73 we were naturally expecting to observe a reaction between CO and the formally Mo(III) dialkyl derivatives reported here. However, despite the oxidation state, the metal has a high electron density, as demonstrated by the electrochemical studies described above, which probably renders ligand coordination and insertion/elimination processes thermodynamically and/or kinetically less favorable. A paramagnetic alkyl complex undergoing a CO insertion process is the 15-electron Ti(III) system (η 5 -C5R'5)2Ti-R (R' = H, Me). [74][75][76] The 17-electron complex CpCr(NO)(PPh3)(CH2SiMe3) is equally unreactive toward CO, whereas the less sterically hindered CpCr(NO)(pip)(CH2Ph) does react, but an insertion process does not occur. 14 In light of the latter observation, steric hindrance toward the ligand addition process to 2a may also be an important factor.

Inertness toward Brønsted Acids.

Reactivity studies of 2a with Brønsted acids also reveal the inertness of the Mo-CH3 bond toward heterolytic cleavage. EPR monitoring reveals that no reaction takes place with the following acids, in order of increased strength (all in THF solution): MeOH, H2O, CH3COOH, H3PO4, and H3O + (from aqueous HCl). On the other hand, gaseous HCl in toluene converts 2a to the parent dichloride compound 1 (eq 5). Heterolytic cleavage is also observed upon interaction with excess HBF4 and CF3COOD in C6D6. The formation in these reactions of CH4 and CH3D, respectively, was evidenced by 1 H NMR spectroscopy.

CpMo(η 4 -C4H6)(CH3)2 + 2 HCl  CpMo(η 4 -C4H6)Cl2 + 2 CH4

(5)

2a

The CF3COOD reaction allowed the observation of an EPR spectrum which is assigned to compound CpMo(η 4 -C4H6)(O2CCF3)2. This compound was not isolated. However, the corresponding reaction of 2"a with CF3COOD has allowed the isolation and full characterization of a mixed methyl-trifluoroacetato derivative, 5, and a bis(trifluoroacetato) derivative, 6, by the use of 1 or 2 equiv of the acid, respectively (Scheme 2). An EPR monitoring of the reaction carried out with 2 equiv of acid clearly indicates that the first protonolysis process is much faster than the second one (see Figure 6). Analogously to the chloro system, the EPR signal of the mixed methyl trifluoroacetato derivative (g = 1.998) is shifted to high g relative to the bis(trifluoroacetato) analogue (g = 1.982). The coupling pattern and H hyperfine splittings for 5 parallel those of the corresponding chloro analogue, namely, an apparent binomial octet, while 6 exhibits an apparent quintet pattern. relative to the dimethyl parent system (see Table 3), each Me/O2CCF3 substitution having quantitatively a similar effect as a Me/Cl substitution. It is curious that the reduction wave of 5 is not fully reversible, the (ia/ic) increasing with increasing scan speed (0.43, 0.50, 0.61, 0.65, and 0.68 at 200, 500, 1000, 2000, and 5000 mV/s, respectively), while the reduction waves of 2"a and 6 are fully reversible. In the corresponding chloride series, the dichloro, methyl chloro, and dimethyl complexes (1, 3, and 2a) all show reversible reduction processes.

Compound 6 could be obtained in the form of single crystals, which were investigated by X-ray diffraction. A view of the molecular structure is shown in Figure 7. The most interesting feature is the monodentate coordination for both trifluoroacetato ligands, confirming the indications given by the IR spectrum. Selected bond distances and angles are shown in Table 5.

In comparison with the structure of compound 2"a, both the cyclopentadienyl ring and the diene ligand are closer to the metal center in compound 6, as shown by the Mo-CNT(1) and Mo-CNT(2) distances, respectively. A greater effective positive charge on the metal center resulting from the replacement of the two methyl groups with the more electronegative trifluoroacetato ligands, thereby reducing the metal radius, may be held responsible for this difference. The diene relative arrangement shows a larger fold angle õ (95.91°) and a larger negative d (-0.155 Å) relative to compound 2"a. suggesting that the decomposition mechanism may be more complex that a simple Mo-R bond homolysis. Further studies both at the experimental and at the theoretical level will be devoted to throwing light on this phenomenon. We are also further pursuing reactivity studies of this new class of alkyl derivatives. All potentials are reported vs the Cp2Fe/Cp2Fe + couple, which was introduced under argon into the cell at the end of each measurement. All the voltammetric results obtained in this study are collected in Table 3. Elemental analyses were performed with a Fisons EA 1108 apparatus. ). An immediate color change from green to brown was observed. Bubbling was stopped and the solution was stirred at room temperature for 4 h under the HCl-containing atmosphere. (c) Compound 4". A green crystal (0.17 x 0.15 x 0.12 mm 3 ) suitable for an X-ray analysis was mounted on an Enraf-Nonius KappaCCD using Mo KR radiation, and 29 175 reflections e Å -3 . Crystal data are reported in Table 6. Computational Details. All electronic structure and geometry optimization calculations were performed using GAUSSIAN 94 83 on an SGI Origin 200 workstation. The LanL2DZ set was employed to perform geometry optimizations with a density functional theory (DFT) approach. The B3LYP functional (B3LYP = the three-parameter form of the Becke, Lee, Yang, and Parr functional) 84 was employed. The LanL2DZ basis set includes both Dunning and Hay's D95 sets for H and C [START_REF] Dunning | Modern Theoretical Chemistry[END_REF] and the relativistic core potential sets of Hay and Wadt for the heavy atoms. [START_REF] Hay | [END_REF][87][88] Electrons outside the core were all those for H and C, the 4s, 4p, 4d, and 5s electrons for Mo, and the 3s and 3p electrons for Cl. The mean value of the first-order electronic wave function, which is not an exact eigenstate of S 2 for unrestricted calculations on the triplet systems, was considered suitable for the unambiguous identification of the spin state. Spin contamination was carefully monitored, and the value of <S 2 > for the UB3LYP (UB3LYP = unrestricted B3LYP) calculations on the doublet CpMo(η 4 -C4H6)XY systems at convergence (XY = Cl2, 0.7655; ClMe, 0.7644; Me2, 0.7636) indicated minor spin contamination.

Experimental Section

General

2

  "c), whereas in all the other cases they are evident only at low temperature. The spectra of complexes 2a and 2a-d6 are shown in the figure of the preliminary communication.20 Compounds with identical alkyl ligands and different substitution on the diene ligand show similar spectra (e.g., 2a, 2'a, and 2"a). Representative spectra for the bis(trimethylsilylmethyl) and dibenzyl derivatives are shown in Figure1.

Figure 1 .

 1 Figure 1. Experimental (left) and simulated (right) EPR spectra of compounds CpMo(R 3 )2(η 4 -CH2=CR 1 -CR 2 =CH2). (a) 2b in pentane at -80 °C; (b) 2c in pentane at -80 °C.

Figure 3 .

 3 Figure 3. Cyclic voltammograms at different times of a THF solution obtained by mixing equimolar amounts of compounds 1 and 2a.

Figure 4 .

 4 Figure 4. ORTEP 89 view of compound 4 with thermal ellipsoids drawn at the 30% probability level.

  (18) C(6)-C(7) 1.434(11) C(7)-C(7)# 1.402(4) C(7)-C(8) 1.367(11) C(7)-C(8) 1.440(3) C(8)-C(9) 1.428(11) C(10)-C(10)# 1.403(4) C(10)-C(11) 1.447(2)

mentioned [CpMo(η 4 -

 4 C4H6)(µ-Br)0.89(µ-Cl)0.11]2 rather than the pure chloro species. The reason for this result was traced to the relative preference of the two acidic centers Mo(II) and Mg(II) for the Lewis bases chloride and bromide, according to the HSAB theory. Thus, the 16-electron product of reduction, CpMo(η 4 -C4H6)Cl, and MgClBr would exchange halides to afford CpMo(η 4 -C4H6)Br and MgCl2 in an equilibrium process. Now, the Grignard reagent employed to synthesize compound 3 was CH3MgBr, while the X-ray analysis indicates that compound 4 is a pure dichloro species. This means that compound 4 must necessarily form only after removal of the magnesium salts from the reaction mixture.A competition between metathesis and reduction is observed in arylation reactions of compounds 1 and 1". The reactions between compounds 1 or 1" with PhLi, PhMgI, or (C6H2Me3-2,4,6)MgBr lead to the formation of solutions containing new EPR active Mo(III)

  -diene) moiety. A greater tilting of the diene function is observed for 4", leading to longer Mo-C bonds for the internal C atoms and shorter ones for the external C atoms. This effect may be attributable to a steric repulsion between the methyl substituents at the diene 2 and 3 positions on one Mo atom with the Cp ligand on the adjacent Mo atom, rather than to a modification in the electronic structure of the Mo-diene bond (i.e., degree of back-bonding). In fact, the C-C bond alternation is similar in the two compounds.

