Haley Blackburn

James C Fettinger

Heinz-Bernhard Kraatz

Rinaldo Poli
email: poli@u-bourgogne.fr

Raymund C Torralba

Preparation of trichloro-and tribromocyclopentadienyltungsten(IV)

Keywords: CO-free compounds, Diamagnetism, Metal-metal bonds, Tungsten, X-ray structures

) and [Cp*WCl 3 ] 2 (m-O) (7).

Introduction

Monoyclopentadienylmetal halide systems are excellent synthons to a variety of other organometallic compounds [1]. For the heavier Group 6 metals (Mo and W), systems with the metal in the oxidation states V, (Ring)MX 4 (Ring= Cp or substituted analogue) [2 -5], and III, [(Ring)MX 2 ] 2 [5 -8], have been known for some time. The corresponding systems of Mo(IV), [(Ring)MoX 3 ] 2 , have been more recently developed [9][10][11][12][13]. Here, we report on the corresponding development of the previously unknown systems of W(IV). These systems have attracted our attention not only because of their potential utility as starting compounds for further organometallic transformations, but also for their structural and bonding features. Compounds of Mo and W having the same stoichiometry are not always necessarily isostructural. This is exemplified by the structure of [(h 5 -C 5 H 4 i Pr)MCl 2 ] 2 (type I [6] for M=Mo and type II [7] for M=W). The W compound has a tendency to maximize the metal-metal bonding at the expense of metal-ligand bonding. The structure of [Cp*MoX 3 ] 2 (X=Cl, Br) was shown to be of type III with no direct metal-metal bonding interaction and the compounds show magnetic properties consistent with antiferromagnetic coupling between two S= 1 d 2 metal centers [11,12]. This structure is also found for the corresponding d 1 -d 1 Ta(IV) system, [(h 5 -C 5 Me 4 Et)TaBr 3 ] 2 [14], which similarly has no metalmetal bond. However, the corresponding d 3 -d 3 [(h 5 -C 5 Me 4 Et)ReCl 3 ] 2 is diamagnetic and adopts a different structure, i.e. IV, with a metal-metal bonding interaction. We have previously advanced the hypothesis [11] that an analogous compound of W(IV) could adopt a structure analogous to that of the Re system, since W is known to form stronger metal-metal bonds than Mo. In this contribution, we present a simple synthetic procedure for the preparation of these W(IV) systems and the structure of their phosphine addition products. The diamagnetism found for [(Ring)WX 3 ] 2 suggest that they indeed adopt a metal -metal bonded structure. The high oxophilicity of these materials leads to the formation of oxygen-containing by-products, two of which have been crystallographically characterized and are also described here.

Experimental

General data

All operations were carried out under an atmosphere of argon with standard Schlenk-line techniques. All glassware was silylated with a 1:10 v/v solution of dichlorodimethylsilane in CCl 4 . Solvents were distilled directly from the drying agent (toluene and heptane from Na, THF from Na/K/benzophenone, CH 2 Cl 2 from P 4 O 10 ) under argon prior to use. FT-IR spectra were recorded with KBr cells on a Perkin -Elmer 1800 spectrophotometer. NMR spectra were recorded on Bruker WP200, AF200, and AM400 spectrometers; the peak positions are reported with positive shifts downfield of TMS as calculated from the residual solvent peaks ( 1 H), or downfield of external 85% H 3 PO 4 ( 31 P). For each 31 P-NMR spectrum a sealed capillary containing H 3 PO 4 was immersed in the same NMR solvent used for the measurement and this was used as the reference. The elemental analyses were carried out by M-H-W Laboratories, Phoenix, AZ, Galbraith Laboratories, Knoxville, TN, and Desert Analytics Laboratory, Tucson, AZ. Compounds (h 5 -Ring)WX 3 (CO) 2 (Ring= C 5 H 5 , C 5 H 4 Me, C 5 Me 5 ; X=Cl, Br) were prepared as previously described [15]. Dppe (Strem) was used without any further purification. The products of these reactions are only sparingly soluble in all common solvents and cannot be easily recrystallized. The spectroscopic and analytical data have been obtained for the crude materials and have been tabulated in Table 1.

(a) [(h 5 -C 5 H 5 )WCl 3 ] 2 (1a). (h 5 -C 5 H 5 )WCl 3 (CO) 2 (534 mg, 1.30 mmol) was placed in a silylated Schlenk tube equipped with a magnetic stir bar and a silylated reflux condenser. Toluene (20 ml) was added and the resulting yellow-green slurry was refluxed for 3 h, yielding a dark emerald-green slurry. An IR spectrum on an aliquot of the mother solution showed no carbonyl stretching frequencies. The solution was evaporated under reduced pressure to ca. 2-3 ml and the precipitation of the product was completed by the addition of heptane (50 ml). After filtration, the solid was washed with heptane (3× 10 ml) until the washings were colorless. The washings were cannulated off and discarded. The solid was then dried under vacuum and stored in flame-sealed vials under dinitrogen. An IR spectrum on the residual solid showed no CO stretching vibrations. Yield: 463 mg (87%).

(b) [(h 5 -C 5 H 4 Me)WCl 3 ] 2 (2a). By a procedure identical with that described in the preceding section, (h 5 -C 5 H 4 Me)WCl 3 (CO) 2 (525 mg, 1.24 mmol) gave 442 mg of product (84%).

An identical reaction with similar quantities of all materials was performed. After the reflux in toluene, the solution was filtered and cooled to -20°C, affording a few dark-green crystals. A single crystal of this batch was used for the X-ray analysis, revealing the trinuclear by-product (h 5 Schlenk tube as described above in (a). The Schlenk tube was then equipped with a magnetic stir bar and a silylated reflux condenser. Toluene (20 ml) was added and the resulting brown -yellow slurry was refluxed overnight, yielding a red -brown slurry. An IR spectrum on an aliquot of the mother solution showed no carbonyl stretching frequencies. The solution was evaporated under reduced pressure to ca. 2 -3 ml. The precipitation of the product was completed by the addition of heptane (50 ml). The solid was washed with heptane (3× 10 ml) until the washings were colorless, dried under vacuum, and stored in flame-sealed vials under dinitrogen. An IR spectrum on the residual brown solid did not show CO stretching vibrations. Yield: 434 mg (57%).

(d) [(h 5 -C 5 H 4 Me)WBr 3 ] 2 (2b). By a procedure identical with that described in (a) above, (h 5 -C 5 H 4 Me)WBr 3 (CO) 2 (283 mg, 0.507 mmol) gave 160 mg of brown solid (63%).

2.3.

Thermal decarbonylation of (p 5 -C 5 Me 5 )WCl 3 -(CO) 2 . Formation of [(p 5 -C 5 Me 5 )WCl 3 ] 2 (v-O) (7) (h 5 -C 5 Me 5 )WCl 3 (CO) 2 (352 mg, 0.731 mmol) was placed in a silylated Schlenk tube equipped with a magnetic stir bar and a silylated reflux condenser. Toluene (20 ml) was added and the resulting dark-yellow slurry was refluxed for 3 h, yielding a dark olivegreen slurry. An IR spectrum on an aliquot of the mother solution showed no carbonyl stretching frequencies. After evaporation to dryness at reduced pressure, the residue was extracted with dichloromethane affording an emerald-green solution and an olive-green residue. The olive-green residue was sparingly soluble in toluene and moderately soluble in dichloromethane. 1 H-NMR (CD 2 Cl 2 ): l 2.24 (s). The emerald-green solution was evaporated to dryness under reduced pressure and redissolved in toluene. The solution was heated to reflux and filtered while hot. The filtrate was allowed to cool slowly to room temperature affording dark greenblack crystals. Yield: 64 mg. 1 H-NMR (CD 2 Cl 2 ): l 1.30 (s); (C 6 D 6 ) l 1.73 (s). A single crystal of this batch was used for the X-ray analysis.

2.4.

Reaction between [(p 5 -Ring)WCl 3 ] 2 (Ring=C 5 H 5 , C 5 HMe) and dppe. Formation of (p 5 -Ring)WCl 3 (dppe) (a) (h 5 -C 5 H 5 )WCl 3 (dppe) (4). [(h 5 -C 5 H 5 )WCl 3 ] 2 (358 mg, 1.01 mmol) was placed in a Schlenk tube with 20 ml dichloromethane and to the resulting green suspension was added solid dppe (402 mg, 1.01 mmol). The Schlenk tube was equipped with a reflux condenser and magnetic stir bar and the mixture was refluxed for 3 h. The starting material dissolved to yield a dark-brown solution. An aliquot of this solution was evaporated to dryness and the residue was redissolved in CD 2 Cl 2 and investigated by 1 H-NMR spectrometry. 1 (5). By using a procedure identical with that described above for the corresponding Cp compound, [(h 5 -C 5 H 4 Me)WCl 3 ] 2 (610 mg, 1.65 mmol) and dppe (658 mg, 1.65 mmol) yielded a brown solution, from which 925 mg of crude product (79%) was obtained after filtration, evaporation to ca. 5 ml, and addition of pentane (50 ml). 1 H-NMR (CD 2 Cl 2 ): l 7.70-7.27 (m, Ph, 20H), 4.44 (br, Cp%, 4H), 3.47 (br, CH 2 , 2H), 3.11 (br, CH 2 , 2H), 2.17 (s, Cp%, 3H); 31 P-NMR (CD 2 Cl 2 ): l 12.96 (s, 1 J PW = 226 Hz). A single crystal of this compound was obtained by slow diffusion of an n-pentane layer into a CH 2 Cl 2 solution.