Figure 5 .

 5 Figure 5. ORTEP 89 view of compound 4" with thermal ellipsoids drawn at the 30% probability level. The bridging atom X is a compositional disorder of Br (77%) and Cl (23%). Only the major species (X = Br) is shown for clarity.

Figure 6 .

 6 Figure 6. Room-temperature EPR spectra of a toluene solution obtained by mixing compounds 2"a and 2 equiv of CF3COOD: (a) immediately; (b) after 50 min; (c) after 24 h.

Figure 7 .Table 5 .

 75 Figure 7. ORTEP 89 view of compound 6 with thermal ellipsoids drawn at the 30% probability level.

  Procedures. All reactions involving air-and moisture-sensitive organometallic compounds were carried out in a Jacomex glovebox or by the use of standard Schlenck techniques under an argon atmosphere. Toluene and diethyl ether were purified by distillation under argon after drying over sodium benzophenone ketyl. THF was dried over sodium benzophenone ketyl and then over Na/K alloy. Pentane was dried over sodium. 1H NMR measurements were carried out on a Bruker AC200 spectrometer. The peak positions are reported with positive shifts in ppm downfield of TMS as calculated from the residual solvent peaks. The IR spectra were recorded on a Bruker IFS 66V spectrophotometer with NaCl optics. EPR measurements were carried out at the X-band microwave frequency on a Bruker ESP 300 spectrometer, equipped with a ER 4111 VT unit. The spectrometer was calibrated with DPPH (g )2.0037). EPR spectra simulations and fittings were carried out with WinSim.77 Cyclic voltamograms were recorded with an EG&G 362 potentiostat connected to a Macintosh computer through MacLab hardware/software. The electrochemical cell is a locally modified Schlenk tube. The cell is fitted with a Pt counter electrode, a Ag/AgCl reference electrode, and a Pt working electrode. Bu4NPF6 was used as supporting electrolyte at a concentration of 0.1 M.

Isoprene and 2 , 3 - 4 , 3 .

 2343 dimethylbutadiene were purchased from Aldrich Chemical Co, degassed, and alkylation reactions. The resulting solutions are extremely sensitive, and attempts to recover pure solid materials from these solutions failed. EPR (pentane, -80 °C): product of the 1-PhLi or PhMgBr reaction: apparent quintet with Mo satellites; g = 2.009; aMo = 32.4 G; aH = 6.3 G (2H); aH = 5.2 G (2H); product of the 1"-PhLi or PhMgBr reaction: apparent quintet with Mo satellites; g = 2.009; aMo = 32.3 G; aH = 6.3 G (2H); aH = 5.1 G (2H); product of the 1"-(C6H2Me3-2,4,6)MgBr reaction: apparent quintet with Mo satellites; g = 2.010; aMo = 32.4 G; aH = 6.1 G (2H); aH = 5.2 G (2H). Crystallization of the product of the 1"/mesityl Grignard reaction by diffusion of pentane into a concentrated THF solution at room temperature provided crystals of compound To a stirred, brown suspension of 1 (858 mg, 3 mmol) in THF (40 mL) at -80 °C was added dropwise CH3MgCl in Et2O (1.1 mL, 3.0 M, 3.2 mmol) via a syringe. After the addition of the Grignard reagent, the reaction mixture was stirred for 4 h while being allowed to warm slowly to room temperature. During this time a clear solid precipitated, while the red-brown solid dissolved to afford a green solution. The solvent was removed in vacuo, and the residue was extracted with pentane (60 mL). The extract was filtered through Celite (1.5 x 2 cm). The pentane was removed from the filtrate under reduced pressure to obtain 3 as a dark green powder. Yield: 550 mg, 69%. Anal. Calcd for C10H14ClMo: C, 45.22; H, 5.31. Found: C, 45.22; H, 5.63. EPR (toluene, room temperature): apparent octet with Mo satellites, g = 2.012; aMo = 32.8 G; aH = 6.4 G (2H); aH = 5.4 G (3H); aH = 4.5 G (2H). In a separate preparative experiment, a concentrated pentane solution of the product 3 was set for crystallization at room temperature without protection from light. Single crystals of compound 4 which were suitable for an X-ray analysis formed over 1 week. Formation of 3 by Ligand Redistribution from 1 and 2a. A solution of 2a (78 mg, 0.31 mmol) in 10 mL of THF was cannulated onto a suspension of 1 (90 mg, 0.31 mmol) in 40 mL of THF at room temperature. The mixture was then protected from light and stirred for 3 weeks. Aliquots from this mixture were withdrawn from time to time by cannula directly into an electrochemical cell which already contained THF and supporting electrolyte for a monitoring of the reaction by cyclic voltammetry (see Results section). Attempt to React Compound 2a with Lewis Bases. (a) With CO. Compound 2a (187 mg, 0.76 mmol) was dissolved in THF (80 mL). An initial EPR spectrum was recorded. The solution was introduced into a 0.5 L Parr autoclave and pressurized with CO (600 psi). The solution was stirred at room temperature overnight. An aliquot the solution was withdrawn and exhibited no change in color and intensity of the EPR spectrum. No ν(C=O) could be observed by IR. The autoclave was then depressurized to 400 psi and warmed to 65 °C for 1 h (the pressure increased again to ca. 600 psi). No change in the properties of the solution was again observed. (b) With PMe3. Compound 2a (166 mg, 0.66 mmol) was dissolved in THF (20 mL). PMe3(1 M in THF, 0.8 mL, 0.8 mmol) was added by syringe, and the resulting solution was stirred at room temperature for 24 h. The EPR spectrum showed no decrease in intensity relative to the initial solution.(c) With PEt3. Compound 2a (90 mg, 0.36 mmol) was dissolved in THF (10 mL). PEt3 (1 M in THF, 0.5 mL, 0.5 mmol) was added by syringe, and the resulting solution was heated to reflux under stirring for 24 h. The EPR spectrum showed no decrease in intensity relative to the initial solution.Reaction of Complex 2a with Brønsted Acids. (a) With HBF4. In a NMR tube, a solution of 2a (46 mg, 0.19 mmol) in 0.5 mL of C6D6 was prepared. HBF4•OEt2 (54%, 24 µL, 0.17 mmol) was then added via a microsyringe. A dark precipitate immediately formed while gas evolution was noted. The 1 H NMR spectrum revealed the formation of CH4 (δ = 0.15 ppm), while an EPR spectrum of the surpernatant solution was featureless. (b) With CF3COOD. In an NMR tube, a solution of 2a (10 mg, 0.04 mmol) in 0.5 mL of C6D6 was prepared. An excess (20 µL, 0.26 mmol) of CF3COOD was then added via a microsyringe. The color immediately changed from dark green to yellow-brown. The 1 H NMR spectrum revealed the formation of CH3D (t, δ = 0.14 ppm, 2 JHD = 0.2 Hz). No other resonances were noticed except for those of the solvent. The EPR spectrum of this solution showed a broad singlet (g = 1.985; aMo = 41 G). Identical results were obtained upon reacting compound 2"a with CF3COOD under the same conditions. (c) With HCl in THF. In a Schlenk tube, a solution of 2a (17 mg, 0.07 mmol) in 10 mL of THF was prepared. Through this solution was bubbled via a Pasteur pipet an excess amount of gaseous HCl (generated in situ in a separate Schlenk tube by adding a 37% HCl solution to P4O10

(a) Compound 2 " 63 Å - 1 .