X-ray crystallography

(a) Compound 5. A dark-red parallelepiped crystal with dimensions 0.20× 0.20× 0.40 mm was placed on the Enraf-Nonius CAD-4 diffractometer. The cell parameters and crystal orientation matrix were determined from 25 reflections in the range 6.1B q B 13.9°a nd confirmed with axial photographs. Data were collected [Mo-K a ] with -2q scans over the q range 1.6-22.5°with a scan width of (1.26+ 1.05 tan q)°and a fixed scan speed of 4.12°min -1 with each scan recorded in 96 steps with the outermost 16 steps on each end of the scan being used for background determination. No decay correction was necessary (B3% decay). Intensity statistics clearly favored the non-centrosymmetric case. The structure was determined by direct methods with the successful location of the tungsten atom in the space group P2 1 . Subsequent difference-Fourier maps revealed the location of all remaining non-hydrogen atoms within the main molecule and also the two methylene chloride molecules in the asymmetric unit. Hydrogen atoms were placed in calculated positions as described in section (a) above. After convergence, a final difference-Fourier map exhibited three peaks in the vicinity, i.e. within 1.2 A , , of the tungsten atom with Dr5 2.71 e A , -3 . Selected crystal data and refinement parameters are collected in Table 2, and relevant bond distances and angles are listed in Table 3.

(b) Compound 6. A black crystal with dimensions 0.13× 0.08×0.08 mm was placed and optically centered on the Enraf-Nonius CAD-4 diffractometer. The cell parameters and crystal orientation matrix were determined from 25 reflections in the range 8.6B qB 20.7°and confirmed with axial photographs. Data collection and reduction was carried out as described in (a) above. Two forms of data were collected in the hemisphere 9 hk9l, resulting in the measurement of 5392 reflections, 2707 of which were unique. Minor (1-4%)

R a [I\2|(I)] 0.0825 R w b [I\2|(I)] 0.0746 0.1964 a R =SF o -F c /SF o . b R w = [Sw(F o -F c ) 2 /SwF o 2 ] 1/2 ; w= 1/| 2 (F o ).
variations in intensity were observed for three nearly orthogonal standard reflections monitored at 1-h intervals of X-ray exposure, thus the data were not corrected for decay. The data were corrected for Lorentz and polarization factors, and for absorption on the basis of the c-scan method. Intensity statistics clearly determined the centrosymmetric monoclinic space group P2 1 /c (no. 14). The structure was solved by direct methods with the successful location of the three tungsten atoms. Subsequent difference-Fourier maps revealed the location of all remaining non-hydrogen atoms within the molecule. Hydrogen atoms were placed in calculated positions, with the aromatic hydrogens at d(C H)= 0.950 A , and U H =1.2 U (parent) while methyl hydrogens were initially determined with a rotational Fourier about the central carbon atom that was further optimized for tetrahedral geometry with d(C H) =0.980 A , and U H =1.5 U (parent). After convergence, a final difference-Fourier map exhibited several peaks in the vicinity, i.e. within 1.2 A , of the tungsten atoms with Dz 51.58 e A , -3 . Selected crystal data and refinement parameters are collected in Table 2, and relevant bond distances and angles are listed in Table 4.

(c) Compound 7. A reddish -black crystal with dimensions 0.20× 0.20×0.25 mm was placed on the Enraf -Nonius CAD-4 diffractometer. The cell parameters and crystal orientation matrix were determined from 25 reflections in the range 12.9B q B15.2°and confirmed with axial photographs. Data collection and reduction was carried out as described in (a) above. No decay correction was necessary (B3% decay). Systematic absences clearly indicated the centrosymmetric space group Pbca (no. 61). The structure was deter-mined by direct methods with the successful location of the tungsten and chlorine atoms. Subsequent difference-Fourier maps revealed the location of all remaining non-hydrogen atoms within the molecule. Hydrogen atoms were inserted and handled as described in the previous section. After convergence, a final difference-Fourier map possessed three peaks in the vicinity, i.e. within 1.2 A , of the tungsten atom with Dz51.29 e A -3 and the largest remaining peaks had Dz5 0.65 e A -3 . Selected crystal data and refinement parameters are collected in Table 2, and relevant bond distances and angles are listed in Table 5. pared with refluxing CH 2 Cl 2 for the Mo systems), is somewhat remarkable because the strength of the M CO bond increases upon descending a group of transition metals. For instance, while thermal decarbonylation of MoI 2 (CO) 3 (dppm) cleanly affords Mo 2 I 4 (dppm) 2 [16], the corresponding WI 2 (CO) 3 -(dppm) affords only WI 2 (CO)(dmpm) 2 in small yields [17].

The solubility of the W(IV) trichloride products increases, as expected, in the order CpBCp%. 1 H-NMR characterization for the Cp compounds could only be carried out in dilute CD 2 Cl 2 solutions, whereas the Cp% derivatives are sufficiently soluble in aromatic hydrocarbons. The 1 H-NMR properties of the isolated materials (see Table 1) indicate diamagnetism, in contrast with the corresponding Mo analogues [11,12], this hinting to a different structure for the Mo and W analogues (see Section 1). Unfortunately, suitable single crystals could not be obtained for any of these derivatives. During one of the various crystallization attempts, crystals of an oxygenated by-product were obtained for the Cp%-chloro system (Cp% 3 W 3 Cl 5 O, compound 6). The structure of this compound, showing several remarkable features, is discussed later. It is noteworthy that oxygen incorporation in these systems is associated with formal metal reduction. We have not been able to elucidate the nature of the concomitant oxidation products.

In an attempt to facilitate the crystallization process, the synthesis of the more methyl-substituted (thus presumably more soluble) Cp*-chloro derivative was attempted by the same decarbonylation method shown in Eq. (1). Indeed, the decarbonylation of Cp*WCl 3 (CO) 2 yielded an intensely colored green solution, which contains two diamagnetic products according to 1 H-NMR spectroscopy. The major product could be obtained in the form of single crystals and was shown by X-ray crystallography to correspond to the oxo compound [Cp*WCl 3 ] 2 O (7), vide infra. This result was reproduced several times, in spite of our best attempts to rigorously exclude air and moisture (see Section 2). The oxygen atom could be originating from a CO ligand by a process of metal-mediated CO bond breaking, which is not unprecedented for tungsten [18,19]. If this is the case, a carbido ligand could possibly be incorporated into the other observed product which, unfortunately, could not be obtained in a pure, crystalline form suitable to full characterization [20].

For the less methyl-substituted Cp and Cp% systems, only a single compound could be detected by 1 H-NMR investigations of the crude products and satisfactory C, H analyses were obtained. The identity of the compounds is further confirmed by the derivatization reactions discussed in the next section.

Results and discussion

Synthesis and physical properties

The preparation of the trihalocyclopentadienyltungsten(IV) compounds follows one of the strategies previously adopted for the synthesis of the corresponding Mo(IV) compounds [10 -12], i.e. thermal decarbonylation of the dicarbonyl adducts, see Eq. ( 1). The latter compounds are easily available by dihalogen oxidation of the corresponding [(Ring)W(CO) 3 ] 2 compounds [15].

(Ring)WX 3 (CO) 2 D toluene 1/2[(Ring)WX 3 ] 2 +2Co (1)
(Ring =Cp, Cp%; X=Cl, Br)

The occurrence of a complete decarbonylation of these tungsten carbonyl derivatives, albeit under rather forcing conditions (refluxing toluene as com- These reactions show that the W W bonds in these systems are sufficiently weak to allow their rupture and the formation of mononuclear ligand adducts. However, the reactions are quite slow when compared with those of the corresponding [(Ring)MoCl 3 ] 2 complexes in which the metal centers are held together only by two bridging chloride ligands [9,11]. Both products of reaction 2 are diamagnetic compounds and are stoichiometric analogues of the previously reported CpMoCl 3 (dppe), which was shown to adopt the pseudo-mer-octahedral structure V in the solid state [START_REF] Sta ¨rker | [END_REF]. The same stereochemistry was also established for compound CpMoCl 3 (dmpe) [22]. The NMR properties of the W derivatives, however, indicate equivalence for the two P nuclei (singlet resonance in the 31 P-NMR spectrum and triplet resonance for the Cp protons in the 1 H-NMR spectrum of 4) and are therefore more consistent with the alternative fac stereochemistry illustrated in VI, which is common, on the other hand, to the previously reported Mo(IV) bis-phosphite derivative CpMoCl 3 [P(OCH 2 ) 3 CEt] 2 [9], and to the W(IV) carbonyl derivatives (C 5 H 4 i Pr)WBr 3 (CO) 2 [23] and Cp*WBr 3 (CO) 2 [15]. The fac geometry of compound 5 has been unambiguously confirmed by an X-ray structural determination (see Fig. 1). Selected distances and angles are reported in Table 3. The cyclopentadienyl ring is significantly distorted from the ideal five-fold symmetry, the difference between the shortest and longest W C distance being 0.31(4) A , . The longest distance is to the methyl-substituted C atom [W(1)-C(2), 2.50(3) A , ], while the shortest ones are those to the opposite C atoms, C(4) and C(5), 2.22(3) and 2.19(3) A , , respectively. This distortion could be induced sterically by the proximity of the ring Me substituent and the Cl(1) atom and/or electronically by the back-donation from the metal d xy orbital to one of the ring d acceptor orbitals, as previously described [24]. A similar distortion has been noted for (h 5 -C 5 H 4 i Pr)WBr 3 (CO) 2 , whose stereochemistry is identical to that of compound 6 [23]. We are not aware of other crystal structures of monocyclopentadienyltungsten(IV) chloride or phosphine derivatives. The average W-Cl distance [2.46(3)A , ] is similar to the average Mo-Cl distance in the corresponding CpMoCl 3 (dppe) [2.48(1) A , ] [21], CpMoCl 3 [P(OCH 2 ) 3 CEt] 2 [2.47(4) A , ] [9], and CpMoCl 3 (dmpe) [2.494(7) A , ] [22] compounds. The axial (trans to Cp) W-Cl distance is slightly shorter than the equatorial (trans to P) distances, as previously seen also for CpMoCl 3 [P(OCH 2 ) 3 CEt] 2 [9]. The W-P distances are essentially equivalent and their average [2.568(8) A , ] is similar to the average Mo-P distance in CpMoCl 3 (dppe) [2.604 A , ] [START_REF] Sta ¨rker | [END_REF] and CpMoCl 3 (dmpe) [2.548 A , ] [22], although in the latter cases the two The different solid state structure of the homologous CpMCl 3 (dppe) compounds (V for Mo, VI for W) is an interesting phenomenon that does not have a clear-cut explanation. In solution, structure VI appears to be strongly favored for the W system (no other isomer can be detected by 31 P-NMR), whereas a more delicate balance is present for the Mo compound, an 80:20 mixture of isomers favoring V being established by 31 enyl halides of W(IV). A comparison with the previously developed Mo systems indicates two main differences. The decarbonylation of (Ring)MX 3 (CO) 2 is, as expected, less facile for W than for Mo. It does proceed to completion, however, in refluxing toluene. The structure of the two [(Ring)MX 3 ] 3 systems is quite different, the lack of metal-metal bonding leading to paramagnetism for M=Mo whereas diamagnetic compounds, presumably containing metal-metal bonds [20], are obtained for M=W. These compounds are able, however, to add neutral donors with formation of diamagnetic mononuclear derivatives (albeit more slowly than the corresponding Mo systems), as exemplified by the formation of the dppe adducts (Ring)WCl 3 (dppe) (Ring= Cp, Cp%). Finally, these W(IV) systems are quite reactive toward oxygen containing compounds, leading to the facile formation of oxygen-containing derivatives.

Supplementary material

Crystallographic data for the structural analyses have been deposited with the Cambridge W Cl p bonding. For the same reason, the W-Cl distances to the bridging Cl ligands are shorter on the W(2) and W(3) sides with respect to the W(1) side. All bridging W Cl bonds are, as expected, longer than the terminal ones.

Structure of [Cp*WCl 3 ] 2 (v-O)

A view of this compound is shown in Fig. 3 and selected bond distances and angles are given in Table 5. The compound crystallizes with the oxygen atom sitting on a center of symmetry, thus the linearity of the W-O-W moiety is crystallographically imposed. The geometry around each W atom is of the four-legged piano stool type, the four legs being the three terminal chlorine and the bridging O atoms. The three W -Cl distances are essentially equivalent [average 2.373(3) A , ]. These W-Cl distances and the W -O distance are significantly shorter than the corresponding parameters in the above described Cp% 3 W 3 Cl 5 O, in agreement with the higher metal oxidation state in [Cp*WCl 3 ] 2 O. In addition, W O multiple bonding is allowed in the dinuclear W(V) compound, further shortening the bonds, whereas only s bonds are allowed for the capping oxygen in the trinuclear cluster described above. The W -O distance is comparable with that of the isoelectronic {[Cp 2 W(CH 3 )] 2 (m-O)} 2 + complex (1.904(1) A , ) [33].

Conclusions

The study reported here has allowed us to establish the existence of the hitherto missing monocyclopentadi-

2. 2 .

 2 Thermal decarbonylation of (p 5 -Ring)WX 3 (CO) 2 (Ring=C 5 H 5 , C 5 H 4 Me; X= Cl, Br). Formation of [(p 5 -Ring)WX 3 ] 2

  (15) Cl(12)-W(1)-Cl(13) W(2)-W(3)-O(123

3. 2 .

 2 Formation of ligand adducts and X-ray structure of Cp%WCl 3 (dppe) Compounds [CpWCl 3 ] 2 and [Cp%WCl 3 ] 2 slowly add dppe in refluxing dichloromethane, to afford the 18electron adducts, see Eq. (2): [(Ring)WCl 3 ] 2 + 2dppe 2(Ring)WCl 3 (dppe) (2) (Ring = Cp, 4; Cp%, 5)

Fig. 1 .

 1 Fig. 1. A view of the molecular structure of compound Cp%WCl 3 (dppe).

  (10) Cl(2)-W-CNT 112.2(3) Cl(3)-W-O 85.04(7) Cl(3)-W-CNT 110.1(3) O-W-CNT 112.6(2) 180.0 W-O-W% individual distances are quite different from each other because of the different geometry (e.g. V).

  P-NMR in CD 2 Cl 2[START_REF] Sta ¨rker | [END_REF].

3. 3 .

 3 Scheme 1. with essentially equal distances to the three W atoms [average 2.02(2) A , ]. The average oxidation state of the metal in this compound is 3 and 1/3. A valence-trapped assignment as + 4 for W(1) and +3 for W(2) and W(3) is indicated by the structure. Mononuclear fourlegged piano stool compounds for W(IV) (e.g. having a 16-electron configuration) are not precedented, but this is a common geometry for analogous complexes of Mo(IV), for instance [CpMoCl 2 L 2 ] + and CpMoCl 3 L (L= tertiary phosphine) [25,26]. The two d electrons of the W(1) atom can be imagined to become involved in metal-metal bonding through participation of the d xy orbital, which is of suitable symmetry. If, at this point, the Cp%WCl 2 + unit of atom W(1) is removed from the cluster, the remaining fragment corresponds to a [Cp% 2 W 2 Cl 4 (m-O)] 2 -ion as illustrated in Scheme 1(a). The geometry of this ion strongly resembles that of the dinuclear Mo(III) ion [CpMoCl 4 (m-Cl)] -[27], shown schematically in Scheme 1(b), giving credit to our oxidation state assignment for atoms W(2) and W(3). The two geometries in Scheme 1 differ only in the relative disposition and angular relationships of the ligands (cisoid Cp% and transoid terminal Cl ligands in the W structure; transoid Cp ligands in the Mo structure). This difference probably results from the steric repulsion between the Cp% rings in the Cp% 3 W 3 Cl 5 O structure and by the forced relative configuration of the Cl(12) and Cl(13) ligands, these being locked into place by the need to bridge the W(2) and W(3) atoms, respectively, with W(1). The W(2)-W(3) bond is slightly longer than the Mo Mo bond in [Cp 2 Mo 2 Cl 5 ] -[2.413(1) A , ] [27], which was described as a weakened metal-metal triple bond. Other parallels may be drawn between the structure of 6 and other oxo-capped, trimetallic clusters such as Cp 3 Mo 3 Cl 7 O, [28] (Ring) 3 Nb 3 Cl x (OH) 7 -x O (Ring= Cp, Cp%; X=Cl, OH),[29,30] and Cp* 3 Ta 3 O 4 Cl 4 ,[31,32] but none of these show coordination geometries or a d-electron count as 6.The terminal W(1)-Cl bond is longer than the other terminal W Cl bonds, contrary to what would be expected on the basis of the oxidation state assignment given above. However, while the Cl(1) ligand is bonded to an effectively saturated W(1) center, the Cl(2) and Cl(3) are bonded to electronically unsaturated metal centers. The shorter W(2)-Cl(2) and W(3)-Cl(3) distances may therefore reflect the involvement of some

Fig. 2 .

 2 Fig. 2. A view of the molecular structure of compound Cp% 3 W 3 Cl 5 O.

Fig. 3 .

 3 Fig. 3. A view of the molecular structure of compound [Cp*WCl 3 ] 2 (m-O).

  H-NMR (CD 2 Cl 2 ): l 8.33-7.34 (m, Ph, 20H), 4.85 (t, Cp, 5H, J PH = 4 Hz), 3.41 (br, CH 2 , 2H), 2.77 (br, CH 2 , 2H); 31 P-NMR (CD 2 Cl 2 ): l 12.14 (s, 1 J PW = 229 Hz). (b) (h 5 -C 5 H 4 Me)WCl 3 (dppe)

Table 2

 2 Crystal data for all compounds

	6

Table 3

 3 Selected bond distances (A , ) and bond angles (°) for compound 5•2CH 2 Cl 2

	Bond angles (°)
	W(2)n ˜W(1)n ˜W(3)	56.46(3) W(3)-W(2)-O(123)	49.9(3)
	W(2)n ˜W(1)n ˜Cl(1)	119.82(12)	W(3)-W(2)-CNT2	114.2(3)
	W(2)n ˜W(1)n ˜Cl(12)	55.75(11)	Cl(2)-W(2)-Cl(12)	90.41(16)
	W(2)n ˜W(1)n ˜Cl(13)	109.12(12)	Cl(2)-W(2)-O(123)	81.4(3)
	W(2)n ˜W(1)n ˜O(123)	47.6(3)	Cl(2)-W(2)-CNT2	109.1(3)
	W(2)n ˜W(1)n ˜CNT1	120.5(3)	Cl(12)-W(2)-O(123)	76.3(3)
	W(3)n ˜W(1)n ˜Cl(1)	120.70(11)	Cl(12)-W(2)-CNT2	114.8(3)
	W(3)-W(1)-Cl(12)	110.29(11)	O(123)-W(2)-CNT2 163.9(4)
	W(3)-W(1)-Cl(13)	55.05(12)	W(1)-W(3)-W(2)	61.24(3)
	W(3)-W(1)-O(123)	46.9(3)	W(1)-W(3)-Cl(3)	94.72(12)
	W(3)-W(1)-CNT1	119.5(2)	W(1)-W(3)-Cl(13)	56.52(11)
	Cl(1)-W(1)-Cl(12)	81.67(16)	W(1)-W(3)-O(123)	46.3(3)
	Cl(1)-W(1)-Cl(13)	82.46(16)	W(1)-W(3)-CNT3	158.4(3)
	Cl(1)-W(1)-O(123)	85.2(4)	W(2)-W(3)-Cl(3)	112.27(14)
	Cl(1)-W(1)-CNT1	110.6(3)	W(2)-W(3)-Cl(13)	115.08(11)
		146.21

Table 5

 5 Selected bond distances (A , ) and bond angles (°) for [Cp*WCl 3 ] 2 (m-O)

	Bond distances (A , )
	W-Cl(1)	2.371(3)
	W-Cl(3)	2.372(3)
	W-Cl(2)	2.375(3)
	W-CNT	2.031(11)
	W-O	1.8709(4)
	Bond angles (°)
	Cl(1)-W-Cl(2)	80.17(11)
	Cl(1)-W-Cl(3)	139.45(12)
	Cl(1)-W-O	84.69(8)
	Cl(1)-W-CNT	110.1(3)
	Cl(2)-W-Cl(3)	79.96(12)
	Cl(2)-W-O	135.17

Acknowledgements

Support of this work from the US Department of Energy-Office of Energy Research (DEFG0295ER-14550) and the Re ´gion Bourgogne is gratefully acknowledged. The authors thank Dr Peter Burger for disclosing to us the results of his work on [Cp*WCl 3 ] 2 [20].