 2631 a. A dark green crystal (0.32 x 0.30 x 0.25 mm 3 ) was placed in a flamesealed capillary under Ar and mounted on an Enraf-Nonius CAD4 diffractometer. A total of 4687 reflections (of which 4561 were unique) were collected at room temperature up to sin(θ)/λ = 0.The data were corrected for Lorentz and polarization effects and for absorption (psi-scan method).[START_REF] North | [END_REF]80 A 12.5% decay was linearly corrected. The structure was solved via a Patterson search program 81 and refined (space group P21/c) with full-matrix least-squares methods 81 based on |F 2 |. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of the cyclopentadienyl anion were included in their calculated positions and refined with a riding model. The methyl hydrogen atoms were located in the Fourier difference map and refined, after idealization, as rigid rotating groups. The hydrogen atoms attached to the butadiene ligand were located in the Fourier difference map and were refined freely. At the end of this refinement the agreement indices were wR2 = 0.0747 for all data and R1 = 0.0276 for 4053 intensities with I > 2σ(I). The final difference electron density is featureless: Δρ = 0.33 and -0.53 e Å -3 .(b) Compound 4.A brown crystal (0.36 x 0.28 x 0.25 mm 3 ) was mounted on an Enraf-Nonius CAD4 diffractometer. A total of 1920 reflections (1827 unique) were collected up to sin(θ)/λ = 0.62 Å -1 . The data were corrected for Lorentz and polarization effects and for absorption (psi-scan method).[START_REF] North | [END_REF]80 The structure was solved via a Patterson search program 81 and refined (space group P21/c) with full-matrix least-squares methods based on |F 2 |. All nonhydrogen atoms were refined with anisotropic thermal parameters. The cyclopentadienyl ring was refined as a variable metric group. Except for the CH2 hydrogen atoms of the butadiene ligand, which were located on the difference Fourier map, all the hydrogen atoms were included in their calculated positions. All H atoms were refined with a riding model. The final agreement indices were wR2 = 0.140 for all data and R1 = 0.046 for 1639 intensities with I > 2σ(I). Final difference electron density: Δρ = 1.0 and -0.59 e Å -3 .

(

  4810 unique) were collected up to sin(θ)/λ = 0.807 at 110 K. Absorption correction was applied to the data during integration by the SCALEPACK 82 algorithm. The structure was solved via a Patterson search program81 and refined (space group Pbcm) with full-matrix least-squares methods based on |F 2 |. Except for the chlorine atom, all non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were found by Fourier difference synthesis and freely refined with isotropic temperature factors fixed to 1.3 times those of the corresponding carbon atoms. The halide atom position was found to be compositionally disordered. Two positions were independently refined as Br and Cl with no constraints except that the sof's were imposed as x and (1 -x), respectively, with x refining to 0.77. The Cl atom was isotropically refined. The Mo-X distances are consistent with a Mo-(µ-Br) and a Mo-(µ-Cl). The final agreement indices were wR2 = 0.0641 for all data and R1 = 0.0368 for 4230 intensities with I > 2σ(I). Final difference electron density: Δρ = 1.49 and -1.14 e Å -3 .(d) Compound 6. A dichroic green-yellow crystal (0.50 x 0.35 x 0.20 mm 3 ) was placed in a flame-sealed capillary under Ar and mounted on an Enraf-Nonius CAD4 diffractometer. A total of 3964 reflections (3633 unique) were collected at room temperature up to sin(θ)/λ = 0.62 Å -1 .The data were corrected for Lorentz and polarization effects and for absorption (psiscan method).[START_REF] North | [END_REF]80 A 7% decay was linearly corrected. The structure was solved via a Patterson search program3 and refined (space group Pbca) with full-matrix least-squares methods81 based on jF2j. All non hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of the cyclopentadienyl anion were included in their calculated positions and refined with a riding model. The methyl hydrogen atoms were located in the Fourier difference map and refined, after idealization, as rigid rotating groups. The hydrogen atoms attached to the butadiene ligand were located in the Fourier difference map and were refined freely. At the end of the refinement the thermal ellipsoids of the fluorine atoms were quite large and indicated either a disorder on the fluorine atom positions or very pronounced thermal librations. Any attempt to impose a disorder model lowered the agreement indices but did not lead to reasonable structural parameters. The final agreement indices were wR2 = 0.113 for all data and R1 = 0.042 for 2727 intensities with I > 2σ(I). The final difference electron density is featureless: Δρ = 0.79 and -0.55

  and -0.549 a R1=(|Fo| -|Fc|)/|Fo|. b wR2 = [w(Fo 2 -Fc 2 ) 2 /[w(Fo 2 ) 2 ] 1/2 where w = 1/[σ 2 (Fo 2 ) + (uP) 2 + vP] where P = (Max(Fo 2 ,0) + 2Fc 2 )/3, for 2a, u = 0.0418 and v = 0.98; for 4, u = 0.0872 and v = 2.11; for 4, u = 0.03, v = 2.35; and for 6, u = 0.0739 and v = 3.82. c Goodness of fit = [w(Fo 2 -Fc 2 ) 2 /(No -Nv)] 1/2 .

Table 1 .

 1 Selected Bond Distances (Å) and Angles (deg) for Compound 2

	are -

"a a Figure 2. ORTEP89 view of compound 2"a with

  thermal ellipsoids drawn at the 30% probability level.

Table 2 .

 2 Selected Mo(III)-C Bond Distances

	Compound	Mo-C distance (Å)	ref
	Mo2(CH2SiMe3)6	2.131	28
	Mo2(CH2Ph)6	2.162(2)	29
	Mo2Me2(OtBu)4(py)2	2.167(7)	30
	Mo2(CH2SiMe3)4(O t Bu)2	2.134(14) a	31
	Mo2(CH2SiMe3)4(OC6H3Me2)2	2.090(7), 2.112(6)	32
	Mo2(CH2Ph)2(O i Pr)4(PMe3)	2.216(6) a	33
	Mo2(CH2Ph)2(O i Pr)4(dmpm)	2.206(10)	33
	Mo2Et2(NMe2)4	2.164(7) a	34
	Mo2(CH2Ph)2(NMe2)4	2.19(1) a	35, 36

Table 3 .

 3 Cyclic Voltammetric Data at Room Temperature in THF for All Compounds a

	Compound	E1/2 (Mo II /Mo III ) (V)	Ep,a (Mo III /Mo IV ) b (V)
	1 c	-1.11	0.73
	1'	-1.19	0.75
	1	-1.24	0.66
	2a	-2.09	-0.17
	2'a	-2.13	-0.18
	2"a	-2.14	-0.23
	2b	-1.85	-0.38
	2"b	-1.85	-0.34
	2c	-2.01	-0.26
	2"c	-2.07	-0.28
	3	-1.62	d
	5	-1.61 e	0.27
	6

Table 4 .

 4 The basic geometry of the molecule is identical with that previously

	established for the isomorphous [CpMo(η 4 -C4H6)(µ-X)]2, X being a compositional disorder of Br
	(89%) and Cl (11%).21 The metals are separated by the nonbonding distance of 3.96 Å (cf. 4.08
	Å for the Br0.89Cl0.11 analogue). The Mo-Cl distances are much shorter than the Mo-X distances
	in the above-mentioned X analogue (2.6613(6) and 2.6570(6) Å), in agreement with the different
	nature of the bridging halide atom. The reasonable thermal ellipsoid of the bridging Cl atom

suggests that no significant compositional disorder (e.g., with Br) is present in this case. All other structural parameters are rather similar to the compositionally disordered Br0.89Cl0.11 analogue, including the previously defined

26

Δd and θ values, these being -0.075 Å and 91.77° for compound 4. These values are also similar to those reported above for compound 2"a, indicating that both compounds adopt a similar Mo-diene bonding situation.

Table 4 .

 4 Selected

	Mo-CNT(1)	1.922(6)	Mo(1)-CNT(1)	1.945(3)
			Mo(2)-CNT(3)	1.930(3)
	Mo-CNT(2)	1.911(10)	Mo(1)-CNT(2)	1.949(2)
			Mo(2)-CNT(4)	1.946(2)
	Mo-Cl	2.5712(12)	Mo(1)-Br	2.6677(15)
	Mo-Cl#	2.5766(12)	Mo(2)-Br	2.6667(15)
			Mo(1)-Cl	2.572(13)
			Mo(2)-Cl	2.607(13)
	Mo-C(6)	2.246(6)	Mo(1)-C(7)	2.4144(16)
	Mo-C(7)	2.308(7)	Mo(1)-C(8)	2.2206(19)
	Mo-C(8)	2.330(7)	Mo(2)-C(10)	2.4105(16)
	Mo-C(9)	2.242(7)	Mo(2)-C(11)	2.2206

Bond Distances (Å) and Angles (deg) for Compounds 4 and 4" a

Table 6 .

 6 Crystal and Structure Refinement Data

		2"a	4	4"	6
	formula	C13H21Mo	C18H22Cl2Mo2	C22H30Br1.54Cl0.46Mo2	C15H15F6MoO4
	M	273.24	501.14	625.76	469.21
	T; K	293(2)	293(2)	110(2)	293(2)
	diffractometer	CAD4	CAD4	Kappa CCD	CAD4
	cryst syst	monoclinic	monoclinic	orthorhombic	orthorhombic
	space group	P21/c	P21/c	Pbcm	Pbca
	a; Å	15.092(1)	10.636(1)	8.7030(2)	14.566(1)
	b; Å	7.458(1)	7.635(1)	19.5210(3)	14.717(1)
	c; Å	22.186(2)	11.912(1)	12.4410(3)	16.700(1)
	β; deg	91.248(8)	111.454(6)
	Z	8	2	4	8
	F(000)	1128	496	1230.88	1864
	Dcalc; g/cm 3	1.454	1.849	1.966	1.741
	λ; Å	0.71073	0.71073	0.71073	0.71073
	µ; mm -1	1.013	1.683	4.16	0.809
	cryst size; mm 3	0.32 x 0.30 x 0.25	0.36 x 0.28 x 0.25	0.17 x 0.15 x 0.12	0.50 x 0.35 x 0.20

Acknowledgement. We knowledge the Conseil Régional de Bourgogne, the MENRT, and the CNRS for financial support of this work, the European Commission for an Erasmus exchange undergraduate student program concerning J.D. from the University of Kaiserslautern, Germany, and the MENRT for a student followship to J.M.C.

stored on 4 Å molecular sieves before use. CH3MgBr (3.0 M in Et2O), CD3MgI (1 M in Et2O), (CH3)3SiCH2MgCl (1 M in Et2O), and (C6H2Me3-2,4,6)MgBr (1 M in THF) were purchased from Aldrich Chemical Co. and used as received. Diethyl ether solutions of PhLi, PhMgI, and PhCH2MgBr [START_REF] Vogel | s Textbook of Practical Organic Chemistry[END_REF] and compounds {CpMoCl2}n 54 and CpMo(η 4 -C4H6)Cl2 (1) 21 were prepared as described in the literature. 4 -diene)Cl2. Diene = Isoprene, 1'. This synthetic procedure is an adaptation of that previously reported for compound 1. 21 {CpMoCl2}n (2.216 g; 9.55 mmol of Mo) was suspended in 40 mL of THF. Isoprene (1.05 mL; 10.5 mmol) was added to the mixture via a syringe, followed by heating to 70 °C for 4 h, yielding 1' as a fine red-brown powder. The suspension was concentrated to about half volume, followed by addition of pentane (20 mL).

Synthesis of CpMo(η

Following filtration, the solid was washed with pentane (2 x 10 mL) and dried under vacuum. Yield: 2.222 g, 80%. EPR (THF, room temperature): singlet, g = 1.994 (cf. lit. 18

Synthesis of CpMo(η 4 -diene)R2.

The synthesis of all the dialkyl complexes was carried out under identical experimental conditions. The preparation of CpMo(η 4 -C4H6)(CH3)2 (2a) is described below as a representative example. To a suspension of 1 (534 mg, 1.86 mmol) in 30 cm3 of Et2O at -78 °C was added 1.4 mL of MeMgBr in Et2O (3.0 M, 4.1 mmol; Grignard/Mo = 2.2). The resulting mixture was stirred while being allowed to warm slowly to room temperature.

During this time a white solid precipitated, while the red-brown solid dissolved to afford a green solution. The solvent was removed under reduced pressure, and the residue was extracted with pentane (2 x 20 mL). The extracts were combined and filtered through a column of Celite (1.5 x 3 cm). The column was washed with pentane until the washings were colorless. The solution was evaporated under reduced pressure to dryness, yielding compound 2a as a dark green powder. Arylation Reactions. Reactions between compounds 1 and 1" and PhLi, PhMgBr, or (C6H2Me3-2,4,6)MgBr were carried out under conditions similar to those described above for the Supporting Information Available: A listing of positional coordinates, bond distances and angles, anisotropic displacement parameters, and calculated H atom positions for compounds 2a, 4, 4, and 6. This material is available free of charge via the Internet at http://pubs.acs.org.


Rinaldo Poli

Kevin M Smith

Theoretical Studies of the Reactivity of Cyclopentadienyl Nitrosyl Alkyl Species of Molybdenum and Tungsten

The reactivity differences observed experimentally for Cp*W(NO)(CH2CMe3)2 and CpMo(NO)(CH2CMe3)2 are investigated using density functional theory (DFT) techniques. The reactions of the CpW(NO)(CH2) model complex with NH3 and CH4 are more exothermic and have lower activation barriers than the corresponding processes for

can undergo two competitive processes: CH activation to afford CpM(NO)(CH3)2 or loss of methane. The relative barrier heights are almost identical for M = W, whereas the formation of CpM(NO)(CH3)2 is significantly disfavored for M = Mo. The activation of CH and NH bonds proceeds via a direct, metal-assisted 1,2 addition across the M=CH2 bond without a distinct carbene-hydride intermediate. The relative stability order of the products can be rationalized in part through metalligand -bonding considerations. The relative orbital energies, which are consistently lower for Mo than for W, can be used to rationalize various reactivity differences observed experimentally for the CpM(NO)(alkyl)2 species, including the selective coordination of small molecules. The absence of sufficient steric bulk in the model complexes likely contributes to the overestimation of the calculated activation energies, and the discrepancy between some of the calculated and experimental structural parameters.

Introduction

The activation of CH and NH bonds by transition metal complexes constitutes a critical step in a variety of stoichiometric and catalytic reactions. [START_REF]For recent reviews of catalytic reactions involving CH ae and NH f,g activation by transition metal complexes, see: (a) Sen[END_REF] Many investigations have been undertaken in order to understand this fundamental process, with theoretical studies 2 proving particularly useful when the key intermediates are too fleeting to be observed by standard spectroscopic methods. 3 The studies of stoichiometric alkane CH activation by non-d 0 complexes have largely focused on late transition metals, especially Rh and Ir. 4,5 Recently, however, W-based systems capable of alkane CH activation have been generated via thermolytic elimination of SiMe4 or CMe4 from Cp*W(NO)(CH2CMe3)2 or Cp*W(NO)(CH2SiMe3)(CPh=CH2), respectively. 6,7 Scheme 1 Scheme 1 illustrates the -hydrogen abstraction reaction for both Cp*W(NO)-(CH2CMe3)2 and CpMo(NO)(CH2CMe3)2, and the characteristic reactivity of the resulting transient alkylidene species. 6,8,9 While the thermal elimination of CMe4 requires heating to 70°C for the Cp*W bis(neopentyl) species, the same process occurs readily at 25°C for CpMo(NO)(CH2CMe3)2. In the presence of a trapping ligand L (e.g. phosphines, pyridine), the alkylidene can be isolated as a Cp*W(NO)(CHCMe3)(L) or CpMo(NO)(CHCMe3)(L) adduct. 6,8 Both Cp*W and CpMo systems react with primary or secondary amines to give alkyl amido compounds. 9,[START_REF] Tran | unpublished results. (11) For further examples of intermolecular CH bond activation by neopentylidene species, ac and alkane CH bond activation by tungsten complexes, de see[END_REF] However, while Cp*W(NO)(CHCMe3) activates even normally unreactive CH bonds in substrates such as cyclohexane, CpMo(NO)(CHCMe3) does not display any CH activation with alkanes or arenes. 11 Note that CpW(NO)(CH2CMe3)2 is also capable of solvent CH activation upon thermolysis, [START_REF] Tran | unpublished results. (11) For further examples of intermolecular CH bond activation by neopentylidene species, ac and alkane CH bond activation by tungsten complexes, de see[END_REF] confirming that the reactivity differences between the W and Mo systems is not simply due to variations in the ancillary ligands. 12 Previous theoretical studies of CpW(NO)-and CpMo(NO)-containing compounds have been performed using Extended Hückel and FenskeHall molecular orbital calculations. [13][14][15][16] Many of the distinctive properties of Cp'M(NO)X2 complexes (Cp' = Cp or Cp*; X = alkyl, amido, or other monoanionic ligand) [START_REF] Green | For a review of the LX ligand classification system[END_REF] can be attributed to bonding effects on the frontier orbital energies, as illustrated schematically in Figure 1.