Data Centre, CCDC No. 116644 for 5•2CH 2 Cl 2 , CCDC No. 116646 for 6, and CCDC No. 116645 for 7. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: + 44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam. ac.uk).


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

Kevin M Smith

Pairing Energy Effects in Cyanide Complexes of CpCr III

Keywords: Cyanides, Density-functional calculation, Electronic structure, Spin state

Chromium /

Introduction

[CpCrX 2 (PH 3 ) n ] (X ϭ Cl, CH 3 ; n ϭ 1, 2) and The study of transition metal compounds containing cy-[CpMoCl 2 (PH 3 ) n ] systems in the doublet and quadruplet anide ligands has experienced a recent resurgence due in spin states. [12] In this study, DFT calculations were used to large part to the ability of these ligands to form determine the relative importance of the possible factors MϪCNϪMЈ bridges. [1] Extending these bridges in one, which contribute to the anomalous stability of the 17-electwo, or three dimensions results in novel materials often tron doublet CN species relative to the Cl analogues. Pospossessing microporous structures and interesting magnetic sible factors which we have previously considered include: properties. [2] As an alternative to homoleptic cyanometal-(a) steric considerations (since CN is smaller than Cl), (b) ates, anionic [CpM(CN) n ] Ϫ complexes have also been investhe CrϪL bond strength (the relative electron-withdrawing tigated in which the Cp or Cp* ligand serves to enhance the ability of CN could increase the relative strength of the adoverall inertness of the complex as well as to limit polymerditional CrϪPR 3 bond in the 17-electron species), (c) ization. [3] [4] changes in ∆E DϪQ (by modifying the orbital splitting and/ We have recently examined the reactivity of cyanide comor the pairing energy, CN could influence the relative staplexes of CpCr III as part of our ongoing investigation of bilities of the doublet and quadruplet spin states). [START_REF] Mattamana | [END_REF] spin-state effects in organometallic chemistry. [5] Cyanide ligands confer a greater stability on the resulting highly coordinated monometallic compounds than the halide ligands Results and Discussion on the corresponding, relatively unsaturated analogues. For example, in the case of complexes of CpMo IV , the cyanome-Optimized LanL2DZ Geometries and Energies talate exists as the 18-electron dianion, [6] [CpMo(CN) 5 ] 2Ϫ , while with the chloride ion the 16-electron monoanion, [7] The geometries of doublet [CpCr(CN) 2 (PH 3 ) 2 ], and [CpMoCl 4 ] Ϫ , is observed. The same trend between CN and doublet and quadruplet [CpCr(CN) 2 (PH 3 )] were optimized Cl ligation is observed for CpCr III complexes. [START_REF]Diamagnetic, 18-electron CpCr(CN)-containing compounds have long been known for Cr[END_REF] For cyanin the C s symmetry with DFT using the three-parameter ide, equilibrium concentrations of the 17-electron spinform of the Becke, Lee, Yang and Parr functional doublet [CpCrL 4 ] species [CpCr(CN) 2 (PR 3 ) 2 ] and (B3LYP) [13a] and the LanL2DZ basis set. [13bϪd] The bond [CpCr(CN) 4 ] 2Ϫ have been detected in solution by ESR speclengths and angles are shown in Table 1. A comparison of troscopy, [START_REF] Mattamana | [END_REF] whereas similar 17-electron species were not the two 15-electron monophosphane compounds reveals spectroscopically observed for the analogous chloride comthat the doublet complex has slightly shorter bond lengths plex. [START_REF] Fettinger | Or-viously observed for doublet[END_REF] A kinetic and theoretical investigation of the PR 3 and deviates further from an idealized three-legged pianoexchange reactions for [CpCrCl 2 (PR 3 )] complexes demonstool structure. [14] The relative difference between the two strated that, although exchange proceeds by an associative geometries is comparatively small and is much less propathway, the reaction proceeds solely on the quadruplet nounced than the difference between doublet and quadspin hypersurface by an S N 2-type mechanism. [11] Preruplet [CpCrCl 2 (PH 3 )]. [11,12b] viously, density functional theory (DFT) calculations have

The relative energies calculated for the CpCr III cyanide compounds are depicted in Figure 1. As expected, based on 15-electron CpCr III monophosphane compounds contain- [a] Bond lengths in A ˚and angles in degrees. Ϫ [b] CNT ϭ center of gravity of the Cp ring.

of 29.9 kcal mol Ϫ1 previously calculated for LanL2DZ level were recalculated with the larger 6Ϫ311ϩG(2d,2p) basis set; the resulting energies are indi-[CpCrCl 2 (PH 3 )] with the same functional and basis set. [11] Similar differences between cyanide and chloride complexes cated in parentheses in Figure 1. The energy difference between the quadruplet and doublet 15-electron compounds are observed when comparing the energy of doublet [CpCrX 2 (PH 3 ) 2 ] to the sum of quadruplet [CpCrX 2 (PH 3 )] decreased slightly to 19.3 kcal mol Ϫ1 , and the expected decrease in ∆E CrϪP was also observed, from 10.3 to 5.0 kcal and free PH 3 . This energy difference, labelled ∆E CrϪP in Figure 1, provides an indication of how readily the quad-mol Ϫ1 . For comparison, the B3LYP/LanL2DZ geometries previously obtained for [CpCrCl 2 (PH 3 ) n ] were used to recal-ruplet species will accommodate another phosphane ligand. In qualitative agreement with experimental results, ∆E CrϪP culate their energies at the B3LYP/6Ϫ311ϩG(2d,2p) level, and the resulting values are also displayed in Figure 1. is smaller for X ϭ CN (10.3 kcal mol Ϫ1 ) than for X ϭ Cl (∆E CrϪP ϭ 21.2 kcal mol Ϫ1 ).

Fixed Geometry Calculations

In order to analyze the various components that make up ∆E CrϪP , single-point calculations were performed at the B3LYP/LanL2DZ level as illustrated in Figure 2. One PH 3 ligand was removed from the optimized structure of doublet [CpCr(CN) 2 (PH 3 ) 2 ] without altering any of the other structural parameters. The single-point calculations on these complexes with one or three unpaired electrons yield energies for the doublet-fixed and quadruplet-fixed complexes, respectively, as indicated in Table 2. The Bond Energy Term (BET) is equal to the difference between the 17electron doublet-optimized and 15-electron doublet-fixed structures plus free PH 3 , and provides an indication of the CrϪPH 3 bond strength without additional factors due to spin-state change. Since the doublet-fixed and quadrupletfixed energies are based on exactly the same geometry, the from the difference between quadruplet-fixed and quadenergies in italics ruplet-optimized energies, accounts for the reorganization required for the 15-electron species to accommodate an additional phosphane ligand, and is directly related to the steric bulk of the adjacent cyanide ligands. These energies

Larger Basis Set Calculations

were also calculated for the chloride species, and the values obtained are collected in Table 2. Recent work with [CpCo(PH 3 ) n ] complexes (n ϭ 1, 2) has showed the importance of the size of the basis set on [ML n ] Direct comparison of the factors that contribute to ∆E CrϪP for the two [CpCrX 2 (PH 3 ) n ] systems helps identify ligand dissociation energies. [16] Smaller basis sets tend to underestimate metalϪligand bond strengths, thereby caus-the ligand properties responsible for the observed experimental differences between the cyanide and chloride com-ing the relative stability of unsaturated compounds to be overstated. To account for this effect, the energies of the pounds. The BET is only slightly greater for X ϭ CN than for X ϭ Cl, a variation which might be attributable to the [CpCr(CN) 2 (PH 3 ) n ] compounds optimized at the B3LYP/ by decreasing the pairing energy, consistent with the known large nephelauxetic effect of the cyanide ligand. [START_REF] Hanusa | Encyclopedia of Inorganic Chemistry[END_REF] were not detected. Calculations at the DFT/B3LYP level indicate that the steric differences and variation in CrϪP [a] ∆E (in kcal mol Ϫ1 ) calculated at the B3LYP/LanL2DZ level of theory.

bond strength are not adequate to account for this discrepancy. The observed differences are best explained by the relative ability of the CN group to encourage spin pairing, different electron-withdrawing capacity of the two X lias demonstrated by the smaller ∆E DϪQ than in the chloride gands which affects the ability of the metal center to accomsystem for both fixed and optimized geometries. modate the strong σ-donor phosphane ligand. The anticipated difference in steric requirements for CN vs. Cl apparently does not play an appreciable role, since the RE is Experimental Section roughly the same in each case. The key factor seems to be General Remarks: All calculations were performed with GAUS- the ability of the cyanide ligand to facilitate the electron SIAN 94 [19] on an SGI Origin 200 workstation. The geometries of pairing required to cross over from the quadruplet-spin to the [CpCrX 2 (PH 3 ) 2 ] and [CpCrX 2 (PH 3 )] species and free PH 3 were the doublet-spin potential energy surface, as reflected in the optimized with B3LYP, [13a] a DFT type of calculation, employing the LanL2DZ basis set. The LanL2DZ basis set includes both PE which is Ϫ17.1 kcal mol Ϫ1 for X ϭ CN in contrast to Dunning and HayЈs D95 sets for H and C and the core potential Ϫ25.5 kcal mol Ϫ1 for X ϭ Cl. The same trend is seen in sets of Hay and Wadt for the heavy atoms. [13bϪd] Electrons outside the values obtained for ∆E DϪQ in the two systems.

the core were all those for H and C, the 3s and 3p electrons for P,

The relative energies of the frontier orbitals for the and the 3s, 3p, 3d and 4s electrons for Cr. All geometries were [CpCrX 2 (PH 3 )] (X ϭ CN, Cl) fixed-doublet and fixedoptimized at the B3LYP/LanL2DZ level with imposed C s symquadruplet species are shown in Figure 3. While it is immetry. The B3LYP energy calculations were performed separately prudent to assign too much physical significance to the with both the LanL2DZ basis set and the larger 6Ϫ311ϩG(2d,2p) Kohn-Sham orbitals arising from the current DFT calcubasis set, which has two sets of polarization functions on all atoms lations, [17] the qualitative trends in orbital energies appear including two f-sets on the Cr, and also diffuse functions. to support our interpretation. The significantly greater en-The mean value of the first-order electronic wavefunction, which is ergy gap for X ϭ CN than for X ϭ Cl between the highest not an exact eigenstate of S 2 for unrestricted calculations on the and lowest energy frontier orbitals for both spin states acopen shell systems, was considered suitable for unambiguous counts for the enhanced stability of the doublet spin-state identification of the spin state. Spin contamination was carefully for the cyanide complex. As well as influencing the orbital monitored and the value of <S 2 > for the unrestricted B3LYP (UB- 3LYP) calculations on the [CpCrX 2 (PH 3 ) n ] species indicated minor splitting, the CN group may also favor the doublet species

Figure 1 .