In CpM(NO)(CH3)2, two of the three metal d  orbitals are engaged in  bonding to the nitrosyl ligand, and are much lower in energy than the metal-centered, non-bonding orbital perpendicular to the MNO axis. This splitting pattern is responsible for the stability of these diamagnetic 16 electron (16e) bis(alkyl) species, since their electronic unsaturation does not result in any reduction of metalligand binding. 14,15 The relatively low energy of the LUMO renders these complexes prone to coordinate small molecules which may then proceed to react with the metalalkyl bonds. 18 Replacing one CH3 group with an NH2 ligand allows the empty d orbital on the metal center to form a  bond with the filled p orbital on the amido N atom, as shown in Figure 1 (b). In order to maintain this  interaction, the planar MNH2 group must remain aligned with the MNO axis, resulting in pronounced energetic barriers to ligand rotation. 13,16 This preferred orientation has been observed experimentally in X-ray crystallographic studies of amido, thiolato, alkoxo, and alkylidene CpM(NO) complexes of Cr, Mo and W. 19 The Cp'M(NO)X2 compounds which possess these MX -bonding interactions are effectively electronically saturated, since the higher LUMO energy renders the metal center less susceptible to nucleophilic attack.

In this paper, we use density functional theory (DFT) to investigate the reactivity of Cp'M(NO)(CH2R)2 complexes of Mo and W. The activation of CH and NH bonds by transient alkylidene species is addressed in the initial sections of paper, followed by a discussion of adduct formation. Simplified models of both the substrates and organometallic reactants are employed (i.e. CH4, NH3, and CpM(NO)(CH3)2) in order to minimize computational expense. While this approach neglects steric effects operating in the actual compounds, the electronic effects that are highlighted provide a basis for a qualitative analysis of these systems. In particular, many key features in the reactivity of Cp'M(NO)(CH2R)2 complexes are shown to be rationalized by examining the bonding character and relative energies of the frontier molecular orbitals, especially when accounting for the divergent reactivity modes exhibited by the Mo and W systems. . 20 This type of geometric distortion has been analyzed for other 16e CpML2 d 6 and CpML3 d 4 compounds. [21][22][23][24][25] The large CMC angles had also previously been noted for CpM(NO)R2 and Cp*M(NO)R2 complexes (R = alkyl or aryl) based on X-ray crystallographic studies, 18 but the comparative bulk of the R groups employed experimentally obscured the electronic origin of the deformation. smaller than for the others, which range from 113.1 to 114.9° in both structures. The possibility of some  CHW interaction was invoked in the initial report of CpW(NO)(CH2CMe3)2 based on coupling constants obtained via 13 C-gated decoupled NMR spectroscopy. It was also noted that "these CHM links, if they do exist, do not apparently affect the characteristic chemistry of these compounds". 26 Similar double  agostic interactions have been proposed in d 0 bis(alkyl) compounds on the basis of X-ray crystallographic 27 and NMR data. 28 In light of the interpretation of Schrock and coworkers of the large 4 JHH coupling in CpW(CR)(CH2CR)(CH2R') complexes in terms of alkyl group orientation and  agostic interactions, 28 it now seems likely that the same considerations can be applied to Cp*M(NO)(CH2R)(CH2R') and CpW(NO)(CH2R)-(CH2R') compounds which were reported to display similar 4 JHH couplings. 29,30 The tungsten bis(methyl) compound is calculated to be much more thermodynamically stable with respect to CpM(NO)(CH2) and free CH4 than the corresponding molybdenum complex, by 35.7 and 24.9 kcal/mol, respectively. respectively). 33 The relative barrier heights from the CpM(NO)(CH2)(CH4) complexes to either the CH activation transition state or the 16e alkylidene plus free CH4 is strongly metal dependent. Both competing processes have almost identical energetic barriers for W.

Results and Discussion

CH

CpM(NO)(H2CHCH3

For Mo, however, the barrier for CH activation from the alkane complex is considerably higher (14.3 kcal/mol) than loss of CH4 (6.5 kcal/mol). This variation in relative energies has important implications for the reactivity differences observed experimentally for the Mo and W systems and, as discussed in the final section of this paper, can be correlated species and other 16e CpML2 compounds, [22][23][24][25]34 singlet CpMo(NO)(CH2) is highly pyramidal at metal, with a ONMoCPCH2 dihedral angle of 120.2°. It should be noted that since the bond lengths and angles are relatively invariant between the free 16e alkylidene species and the CpMo(NO)(CH2) fragment in the CH4 (or NH3) adducts, very little reorganization is expected to be required in order to bind these molecules.

NH Bond Activation. CpM(NO)(CH2)(NH3).

The optimized geometries calculated for the reaction of CpM(NO)(CH2) with NH3 are illustrated in Figure 6. The coordinated NH3 group in the ammonia adduct depicted in Figure 6(a) has one H that is bent slightly towards the metal, leading to a difference in HNM angles analogous to those seen in the CpM(NO)(CH3)2 complex. In the NH3 group, this discrepancy between the angles (104.0 and 103.6° vs. 111.0112.4° for Mo and W, respectively) is not as pronounced as in the CH3 groups discussed previously. Unlike the 16e bis(methyl) complex, the metalalkylidene  bond in CpM(NO)(CH2)(NH3) results in an 18e compound which is less prone to seek additional electron density from the H atoms of the ancillary ligands. structural similarity between adduct and transition state is not as pronounced as that described for the CH activation process.

This difference between the two transition states can be traced to their relative energies on their respective reaction coordinates. Unlike the CpM(NO)(CH2) + CH4 reaction, the transition state for NH activation is not close in energy to either reactants or products, and so is not expected to bear a striking resemblance to either species.