 1 Figure 1. Relative energies of doublet and quadruplet difference in energy between these species reflects the Pair-[CpCr(CN) 2 (PH 3 ) n ] (n ϭ 1, 2); energies calculated at the B3LYP/ ing Energy (PE). The Relaxation Energy (RE), obtained LanL2DZ level of theory, with B3LYP/6Ϫ311ϩG(2d,2p) values indicated in parentheses, and the corresponding [CpCrCl 2 (PH 3 ) n ]

Figure 2 .

 2 Figure 2. Schematic representation of fixed and optimized geome-Figure 3. Orbital energies (α orbitals) and occupancies of fixed getries used for B3LYP/LanL2DZ energy calculations of ometry [CpCrX 2 (PH 3 )] (X ϭ CN, Cl) species in doublet and qua-[CpCrX 2 (PH 3 ) n ] (X ϭ CN, Cl; n ϭ 1, 2) druplet spin statesTable 2. Relative energies of [CpCrX 2(PH 3 ) n ] (X ϭ CN, Cl; n ϭ 1, 2) complexes in optimized and fixed geometries[a]

Table 1 .

 1 Optimized geometries for doublet and quadruplet [CpCr(CN) 2 (PH 3 ) n ]( n ϭ 1, 2)[a]

	Structural Parameter [b]	[CpCr(CN) 2 (PH 3 ) 2 ] doublet	[CpCr(CN) 2 (PH 3 )] doublet	[CpCr(CN) 2 (PH 3 )] quadruplet
	CNTϪCr	1.918	1.958	1.979
	CrϪCN	2.004	1.986	1.993
	CrϪP	2.470	2.495	2.521
	CNTϪCrϪCN	116.50	121.44	122.72
	CNTϪCrϪP	115.58	129.30	128.00
	NCϪCrϪP	78.39	85.53	87.23
	NCϪCrϪCN	126.98	104.06	98.92
	NCϪCrϪCNTϪP	90.6	112.5	115.4
	NCϪCrϪCNTϪCN	178.8	135.0	129.2

Acknowledgments


Rinaldo Poli
email: poli@u-bourgogne.fr

Different Approaches to Mid-Valent, Paramagnetic Half-Sandwich Complexes of the Heavier Group 6 Metals

Keywords: molybdenum, tungsten, cyclopentadienyl complexes, phosphine complexes, paramagnetic complexes

The synthesis of deceptively simple carbonyl-free, mid-valent half-sandwich Mo and W compounds containing mainly phosphine, halide, and hydride ligands actually involves a number of complications requiring several tricks or synthetic detours to be overcome. The stability of some of the target systems also necessitates particular synthetic conditions. 1 Introduction 2 A Metathetical Route to CpMo(III) Complexes 3 A Reductive Route to CpMo(III) Complexes 4 Further Reduction to Noncarbonyl Mo(II) Derivatives 5 Paramagnetic hydride derivatives of Mo(III), Mo(V), and W(V)

Introduction

Over a number of years, interest in my laboratory has been attracted by paramagnetic organometallic complexes, a class of compounds that has developed at a slower pace relative to the diamagnetic counterparts. Two reasons for this state of affairs are the relevance of the EAN rule in organometallic chemistry, dictating a greater stability for electronically saturated, diamagnetic configurations, and the greater difficulty associated with the handling and characterization of paramagnetic compounds. The reasons, on the other hand, to be interested in compounds of this type are the potential for discovering interesting ground state properties and, indeed, the expected greater reactivity, which could lead to interesting new stoichiometric and catalytic transformations.

Early work on organometallic, mostly low-valent, carbonyl-based, radical species had shown a higher lability of the metal-ligand bonds relative to saturated counterparts. However, these lowvalent species are most often unstable, limiting their study and utility. We considered that the EAN rule loses importance as the oxidation states is raised, paralleling a weakening of the metal-ligand back-bonding interaction to typical organic substrates (without necessarily reducing their activation). These considerations led to our interest in mid-valcnt, paramagnetic organometallic compounds, where one could hope to combine an increased stabilization of the odd-electron configuration with substrate activation and lability with respect to a variety of fundamental reactions. In addition, we have chosen to work with half-sandwich compounds, since the cyclopentadienyl ligand forms stable and inert bonds with transition metals, imparting solubility in most common organic solvents. At the same time, coordination by a single cyclopentadienyl ligand, rather than two or three, leaves a sufficient number of other coordination positions available for coordination and activation of potential substrates and reagents for the investigation of chemical reactivity. Most of our studies have been carried out with group 6 metals, especially molybdenum, given the involvement of these metals in a variety of catalytic processes such as polymerization, hydrodesulfurization, and dinitrogen reduction, and the availability of several oxidation states and electronic configurations for these metals.

Our studies have indeed allowed us to contribute to a better understanding of fundamental questions, such as how a spin state change may affect thermodynamic stabilization, reactions rates, and structural changes in open-shell organometallic chemistry. 1 We have also discovered new reaction pathways, such as an electron transfer-catalyzed ligand substitution where, for the first time in organometallic chemistry, the order of reactivity is reversed (faster for even-than for odd-electron configurations). These developments are analyzed in other recent review articles. [1][2][3] In the spirit of this Journal, I shall focus on the synthetic aspects of our research in the present account.

A Metathetical Route to CpMo(lIl) Complexes

We have long been (and still are) interested in developing the chemistry of the 17-electron CpMoX2L2 system. Initial studies were focused on complexes with L = tertiary phosphine, a few examples of which had been previously described briefly by the Green group. 4 The first synthetic approach to these materials consisted in the introduction of the cyclopentadienyl ring on a coordination compound of Mo(HI) by a metathesis process (eq. 1). The first try immediately gave the desired result, namely the synthesis of CpMoX2(PMe3)2 (X = Cl, Br or I) from the corresponding MoX3(PMe3)3 and TlCp in THF or toluene. 5 MoX3L3 + CpM --> CpMoX2L2 + MCl + L Equation 1The Mo(III) precursors are available from the corresponding tetrahydrofuran complexes, MoX3(THF)3. The trichloro complex has long been known and is available in three easy steps from MoCl5, 6 the last one involving separation of the insoluble (in THF) product from the excess tin reagent by extraction into CH2Cl2 and filtration. During our work, we realized that MoC13(THF)3 converts relatively rapidly to the dinuclear metal-metal bonded Mo2Cl6(THF)3 on standing in CH2Cl2, 7 a process that had in fact been previously described by Boyd and Wedd on the basis of an optical study. 8 Our investigation, which was based on a 1 H NMR monitoring, gave additional insights, including the identification of the Mo2Cl6(THF)4 intermediate. The contamination of MoCl3(THF)3 by varying amounts of dinuclear condensation products does not appear to affect most of the subsequent synthetic applications, but we have developed a slight modification of the synthetic procedure whereby the MoCl3(THF)3 and Sn solids are separated mechanically by virtue of their different density, thereby avoiding the dichloromethane extraction. 9 The corresponding tribromo and triiodo derivatives MoX3(THF)3 (X = Br, I) cannot be prepared similarly to the trichloro complex because higher valent MoXn precursors are nonexistent or not easily available, thus we developed alternative procedures consisting of the halogenation of lower-valent carbonyl precursors, suitable to large scale syntheses. [10][11][12] A problem with the synthesis of eq. 1 is the sacrifice of one mole of the phosphine ligand.

Another problem is that phosphine ligands bulkier than PMe3 do not cleanly afford the MoX3L3 precursor, 9 limiting the applicability of this method. Finally, even the PMe3 system afforded less than satisfactory results. The poisonous thallium reagent had to be used to avoid the reductive competitive processes that are prevalent with the sodium reagent. This, however, required forcing conditions for the completion of the metathesis process [several hours reflux in THF (X = Cl, Br) or toluene (X = I)]. While the desired 17-electron half-sandwich complex was the major product in each case, small amounts of reduced materials [MoX2(PMe3)4 and Mo2X4(PMe3)4] were also obtained, especially for X = Cl.

Although this synthetic procedure was also found acceptable for the CpMoX2(dppe) system, 13 an alternative, more selective methodology which could also be of wider applicability was clearly needed. As shown in the next section, such a methodology was rapidly found. Several problems, however, were also encountered during that alternative procedure, as will be accounted in more detail later, leading us to occasionally reconsider a metathetical entry into CpMo(III) chemistry. Subsequent efforts in this area have involved the direct reactions of MoX3(THF)3 with Cp delivering reagents (eq. 1, M = Na, Tl, Me3Si) in order to access either CpMoCl2 (which is a stable compound, vide infra) directly or an adduct containing a labile solvent molecule such as THF or MeCN. These trials have always ended in big disappointments. Attempts have also miserably failed to introduce the indenyl ring via either the sodium or thallium reagent: a few crystals of [(η 5 -C9H7)MoI2]2 have been obtained 14 and EPR signals consistent with the expected (η 5 -C9H7)MoX2L2 have been observed for L2 = dppe, but the reactions were dominated by the reductive pathway 15 and by other complex ligand exchange pathways 16 and clean products could never be isolated.