Although the transition state is much higher in energy with respect to the adduct than is observed for the CH activation process, the transition state is still significantly more stable than the free CpM(NO)(CH2) and NH3, by 9.2 and 19.8 kcal/mol for Mo and W, respectively. The strength of the metal-amido  bond also provides a rationale for the relative stability of the CpM(NO)(CH3)(NH2) complexes. Compared to free CpM(NO)(CH2) and ammonia, the methyl amido compounds are more stable by 56.0 and 69.9 kcal/mol for Mo and W, respectively. While this enormous stabilization may be due in part to the bonding component of the MNH2 linkage, it seems likely that  donation from the filled p orbital on the amido N atom into the empty metal-based orbital perpendicular to the metalnitrosyl bond also plays a significant role. It is interesting to compare the relative exothermicity of the addition of NH3 and CH4 to CpM(NO)(CH2) to a previous study of CH and NH activation by the 16e CpRh(CO) species. 35 Musaev and Morokuma found that although ammonia formed a much stronger adduct than methane to the CpRh(CO) fragment, the formation of the CH activation product CpRh(CO)(CH3)H was much more exothermic than the generation of CpRh(CO)(NH2)H from NH activation. This difference was attributed to the relative weakness of the RhN bond due to the strong repulsion between the metal d electrons and the ligand lone pair. 35 While such "filledfilled repulsions" is a common phenomenon in late metal amido complexes, 36,[START_REF] Holland | interpretation of bond strengths in late metal amido complexes based on bond polarity considerations rather than  bonding[END_REF] the presence of a stabilizing ligand-to-metal  donation in the CpM(NO)(CH3)(NH2) d 4 complexes results in the opposite energetic trend for CH vs. NH activation in this system. The large barrier to the regeneration of CpM(NO)(CH2) via elimination of ammonia from CpM(NO)(CH3)(NH2) helps explain the enhanced thermal stability of CpMo(NO)(CH2CMe3)(NR2) derivatives compared to the bis(neopentyl) precursors. 9

CpM(NO)(CH3)(NH2). As shown in

Adduct Formation. Since the initial synthesis of the 16e CpM(NO)(alkyl)2 complexes, 26 the reactivity of the metalalkyl bonds with small molecules such as H2, CO, H2O and O2 has been a topic of ongoing research. These reactions are generally presumed to proceed via initial coordination to the metal at the position trans to the NO group. 18 The two possible CpM(NO)(CH3)2(NH3) isomers were examined to explore the selectivity of nucleophilic attack and to gauge the relative Lewis acidities of Mo and W. The first distortion is a well-recognized hallmark of CpML4 chemistry, 21,38 and is controlled by the relative -bonding properties of pairs of mutually trans ligands. [START_REF] Lin | For a complete analysis of the -bonding interactions and z 2 and x 2 y 2 d orbital rehybridization responsible for this pairwise angular distortion of CpML2L'2 complexes[END_REF] The two trans ligands capable of the strongest covalent  bonding will possess small CPML angles approaching a minimum of 90°, while the other two ligands will adopt large CPML angles of up to 135°. The CPML angles shown in Figure 7(a) and 7 (b) are consistent with the relative -bonding abilities of the L groups. The thermodynamic preference of trans-CpM(NO)(CH3)2(NH3) over the cis isomer is attributed to the optimal pairing of the two trans methyl groups while the comparatively weak  donor NH3 is opposite the nitrosyl ligand. The large CPMNO angle is also required for efficient nitrosyl  bonding with the two occupied metal d orbitals. 21,38 The second angular distortion in both CpM(NO)(CH3)2(NH3) isomers induces the groups cis to NO to bend away from the nitrosyl ligand, resulting in a compression of the comparatively large methyl and ammine ligands that is completely contrary to steric expectations. The electronic origins of this deformation in trans-CpM(NO)(CH3)2(NH3) are illustrated schematically in Figure 8. In the idealized structure in Figure 8(a), the metal, the Cp centroid, and the two N atoms lie on the mirror plane of the complex, while the two methyl C atoms, the metal, and the Cp centroid define a second plane perpendicular to the Cs plane. The HOMO of the complex is MNO  bonding in character, and has nodal planes that coincide with the two planes containing the ML axes. In the optimized structure, the movement of the methyl ligands towards the NH3 group moves the MCH3 axes away from the HOMO nodal plane. As shown in Figure Comparison with Experimental Systems. Nitrosyl stretching frequencies provide both a convenient spectroscopic handle and a rough gauge of the electron density at the metal center in CpM(NO)-containing complexes. Table 1 shows the calculated (NO) values for the more stable compounds, along with relevant experimental values. 6,9,[40][41][42] The Mo frequencies are calculated to be ~20 cm 1 higher than for W, accurately reflecting the experimentally-observed trend. 18 However, the calculated values are uniformly higher than the experimental frequencies, by up to 70 cm 1 or more for the methyl amido compounds. This discrepancy is likely due to the neglect of anharmonicity effects in the computational technique, which is known to lead to inflated calculated frequencies. 43 The simplified model compounds may also be partially responsible for the systematic increase in (NO), especially since the large hydrocarbyl groups at the periphery of the alkyl, amido, and Cp ligands typically employed experimentally have been replaced with less-electron donating H atoms. a Nujol-mull IR spectra.

These same structural variations between the actual compounds and the computational models probably also contribute to the anomolously large calculated activation barriers. The conversion of CpMo(NO)(CH3)2 to CpMo(NO)(CH2) and methane has a calculated barrier of 32.7 kcal/mol, compared to the value of 18.7 kcal/mol obtained from kinetic measurements of the elimination of CD3CMe3 from CpMo(NO)(CD2CMe3)2 in a variety of solvents. 9 The proclivity of bulky neopentyl ligands to engage in  abstraction reactions at enhanced rates is well recognized, in stark contrast to the slow formation of methylene species from methyl complexes. 28 While the CH3 group is an acceptable substitute for CH2CMe3 in order to obtain the qualitative comparisons required for the present study, a more accurate model is apparently necessary to achieve reasonable quantitative agreement with experiment. 44 The relatively subtle electronic effects that underpin the structural deformations illustrated in Figures 4 and8 play a critical and previously unappreciated role in the characteristic reactivity of CpM(NO)(alkyl)2 complexes. Three distinct features of these compounds contribute to their selective binding of small molecules and subsequent intramolecular reactions with the metalcarbon bonds. The large CMC angle renders the metal prone to nucleophilic attack trans to the NO group by available L ligands. The disparity in and -bonding properties between the alkyl and nitrosyl groups thermodynamically favors the isomer with L trans to the NO. However, the compression of the groups cis to NO in CpM(NO)(alkyl)2(L) complexes increases the steric strain in these complexes, particularly when the bulky alkyl groups typical for this system (e.g.

CH2CMe3, CH2CMe2Ph, CH2SiMe3) are employed. The electronically-dictated small CML angles introduces a degree of steric selection in the binding of L to CpM(NO)(alkyl)2 compounds, discouraging the coordination of larger molecules (e.g.

coordinating solvents, the O atom of another CpM(NO)(alkyl)2 molecule) that could otherwise compete for the open coordination site at the metal.

The relative energies of cis and trans isomers of CpM(NO)(X)2(L) complexes is highly dependent on the nature of the X ligands. The trans form is favored experimentally when X = CH2R or H, since the L groups are typically poorer  donors than the alkyl or hydride ligands. However, if X = halide, the phosphine L ligands prefer to coordinate cis to the nitrosyl, as determined by [START_REF]For recent reviews of catalytic reactions involving CH ae and NH f,g activation by transition metal complexes, see: (a) Sen[END_REF] H NMR studies for CpMo(NO)(I)2(PMe2Ph) 45 and related CpM(NO)(X)2(L) and [CpM(NO)(X)(L)2] + complexes, [46][47][48] and by X-ray crystallography for CpMo(NO)(CH3)(Br)(PPh3). 49 Care must therefore be taken before extending the observations based on the bis(alkyl) complexes to other CpM(NO)(X)2(L) systems such as Cp*M(NO)(CH2CMe3)(Cl)-(PMe3). 29 The optimized geometries of the singlet CpM(NO)(CH3)2, CpM(NO)(CH2)(NH3), CpM(NO)(CH3)(NH2), and CpM(NO)(CH3)2(NH3) complexes are in relatively good agreement with corresponding structures characterized by X-ray crystallography as tabulated in the Supporting Information. [START_REF] Lumb | For the X-ray crystallographic studies used for comparisons with the CpM(NO)(CH3)2, a,b CpM(NO)(CH2)(NH3), a[END_REF] The correlation between the DFT and X-ray results is the poorest for bond angles that are susceptible to steric effects, especially when the simplified ligands in the model complex differ markedly from the closest known experimental structure (e.g. Cp vs. Cp*, NH3 vs. PMe3). However, in each complex, bonding parameters that are primarily influenced by electronic effects display particularly good agreement with experimental values, such as the H3CMCH3 angles and the nitrosylalkylidene (or nitrosylamido) orientations discussed above.

Relative Orbital Energies. Many of the interpretations presented in this paper have been based on the energies and bonding characteristics of the frontier orbitals of the optimized structures. A potential problem underlying this approach is the fact that the KohnSham orbitals obtained from DFT calculations are inherently different than the orbitals resulting from HartreeFock or EHMO techniques. Hoffmann and co-workers have recently demonstrated the similarity between these different types of orbitals, indicating that KohnSham orbitals do indeed provide a suitable basis for the qualitative analysis of chemical problems. 51 This study noted, however, that the KohnSham orbital energies require the application of a linear scaling factor in order to obtain quantitative agreement with energies calculated by HartreeFock or EHMO techniques.

Despite these reservations, comparing the relative orbital energies for congeneric Mo and W complexes reveals that with the sole exception of the LUMO of CpM(NO)-(CH2), the frontier orbitals are lower in energy than for Mo than for W in all of the singlet reactants, products, and intermediates. This observation provides an interesting rationale for many of the reactivity differences observed in the present system. The decreased energy of the two highest-lying occupied orbitals, the HOMO and SHOMO, for Mo results in poorer -backbonding with the unoccupied, relatively high-lying nitrosyl NO * orbitals, as reflected in the uniformly higher (NO) values for Mo than for W. The lower energy of the LUMO in CpMo(NO)(CH3)2 is consistent with the marked tendency of the CpMo(NO)(alkyl)2 complexes to form L adducts more readily than the corresponding W species. 18 The larger CMC angles for Mo than for W observed experimentally in the bis(alkyl) compounds may also be traced to the lowerenergy LUMO for Mo, which presumably mixes more readily with occupied orbitals thereby enhancing the second-order Jahn-Teller effect illustrated schematically in Figure 4. 34,52,53 As part of an ongoing investigation into spin state changes in organometallic compounds, 54,55 we have recently used DFT techniques to examine spin state effects in unsaturated CpMLn species, 56 including CpM(NO) complexes of Cr, Mo, and W. 24,57,58 In a previous study, the ligand effects on the relative energies of the singlet and triplet spin states were examined for CpCr(NO)(CH3)X complexes (X = CH3 or NH2); 57 for comparison, these EST (EST = E(triplet)  E(singlet)) results are displayed in Table 2 along with the new values obtained for the heavier Group 6 congeners. compounds. The relative stability of the singlet spin state compared to the triplet again follows the periodic trend W > Mo >> Cr. As expected, the MNO bond is elongated in all of the triplet species due to partial depopulation of the metalnitrosyl  bonding orbitals, although this variation is not as pronounced as that previously noted for the corresponding Cr complexes. 57 In a previous computational study, singlet CpW(NO)(CH2) was calculated to be only 2.4 kcal/mol more stable than triplet CpW(NO)(CH2). 24 Extending these calculations to the Mo complex reveals that CpMo(NO)(CH2) favors the singlet spin state to an even greater extent, namely 9.0 kcal/mol. As shown in Figure 9(c), the plane containing the MoCH2 moiety remains aligned with the MoNO axis, and the MoCH2 length is relatively invariant compared to the singlet state. The MoNO distance is longer in the triplet species, and the geometry at Mo is planar for the triplet as indicated by the ONMoCPCH2 dihedral angle of 176.6°. This difference in conformation at metal between singlet and triplet spin states has previously been observed in computational studies of CpW(NO)(L) complexes, 24,58 as well as for CpMn(CO)2 and

Singlet and

[CpFeL2] + species. 25,34 Although the large EST values calculated for the CpM(NO)(CH2) complexes preclude the direct involvement of the triplet spin state in the CH or NH bond activation processes, it is interesting to compare the singlettriplet energy gaps for the 16e Mo and W alkylidene species and their differences in reactivity. Su and Chu have performed computational studies on several CH bond activating systems involving congeneric Rh/Ir or Pd/Pt compounds, and have noted that the 16e fragments which have large singlettriplet gaps have larger activation barriers and lower exothermicities than fragments with smaller EST values. 59 Other related systems also adhere to this trend, with the third row species possessing a smaller EST value, lower barriers and greater exothermicities than the second row compound. 60,61 While the CH and NH bond activations reported here do not proceed via the same oxidative addition mechanism common to the aforementioned systems, it is worth noting that the correlation between reactivity and EST also holds true for the CpMo(NO)(CH2) and CpW(NO)(CH2) species. The reactivity of CpM(NO)(alkyl)2 complexes with small molecules appears to be governed by previously unappreciated and -bonding effects. The  agostic interactions, angular deformations, size selectivity and preference for trans adducts can all be interpreted in terms of electronic effects. The difference in reactivity between Mo and W in this class of complexes appears to be linked to their relative orbital energies, which are consistently lower for Mo than for W.

Conclusions

Although the simplified model compounds examined here provide useful information regarding qualitative energetic trends and electronic effects, this approach neglects important steric considerations. The current system is quite sensitive to such effects, as demonstrated by the pronounced difference in thermal stability between Cp*W(NO)(CH2CMe3)(CH2SiMe3) and Cp*W(NO)(CH2CMe3)2, for example. 6 The poor quantitative agreement between calculated and experimental activation barriers are presumed to be due to steric differences between methyl and neopentyl groups.

Computational Details

All calculations were performed using Gaussian 94 62 on an SGI Origin200 workstation. The LANL2DZ basis set was employed to perform geometry optimizations with a DFT approach. The three parameter form of the Becke, Lee, Tang and Parr functional (B3LYP) 63 was used in all cases, and no spatial symmetry constraints were imposed. The LANL2DZ basis set includes both Dunning and Hay's D95 sets for H, C, N and O, [START_REF] Dunning | Modern Theoretical Chemistry[END_REF] and the relativistic Electron Core Potential (ECP) sets of Hay and Wadt for Mo and W. [START_REF] Hay | [END_REF][66][67] Separate frequency calculations on the singlet species established that all the transition states, optimized by a quasi-Newton method, possessed one and only one imaginary frequency, and that the reactants and intermediates possessed no imaginary frequencies. Zero-point energy corrections were applied to the singlet species illustrated in Figure 2. The energies reported for the open shell (triplet) systems correspond to unrestricted B3LYP calculations. The mean value of the first-order wavefunction, which is not an exact eigenstate of S 2 for unrestricted calculations on the triplet systems, was considered suitable for the unambiguous identification of the spin state. Spin contamination was carefully monitored and the value of <S 2 > for the unrestricted B3LYP systems at convergence were in the narrow 2.00832.0161 range for triplets.

Frontier orbital energies and spatial plots were obtained from Gaussian 94 output using Molden v3.2. [START_REF] Schaftenaar | CAOS/CAMM Center Nijmegen[END_REF] In order to discern the source of the methyl group angular distortions of CpM(NO)(CH3)2 and trans-CpM(NO)(CH3)2(NH3), single point calculations were performed on idealized pseudo-Oh and pseudo-C2v structures. The frontier orbital energies and bonding characteristics were then compared to those obtained for the corresponding optimized geometries, as illustrated schematically in Figures 4 and8.

Figure 1 .

 1 Figure 1. Qualitative diagram of frontier molecular orbitals and energies for (a) CpM(NO)(CH3)2 and (b) CpM(NO)(CH3)(NH2).

  Bond Activation. The relative energies of the intermediates, transition states and products of the reactions of CpM(NO)(CH2) with CH4 and NH3 are shown in Figure 2. All geometries were optimized without symmetry constraints in the singlet spin state, and the energies (including zero-point energy corrections) are calculated relative to CpM(NO)(CH2) plus free CH4 or NH3.

Figure 2 .

 2 Figure 2. Potential energy profiles of the reaction of CpM(NO)(CH2) with CH4 and NH3.

Figure 3 .

 3 Figure 3. Optimized geometries of (a) CpM(NO)(CH3)2, (b) CpM(NO)(H3CHCH2), and (c) CpM(NO)(CH2)(CH4).

Figure 4 .

 4 Figure 4. Qualitative diagram of frontier MO energies for (a) CpM(NO)(CH3)2 in ideal CpML3 geometry and (b) CpM(NO)(CH3)2 with increased H3CMCH3 angle. Pictorial representation of LUMO for (a) and (b) in M(CH3)2 plane, as viewed down the MNO axis.