A Reductive Route to CpMo(III) Complexes

One often gets inspired from reading the literature, especially when the simplest, more logical avenues end up in unsatisfactory results or in total failures. The l7-electron complexes previously reported by Green had been made by ligand addition to [(η 5 -C5H4Pr i )Mo(µ-Cl)2]2, 4 which was made by HCl addition to an unusual reagent, (η 6 -C6H5)(η 5 :η 1 -C5H4CMe2)Mo, this in turn being available from a metal vapor synthetic procedure. We were not equipped to carry out metal vapor reactions, but the existence of the trachloro-bridged species led us to believe that the corresponding unsubstituted Cp analogue could also be made and that this could easily form adducts with a variety of ligands. Shortly prior to our impasse, Green and Mountford had communicated that the reduction of (C5H4R)WCl4 (R = Me, Pr i ) affords the dinuclear W(lII)

system, [(C5H4R)WCl2]2, 17 leaving hope that an analogous reductive procedure could work also for Mo. The needed precursor CpMoCl4 was a known compound, 18 available by reaction of HCl in Et2O with any of a number of oxo or oxochloro derivatives, but described as a highly hydroscopic species. On the other hand, Schrock had previously reported a high yield synthesis of Cp*MCl4 (M = Mo, W) 19 by PCl5 oxidation of easily available Cp*M(CH3)(CO)3.

Contemporarily with the Green group, 20 we were pleased to find out that Schrock's method could conveniently be adapted to the synthesis of CpMoCl4 from CpMo(CH3)(CO)3 and PCl5

(eq 2, X = PCl3). 13 The Green contribution 20 also showed the formation of a tetrabromo analogue, (C5H4Me)MoBr4, by Br2 oxidation of the tricarbonyl dimer [(C5H4Me)Mo(CO)3]2.

We found that the tetrachloro species are equally available from the tricarbonyl dimers precursors (eq 3, X = PCl3), the latter providing the notable advantage of being more easily synthesized than the tricarbonylmethyl reagent of equation 2. The unsubstituted Cp complex is even available from a number of commercial sources. We subsequently further simplified the synthetic procedure by using PhICl2 as the oxidizing agent (eq. 3, X = PhI) [START_REF] Gordon | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF] and extended it to the Cp* system. [START_REF] Keogh | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF] The PhICl2 reagent is easily available from PhI and Cl2 [START_REF] Lucas | [END_REF] and has the advantage over PCl5 of not being sensitive toward moisture, thus it can be handled in air (though it must be kept in the dark in a refrigerator if long term storage is needed).

CpMo(CH3)(CO)3 + 5/2 XCl2 --> CpMoCl4 + 5/2 X + CH3Cl + 3 CO [CpMo(CO)3]2 + 4 XCl2 --> 2 CpMoCl4 + 4 X + 6 CO
Equation 2 and 3

Once the CpMoCl4 was at hand in reproducibly high yields, we considered its reduction to phosphine adducts of Mo(III). While communicating our foray into this area at various conferences, Tom Baker informed us of his synthesis of Cp*MoCl2(PMe3)2 and of the orthometallated Cp*WClH(CH2PMe2)(PMe3) complex by sodium reduction of Cp*MCl4(PMe3), reactions that were eventually published a few years later. 24 Eager to access the desired l7-electron CpMoCl2L2 adducts in a one-pot synthesis from CpMoCl4, we preceded to add phosphine ligands to the Mo(V) precursor, with the intention of carrying out an in situ reduction of the resulting adducts. However, greed often leads to bitter punishment. We found out that the CpMoCl4-PR3 combination leads to a complex reduction process, yielding MoCl3L3, (with loss of the Cp ring !) and other uncharacterized products (rates in the order PMe3 > PMe2Ph > PMePh2). On the other hand, the addition of basic phosphine ligands (even primary ones) to the more encumbered and less acidic Cp*MoCl4 or to tungsten analogues leads to stable adducts. 19,[25][26][27][28] The reduction of Lewis acidic metal halide reagents by basic phosphine ligands is a known nuisance, which we have encountered time and again in other areas, such as the coordination chemistry of iron [START_REF] Walker | Po1i[END_REF][30] and the organometallic chemistry of niobium. 31 Thus, in order to circumvent the phosphine-induced CpMoCl4 decomposition, we decided to first reduce the CpMoCl4 material to CpMoCl2, then add the desired phosphine, in two separate steps. As it turns out, this reduction process has also reserved surprises for us, both good and bad. While, almost contemporarily with our studies, Green, Hubert and Mountford had no trouble obtaining [(C5H4R)MoC2]2 by Na-Hg reduction of (C5H4R)MoCl4 (R = Me, Pr i ), 20 the corresponding reduction of the unsubstituted Cp derivative led to an ionic compound that turned out to be the sodium salt of [Cp2Mo2Cl5] -(Scheme 1). This anion did not react cleanly with phosphine ligands, creating yet another obstacle between us and our target 17-electron system.

The good news, however, is that this anion, subsequently prepared also by addition of Cl -to CpMoCl2 and structurally characterized as the [Ph3PNPPh3] + salt, revealed an unprecedented and fascinating structure featuring a Mo-Mo triple bond bridged by a single Cl -ligand. 32 Evidently, the sodium cations produced during the reduction process are unable to completely abstract chloride from the relatively unencumbered and acidic Cp-containing reduced Mo(III) complex. Later, the Green group showed that analogous anionic Cl -adducts are also formed by the less acidic and more encumbered [(C5H4R)MoCl2]2 compounds and by the W analogues. 33 On the basis of this Lewis acidity argument, we finally arrived at the successful use of Zn as the reducing agent (Scheme 1), affording the desired CpMoCl2 in good yields. 34 Scheme 1

The Zn 2+ ion has a greater Lewis acidity than Na + and is able to fully abstract Cl -, leaving behind the neutral CpMoCl2. Furthermore, the product separation is simplified since CpMoCl2 is essentially insoluble in THF while ZnCl2 forms the soluble ZnCl2(THF)2 complex which can be readily washed away. Even this reduction procedure, however, is not completely fool-proof.

The reaction initially produces the Mo(IV) intermediate CpMoC13, 35 which is rather thermally sensitive and is able to exchange chlorides with the oxygen atom of the THF solvent under mild conditions, affording Cp3Mo3Cl7O and l,4-dichlorobutane (eq. 4). 36 The use of coarse Zn grains leads to slow reactions with extensive decomposition, while the use of a fine Zn powder at room temperature leads to strong exotherms, also resulting in decomposition. The best results (> 70% yield) are obtained by the use of a fine Zn powder and low temperatures (-20°C). Equation 4Incidentally, the CpMoCl3 material had not been previously reported. 37 Since this can be imagined to be a potentially versatile reagent, we have invested some time in its synthesis, which turned out to be possible by the four different synthetic routes outlined in Scheme 2. 35,38 It was particularly amusing to realize that the dicarbonyl precursors CpMoX3(CO)2 (X = Cl, Br, I) had been known since 1966 39,40 and had been described as sensitive compounds that readily lose CO in solution, 39 but no studies of the decarbonylation products had ensued. As it turns out, this is in fact the simplest synthesis of CpMoCl3 [START_REF] Keogh | Synthetic Methods of Organometallic and Inorganic Chemistry[END_REF] and is extendable also to the tribromide and triiodide analogues. 38 Scheme 2

Coming back to Mo(III) chemistry, the last step of the synthetic procedure, namely the addition of the phosphine ligand, proceeded as expected and allowed us to obtain a variety of bisphosphine adducts (eq. 5). 13,34,[START_REF] Poli | [END_REF]43 The corresponding dibromide and diiodide derivatives were subsequently obtained by relatively facile halide metathesis processes. 34,44 The use of sodium salts in THF favored the quantitative conversion of the dichloro to the diiodo species, while soluble salts in dichloromethane strongly favored the opposite process. The exchange of the phosphine ligand is an even more facile reaction and proceeds by a dissociative mechanism.

We have proposed that the 15-electron intermediate of this exchange process is energetically stabilized by a spin change. [START_REF] Poli | [END_REF]45 More recently, the addition reaction of eq. 5 has been extended

to the high-yield synthesis of the butadiene complex CpMoCl2(η 4 -C4H6), 46 previously available only in < 50% yields by a disproportionation procedure. 47 Other derivatives with diamine and with bifunctional ligands (phosphine-amine, phosphine-thioether, etc.) are also being prepared and characterized. 48,49 [CpMoCl2]2 + 4 L --> 2 CpMoCl2L2

Equation 5

A few more words on the versatile CpMoCl2 reagent are in order. Its structure is inferred as dinuclear with four bridging chloride ligands by analogy with the crystallographically characterized [(C5H4Pr i )MoCl2] and [(C5Me4Et)MoCl2]2 compounds. 4,50 The bromo and iodo complexes [Cp*MoX2]2 (X = Br, I) and [(η 5 -C9H7)MoI2]2 also display an analogous structure. 14,[START_REF] Poli | Rheingo1d[END_REF][52][53][54] However, this compound is essentially insoluble in all common solvents, preventing its spectroscopic characterization in solution and its purification. One is stuck, unfortunately, with using the material as obtained from the synthetic procedure. In order to avoid unsatisfactory results in the follow-up reactivity studies (in which we have incurred time and again), it is thus absolutely essential that the synthesis be carried out with a slight defect of Zn and from absolutely pure CpMoCl4. Furthermore, the CpMoCl4 precursor is, itself, . 57 Embarassing confusions in the identity of crystallographically characterized complexes 58 can also be apparently attributed to Zn contamination of the cyclopentadienyl starting material. 59 Given the problems associated with the zinc reduction of CpMoCl4, we have devoted additional attention to the direct, one-pot reductions of CpMoCl4 by Na-Hg in the presence of the desired ligand. As state above, this procedure is unsatisfactory with basic phosphines but other ligands that are not oxidized by CpMoCl4 should be compatible with the reaction conditions. At the same time, their coordination at some stage of the reduction procedure should favor the complete removal of the chloride ions by Na + . The bifunctional ligand Ph2PCH2CH2SMe satisfies all necessary requirements, the one-pot reduction of CpMoCl4 in the presence of this ligand providing a good yield synthesis of CpMoCl2(Ph2SCH2CH2SMe). 49 The intermediate CpMoCl4(MeSCH2CH2PPh2) derivative may even be isolated as a pure product. Diene ligands do not allow a direct one-pot synthesis of CpMoCl2(η 4 -diene) using sodium, because of the parasite anionic diene polymerization under these conditions. The use of zinc eliminates the competitive polymerization process, but the separation of the soluble zinc dichloride by-product from the diene-Mo(III) complex is problematic, thus the diene addition to isolated CpMoCl2 remains more convenient. 60 A combined reduction-addition-metathesis process occurs in a single step to convert CpMoCl4 to CpMo(η 3 -C3H5)2 in good yields (eq. 6). 61 This compound was previously available only from a 5-step procedure from MoCl5 in unreported yields. 62 CpMoCl4 + 4 CH2=CHCH2MgBr --> CpMo(η 3 -C3H5)2 Equation 6Cp* analogues of the above systems are easier to synthesize. As mentioned above, Cp*MoCl2(PMe3)2 was prepared by Baker by one-pot reduction of Cp*MoCl4 in the presence of PMe3 (eq. 7). 24 The same procedure also affords bis-PMe2Ph and dppe analogues. 63,64 Cp*MoCl4 + 2 L + 2 Na --> Cp*MoCI2L2 + 2 NaCl (L = PMe3, PMe2Ph, or L2 = dppe) Equation 7The use of Cp* has advantages and disadvantages over the use of Cp. The direct, one-pot process of eq. 7 does not suffer from the competitive reductive process, and the Cp* products are less air sensitive, more soluble, and easier to crystallize than the Cp analogues. On the other hand, the steric bulk of Cp* does not allow the formation of derivatives with bulky L ligands.

PMe2Ph and dppe are the bulkiest ligands to give stable complexes of this family. The use of bulkier PMePh2 and PPh3 only leads to the formation of [Cp*MoCl2]2. 63 As mentioned above, CpMoCl3 was made in our laboratory for the first time. Its insolubility, however, prevented its structural characterization, although the magnetic moment suggests the presence of antiferromagnetic interactions in a dinuclear or oligonuclear structure without metal-metal bonds. Work on the more soluble Cp* derivative, prepared by the decarbonylation procedure (eq. 8), has allowed to verify the dinuclear structure by X-ray crystallography. 53,65 Equation 8

The solution behavior of the bromide and iodide derivatives, however, turned out more complex than expected, revealing equilibria with or irreversible transformations to more complex structures, for instance eq. 9 for the bromide and eq. 10 for the iodide. All these compounds exhibit rather complex and interesting redox behaviours. 53 2 The chloride compound maintains the neutral dinuclear structure shown in eq. 8 under all conditions, but a chloride ligand abstraction with rearrangement to a Mo-Mo bonded structure is enforced by Lewis acids (eq. 11). 66 Equation 11

From a synthetic point of view, the chloride and bromide derivatives have been used to access a number of l6-electron ligand adducts of formula Cp*MoX3L (eq. 12), including

[Cp*MoX4] -. 25 [Cp*MoX2(µ-X)]2 + 2 L --> 2 Cp*MoX3L Equation 124

Further Reduction to Noncarbonyl Mo(II) Derivatives

In order to open coordination sites and favor chemical reactivity, a high level of electron richness on the compound is helpful. The complete substitution of carbonyl ligands with phosphine donors should accomplish this goal, as illustrated for instance by the greater reactivity of Mo(PMe3)6 relative to Mo(PMe3)x(CO)5-x in C-H bond activation processes. 67 Half-sandwich Mo(II) complexes of the CpMoXL3 type frequently contain π acidic ligands (mostly L = CO, but also L = carbene, L2 = diene or XL = allyl), [68][69][70] or moderately acidic ligands such as phosphites. 71 Fully phosphine-substituted examples were limited to X = H. 67 We wanted to access similar derivatives where X = π-donor ligand, with the idea of destabilizing a saturated structure by filled-filled repulsion and stabilizing the unsaturated one by π-donation, 72 thereby favoring an open-shell configuration. Carbonyl-containing precursors of type CpMoXL3 are abundant and easily accessible, and one could envisage a simple CO substitution process. Unfortunately, as is well known, the substitution of CO ligands is a facile process in CO-rich complexes but becomes progressively more difficult as the number of CO ligand decreases, the last CO being almost impossible to remove, especially in electron-rich systems. Work by Lapinte has shown that CO substitution by phosphine or phosphite on Cp*MoH(CO)3 or [CpMo(CO)3(PR3)] + /LiAlH4 stopped at the monocarbonyl level. 73 Thus, the easily available CpMoX(CO)3 compounds are not suitable precursors to fully phosphinesubstituted CpMoXL3. This left us with the use of a reductive route on the CpMo(III) compounds that we had just synthesized (see previous section) according to eq. 13.

CpMoCl2L2 + [red] + L --> CpMoClL3 + [ox] + Cl - Equation 13The reaction works as expected for L = PMe3 and PMe2Ph and [red] = Na amalgam, 57,74 with one caveat. Once again, one must not be too greedy and take a shortcut with a one-pot reduction of CpMoCl2 in the presence of three equivalents of L. The addition of three equivalents of L to CpMoCl2 in THF (the solvent required for the Na reduction) yields a large amount of precipitate that is no longer able to engage in a clean reduction step. Subsequent investigations 45 have

shown that this precipitate is [CpMoCl(PMe3)3] + Cl -. Good results, however, are obtained by adding two equivalents of L to CpMoCl2, allowing the formation of CpMoCl2L2 to go to completion, then add the third equivalent of L and the reducing agent at the same time.

The study of compounds CpMoClL3 revealed a steric effect controlling the access to the desired open-shell systems: upon warming, CpMoCl(PMe2Ph)3 establishes an equilibrium with the 16electron and paramagnetic bis-phosphine analogue, CpMoCl(PMe2Ph)2, whereas the less encumbered PMe3 system does not engage in a phosphine dissociation process. This open-shell product eluded isolation. On the other hand, the more encumbered Cp* system allowed the synthesis and isolation of Cp*MoClL2 (L = PMe3, PMe2Ph or L2 dppe), the first 16-electron half-sandwich M0(lI) derivatives with a spin triplet ground 74 No particular complication was experienced in the synthesis of these compounds directly from Cp*MoCl4 (eq. 14), besides the fact that the PMe3 derivative requires the use of an argon atmosphere and the use of strictly two equivalents of PMe3. Under dinitrogen, the N2 adduct is obtained as previously reported by Baker, 24 whereas the use of three equivalents of the phosphine affords sterically encumbered Cp*MoCl(PMe3)3. 57 An equilibrium process between this product and the bis-phosphine analogue is very slow. 75 A kinetic, thermodynamic, and computational study of the CO and N2 addition to Cp*MoCl(PMe3)2 has allowed us to gain a considerable insight into the effect of a spin state change on reaction rates for organometallic ligand addition processes. 76 Cp*MoCl4 + 3 [red] + 2L --> Cp*MoClL2 + 3 [ox] + 3 Cl - Equation 14One of our goals was therefore achieved: the use of electron donating ligands (phosphines) and a π donor ligand (chloride) favors ligand dissociation and stabilizes an open-shell configuration.

No particularly interesting chemical reactivity such as C-H activation, however, is displayed by these unsaturated Mo(II) complexes. It is relevant to note here that the reduction of Cp*WCl4/PMe3 was reported to afford the C-H oxidative addition product Cp*W(CH2PMe2)(PMe3)(H)Cl, 24 presumably via a Cp*WCl(PMe3)2 intermediate which is more reactive than our Mo analogue. It is also known that unsaturated (η 5 -C5H4R)MoHL2

intermediates (generated by photoinduced H2 elimination from the stable trihydrides) catalyze H/D exchange reactions between C6D6 and other substrates. 4 It was therefore desirable to generate other CpMoXL2 compounds in order to understand the influence of X on the stability and reactivity of the unsaturated complex. The hydroxide system, obtained by the metathesis reaction 15, has given us unexpectedly interesting results. 77,78 CpMoCl(PMe3)3 + KOH --> CpMo(OH)(PMe3)2 + PMe3 Equation 15The first peculiar observation is the stability of the open-shell structure, even in the presence of PMe3, contrasting with the preference of the chloride system for the saturated tris-phosphine system, although OH is less bulky than Cl. This stabilization is related to the greater π donating ability of OH, as confirmed by computational studies. A second point of interest is the ability of this compound to undergo H/D exchange in C6D6 at the PMe3 positions. Most notably, a diversion of the OH π-donating lone pairs into H-bonding with the protic solvent, thereby rendering the metal less saturated and more reactive. 78 The CpMo(OH)(PMe3)2 compound is exceedingly soluble in pentane and cannot be crystallized from concentrated solutions at -80°C.

By comparison, the CpMoCl(PMe3)3 precursor is easily crystallized under the same conditions.

This solubility enhancement is consistent with a negligible Mo-OH bond polarity and absence of self-association via H-bonding, both resulting from the strong HO-Mo π donation. The next synthetic challenge is to generate less soluble versions of the hydroxo system as well as other compounds of this class where a control of the extent of π donation from X is possible.