  Transition metal alkane complexes31 are often invoked as intermediates in CH bond activation reactions by unsaturated organometallic complexes.[START_REF] Bromberg | For recent kinetic, spectroscopic, and theoretical investigations implicating transition metal alkane complexes[END_REF] As illustrated in Figure3(c), methane adducts of CpM(NO)(CH2) were located as distinct intermediates for both Mo and W. The methane ligand is coordinated in an  2 -C,H mode, with the next closest M•••H distance being > 0.4 Å longer. The most distinctive features of the methane group are the slightly elongated CH length for the bound hydrogen (1.119 and 1.120 Å for Mo and W, respectively, with the remaining methane CH bonds all less than 1.10 Å), and an increased HCH angle between the two hydrogens closest to the metal center (116.5° and 117.2° for Mo and W,

  with the EST values calculated for the CpM(NO)(CH2) species. When CpMo(NO)(CH2CMe3)2 is thermolyzed in alkane or arene solvents in the absence of additional trapping reagents, formation of the bimetallic [CpMo(NO)](- 1 : 2 -NO)(-CHCMe3)[CpMo(CHCMe3)] complex occurs. 8 While the intermediate CpMo(NO)-(CHCMe3) species can presumably undergo facile solvent coordination and exchange, the barrier to 1,2-CH addition across the Mo=CHR bond is comparatively large. Instead of activating solvent CH bonds, the alkylidene species is eventually trapped by another molecule of CpMo(NO)(CHCMe3) to afford the observed bimetallic product. The present calculations suggest that once the corresponding W alkylidene intermediate is bound to an alkane solvent molecule, the competing dissociation and activation processes are almost equally facile. CpM(NO)(CH2). The intermediacy of reactive, unsaturated Cp*W(NO)(CHCMe3) and CpMo(NO)(CHCMe3) in the thermolytic reactions of the corresponding bis(neopentyl) compounds has been established experimentally using kinetics, labelling, and trapping experiments. 6,8-10 The optimized geometry of CpW(NO)(CH2) at the current level of theory was recently reported; 24 the calculated structure of the corresponding CpMo(NO)-(CH2) species is illustrated in Figure 5. As previously observed for singlet CpW(NO)(L)

Figure 5 .

 5 Figure 5. Optimized geometry of CpMo(NO)(CH2).

Figure 6 .

 6 Figure 6. Optimized geometries of (a) CpM(NO)(CH2)(NH3), (b) CpM(NO)-(H2CHNH2), and (c) CpM(NO)(CH3)(NH2).

Figure 6 (

 6 c), the geometry of CpM(NO)(CH3)(NH2) is distorted slightly from the ideal CpML3 geometry. The deformations involve the methyl ligand bond angles, although the distortions are not as pronounced as in the CpM(NO)(CH3)2 or CpM(NO)(CH2)(NH3) species. The H2NM group is essentially planar and is aligned with the MNO axis, as indicated by the HNMH and HNMN(O) dihedral angles, all of which are close to 0 or 180°. The short MoNH2 and WNH2 bond lengths (1.941 and 1.947 Å, respectively) and the planarity and alignment of the amido moiety are all consistent with the presence of a strong MN bonding interaction.

  The optimized geometries of trans-and cis-CpM(NO)(CH3)2-(NH3) are remarkably similar, as shown in Figure7(a) and 7(b), respectively. The favored trans isomers are calculated to be 15.8 and 16.0 kcal/mol more stable than CpM(NO)(CH3)2 and free ammonia for W and Mo, respectively, while the corresponding cis isomers are found to be stabilized by only 10.6 and 9.3 kcal/mol. The MCH3 and MNH3 bond lengths are elongated by roughly 0.10 to 0.05 Å compared to the more coordinatively-unsaturated compounds in Figures3 and 6, with the ligand trans to the nitrosyl typically displaying the most pronounced lengthening. With regard to the bond angles, two distinct deformations from an ideal pseudo-C4v CpML4 geometry can be discerned, involving (i) the CPML angles and (ii) the bending of the groups cis to the NO away from the nitrosyl ligand.

Figure 7 .

 7 Figure 7. Optimized geometries of (a) trans-CpM(NO)(CH3)2(NH3) and (b) cis-CpM(NO)(CH3)2(NH3).

8( b )

 b , this compression allows the HOMO to distort in order to improve the overlap with the NO * orbitals, thereby strengthening the MNO  bond and lowering the HOMO energy.

Figure 8 .

 8 Figure 8. Qualitative diagram of frontier MO energies for (a) trans-CpM(NO)(CH3)2-(NH3) with H3CMCPCH3 dihedral angle = 180° and (b) trans-CpM(NO)-(CH3)2(NH3) with CH3 groups bent towards NH3. Pictorial representation of HOMO for (a) and (b), as viewed down the MCP axis.

Table 2 . 5 a

 25 EST for CpM(NO)(CH3)X (M = Cr, Mo, W; X = CH3, NH2) a EST in kcal/mol. b CpCr(NO)(CH3)X values from ref 57.The optimized geometries of triplet CpM(NO)(CH3)2 are shown in Figure9(a). In the CpM(NO)(CH3)2 compounds, the nitrosyl ligand is bent down with respect to the Cp centroid, while the CMC angle is compressed to very close to the pseudo-octahedral value of 90°. Unlike CpCr(NO)(CH3)2 which exhibits a very small singlet-triplet gap, the Mo and W complexes display a substantial preference for the singlet configuration.

Figure 9 .

 9 Figure 9. Optimized geometries of (a) triplet CpM(NO)(CH3)2, (b) triplet CpM(NO)-(CH3)(NH2), and (c) triplet CpMo(NO)(CH2).

  Computational investigation of CH and NH bond activation by CpM(NO)(CH2)provides a qualitative rationale for the distinct reactivity profiles observed for Mo and W compounds. While the optimized geometries along the calculated pathway are remarkably similar for the two metals, the 1,2-additions of NH3 and CH4 across the metalalkylidene bond of CpMo(NO)(CH2) are less exothermic and have greater reaction barriers than the congeneric W species. This trend is consistent with the energy gap between the singlet and triplet spin states for the CpM(NO)(CH2) intermediate, which is larger for Mo than for W (EST = 9.0 and 2.4 kcal/mol, respectively). The relative barrier heights for methane loss vs. CH activation from the CpM(NO)(CH2)(CH4) alkane complex are especially metal dependent, which offers a possible explanation for the absence of alkane or arene CH activation by CpMo(NO)(CHR) species. The formation of a strong MNH2 -bonding interaction helps make NH3 activation more exothermic than the reaction of CpM(NO)(CH2) with CH4, in contrast to late transition metal systems.

Table 1 .

 1 Calculated and Experimental Nitrosyl Stretching Frequencies

	Calculated	(NO), cm 1	Experimental a	ref.
	CpW(NO)(NH3)(CH2)	1563	1513	Cp*W(NO)(PMe3)(CHCMe3)	6

  Triplet Spin States. Spin state effects are increasingly being recognized as playing a crucial role in element-hydrogen activation reactions by

	unsaturated 16e species.	Ultrafast infrared spectroscopic techniques and DFT
	calculations have demonstrated that triplet CpMLn complexes such as CpMn(CO)2 or
	CpCo(CO) interact only weakly with alkane CH bonds, yet react readily with better
	ligands such as SiH bonds or CO.

Acknowledgment. We are grateful to Elizabeth Tran and Peter Legzdins for providing us with results prior to publication. R.P. thanks the Région Bourgogne for supporting this research, and K.M.S. thanks the European Commission for a TMR Marie Curie Postdoctoral Fellowship.

Supporting Information Available:

A listing of the cartesian coordinates and final energies of all calculated complexes, tables of bond lengths and angles of selected optimized geometries with comparisons to relevant X-ray crystallographic structures, pictures and energies of the frontier orbitals for the ground state singlet structures. This material is available free of charge via the Internet at http://pubs.acs.org.