Paramagnetie hydride derivatives of Mo(III), Mo(V), and W(V)

Since the hydride ligand is involved in a variety of stoichiometric and catalytic processes, it was of interest to generate paramagnetic versions of known hydride complexes and to probe their reactivity. A particular interest consisted in the achievement of the elementary process of H2 reductive elimination from a paramagnetic polyhydride system. This process has the potential of generating highly unsaturated, 15-electron species, analogous to the 16-electron species obtained by H2 reductive elimination from a saturated dihydride complex.

Although reductive elimination of an alkane from a saturated dialkyl complex is known to be accelerated by oxidation to a radical system, the reductive elimination of H2 from an oxidized polyhydride system had never been reported. The reason for this is probably the greater propensity of paramagnetic hydrides to undergo other decomposition processes, namely deprotonation 79 and disproportionation. 80 A large body of work suggests that both of the latter two pathways may be discouraged by employing a sterically protecting coordination sphere. A large ligand bulk would in fact lower the kinetic acidity, thereby slowing down proton transfer to an acceptor base (the rate determining step of the deprotonation mechanism) and at the same time lower the ability of a donor solvent molecule to coordinate the metal center, which seems a prerequisite for the rate determining electron transfer step of the disproportionation pathway.

The generation of paramagnetic hydride complexes by oxidation requires the availability of the saturated diamagnetic precursors. Thus, our work started with the synthesis of stable 18electron hydride complexes.

Group 6 metal complexes of stoichiometry (η 5 -C5R5)MHL3 and (η 5 -C5R5)MH3L2 (M = Mo, W) allow a good control of the steric and electronic properties via the choice of the cyclopentadienyl and phosphine substituents. CpMoH(PMe3)3 was first reported by Brookhart et al. 67 as an uncrystallizable wax, impure from cyclopentadiene oligomers, obtained from the oxidative addition of CpH to Mo(PMe3)6. We have generated this compound from CpMoCl(PMe3)3 and LiBEt3H, eq. 16, therefore in the absence of nonvolatile organic contaminants, without improving the crystallinity of the compound.

CpMoCl(PMe3)3 + LiBEt3H --> CpMoH(PMe3)3 + LiCl + BEt3 Equation 16Every subsequent reactivity study has therefore required the generation of the hydride compound in situ from the chloride precursor. This nuisance did not prevent us from developing interesting oxidation chemistry, 81,82 but it demanded the completion of the metathesis step to avoid complications during the subsequent transformations. For instance, oxidation of this product in a wet organic solvent engages water as both a ligand and a proton source (but not as a base!) to afford the Mo(III)-hydroxo complex [CpMo(OH)(PMe3)3] + , amongst other products.

Occasionally, the use of a hydride source that was contaminated by unconverted chloride precursor afforded crystals that turned out to contain a solid solution of [CpMo(OH)(PMe3)3] + and [CpMoCl(PMe3)3] + tetrafluoroborate. 82 The oxidation in dry solvents has allowed the formation of the relatively stable Mo(Ill) hydride complex [CpMoH(PMe3)3] + . This eventually decomposes by both deprotonation and disproportionation pathways. A kinetic study has allowed (for the first time) a quantitative assessment of both pathways for the same system. 81 The steric protection in this system has also allowed, for the first time, the isolation and X-ray . 4 Since we had at our disposal a number of new (Ring)MoCl2L2 complexes with monodentate phosphine ligands (Ring = Cp, Cp*, see sections 2 and 3), we attempted to generate the corresponding (Ring)MoH3L2 by adapting the published procedure. 4 With frustration, however, we discovered by NMR monitoring that the final reaction mixture contained variable amounts of (Ring)MoHL3, (Ring)MoH3L2 and even (Ring)MoH5L depending on conditions. A variation of reaction time, solvent, temperature and work-up conditions has not allowed us to obtain a pure crystalline sample of the desired trihydride species from this procedure. As a matter of fact, the use of a large excess of LiAlH4 in Et2O at 0°C afforded a pure sample of Cp*MoH5(PMe3) in good yield, see eq. 17, 84 a compound later prepared also by Parkin by a more economical synthesis from Cp*MoCl4(PMe3). 54 Equation 17

Eventually, suitable conditions were found for the preparation of CpMoH3(PMe2Ph)2 directly from CpMoCl3, PMe2Ph and LiAlH4 in THF, eq. 18. 85 The crude product contained about 12% of the pentahydride complex, but a recrystallization afforded crystals of the pure trihydride. The difficulties associated with the preparation of this system, however, required the development of other systems. Equation 18We reasoned that the use of a chelating phosphine should favour the formation of the trihydride over the pentahydride, while using Cp*MoCl4, as the metal precursor in the presence of the phosphine would offer the convenience of a more straightforward synthesis. For once, we have been rewarded for being daring, compound Cp*MoH3(dppe) being produced in a single step in a 65% yield according to eq. 19. 86 Small amounts of the pentahydride by-product, CpMoH5(η 1dppe), are also present in solution but do not contaminate the isolated product when using a 80:20 toluene-ether solvent mixture. The use of an ether-richer solvent affords increasing amount of the pentahydride by-product, while using pure toluene requires longer reaction times, eventually leading to other uncharacterized by-products. Of course, this strategy is limited to the Cp* system, given the problems associated with the CpMoCl4-phosphine interaction (see section 3). Equation 19We were also pleased to find that the same strategy of eq. 19 also yields the tungsten analogue.

The only other (to the best of our knowledge) previously reported tungsten compound of this family, CpWH3(PMe3)2, 87 had been synthesized from either W(PMe3)5H2 or W(PMe3)4H4 and cyclopentadiene in poor yields (16%). The use of a toluene-ether mixture is also critical in this case. In addition, the subsequent methanolysis of the excess hydride reagent (which increases the yield without affecting the purity in the Mo case) increases the formation of the pentahydride in the W case. Therefore, the best operating procedure is the immediate isolation of the pure trihydride in ca. 30% yield, followed by methanolysis of the residue to recover additional impure product which can be subsequently recrystallized.

The subsequent chemical and electrochemical oxidation of the Cp*MH3(dppe) compounds (M = Mo, W) has yielded the corresponding l7-electron [Cp*MH3(dppe)] + complexes. [START_REF] Pleune | Biographical Sketch Rinaldo Poli received a Laurea degree in Chemistry from the University of Pisa[END_REF] Careful kinetic and electrochemical studies in a variety of solvents and conditions have revealed that the Mo complex decomposes by any or a combination of three different mechanisms, each of which has been quantitatively assessed. These are the known deprotonation and disproportionation mechanisms (prevalent in MeCN), plus the heretofore unreported H2 reductive elimination. The last mechanism is the only one occurring in CH2Cl2, leading to a stable l9-electron complex containing a bidentate solvent ligand, as unambiguously shown by Cl hyperfine coupling in the EPR spectrum (eq. 20). [START_REF] Pleune | Biographical Sketch Rinaldo Poli received a Laurea degree in Chemistry from the University of Pisa[END_REF] [Cp*MoH3(dppe)] + + CH2Cl2 --> [Cp*MoH(dppe)(η 2 -CH2Cl2)] + + H2 Equation 20Contrary to the Mo complex, the W analogue does not undergo any decomposition under ambient conditions, allowing the growth of single crystals from MeCN/Et2O. We expect that the photochemical investigation of this system as well as other similar sterically protected para magnetic polyhydrides will lead to the discovery of interesting new transformations.

a

  sparingly soluble, difficult to purify, and extremely moisture sensitive compound. It, in turn, must be synthesized from absolutely pure starting materials. The use of crude [CpMo(CO)3]2, as obtained from the classical Manning procedure (Fe 3+ oxidation of [CpMo(CO)3] -), 55 has repeatedly led to inconsistent results and also to some occasional surprises, such as the incorporation or iron to yield the complex structure of [Cp*2Mo2(µ-I)4][Cp*2Mo2(µ-I)2(µ-O)][FeI4]. 56 A reliable procedure involves the purification of [CpMo(CO)3]2 by Soxlet extraction with dichlorornethane. Finally, the thorough washing of the ZnCl2(THF)2 by-product from CpMoCl2 is absolutely essential. The direct addition of PMe3 to the CpMoCl2 suspension in the presence of ZnCl2 led to the formation of a product which, on the basis of elemental analyses and EPR spectroscopy, is formulated as either the zwitterionic CpMo(PMe3)3(ClZnCl3) or the isomeric [CpMoCl(PMe3)3]2[Zn2Cl6]

5 Equation 9 and 10

 510 [Cp*MoBr2(µ-Br)]2 [Cp*2Mo2Br2(µ-Br)4] + [Cp*MoBr4] -+ Cp*MoBr4 [Cp*MoI2(µ-I)]2 --> [Cp*2Mo2(µ-I)4] + (I -)0.5(I3 -)0.

  study of the intermediate of the disproportionation pathway, [CpMoH(PMe3)3-(MeCN)][BF4]2. 81 An even more stable Mo(III) hydride compound could be envisaged starting from Cp*MoH(PMe3)3. Unfortunately, the synthesis of this product according to the protocol of eq. 16 from Cp*MoCl(PMe3)3 affords a mixture of Cp*MoH(PMe3)3 and Cp*MoH3(PMe3)2, 57 thus the generation of [Cp*MoH(PMe3)3] + must await a more selective synthesis of the neutral precursor. Compounds of the (η 5 -C5R5)MH3L2 family are known for both Mo and W. The Mo complexes were reported from the reaction of a variety of hydride sources with the corresponding (Ring)MoCl3L2 (Ring = Cp; L2 = dppe) 83 or (Ring)MoCl2L2 (Ring = C5H4Pr i ; L = PMe3,

Acknowledgement

I wish to acknowledge the generous support of the Petroleum Research Fund, the National Science Foundation, and the Department of Energy during the period of my association with the University of Maryland at College Park. Work in this area at the Université de Bourgogne is being continued with support from the MENRT and the CNRS. I would like to express my particular gratitude to the Conseil Regional de Bourgogne for providing all the necessary funds for the installation of my new laboratory at the Université de Bourgogne.