Mathilde Fort
email: mathilde.frt@gmail.com

Perrine Brusini

M Julia Carbajal

Yue Sun

Sharon Peperkamp

A novel form of perceptual attunement: Context-dependent perception of a native contrast in 14-month-old infants

Keywords:

By the end of their first year of life, infants have become experts in discriminating the sounds of their native language, while they have lost the ability to discriminate non-native contrasts. This type of phonetic learning is referred to as perceptual attunement. In the present study, we investigated the emergence of a contextdependent form of perceptual attunement in infancy. Indeed, some native contrasts are not discriminated in certain phonological contexts by adults, due to the presence of a language-specific process that neutralizes the contrasts in those contexts. We used a mismatch design and recorded high-density Electroencephalography (EEG) in French-learning 14-month-olds. Our results show that similarly to French adults, infants fail to discriminate a native voicing contrast (e.g., [f] vs. [v]) when it occurs in a specific phonological context (e.g.

[ofbe] vs. [ovbe], no mismatch response), while they successfully detected it in other phonological contexts (e.g., [ofne] vs. [ovne], mismatch response). The present results demonstrate for the first time that by the age of 14 months, infants' phonetic learning does not only rely on the processing of individual sounds, but also takes into account in a language-specific manner the phonological contexts in which these sounds occur.

Introduction

Speech is an inherently variable signal: tokens of identical phonemes and words are acoustically distinct. When acquiring their native language, infants have thus to detect the equivalence of these different tokens in spite of their variability. For instance, the acoustic properties of consonants differ as a function of the following vowel. After only a few months of life, infants normalize this acoustic variation [START_REF] Bertoncini | An investigation of young infants' perceptual representations of speech sounds[END_REF][START_REF] Hochmann | The invariance problem in infancy: a pupillometry study[END_REF][START_REF] Mersad | Electrophysiological evidence of phonetic normalization across coarticulation in infants[END_REF]. Infants must also detect which aspects of phonetic variation in the speech signal are meaningful, i.e. reflect distinctions among the phonemes and hence words of their native language. Numerous behavioral studies report that by the end of the first year of life, infants have learned to interpret which phonetic distinctions are relevant (or not) to recognize the phonemes of their native language (e.g., [START_REF] Kuhl | Linguistic experience alters phonetic perception in infants by 6 months of age[END_REF][START_REF] Polka | Developmental changes in perception of nonnative vowel contrasts[END_REF][START_REF] Werker | Cross-language speech-perceptionevidence for perceptual reorganization during the 1st year of life[END_REF]. More recently, electrophysiological measures have confirmed these findings [START_REF] Bosseler | Theta brain rhythms index perceptual narrowing in infant speech perception[END_REF][START_REF] Conboy | Impact of second-language experience in infancy: brain measures of first-and second-language speech perception[END_REF][START_REF] Peña | Earlier speech exposure does not accelerate speech acquisition[END_REF]Rivera-Gaxiola et al., 2007a, 2005a,b). For instance, [START_REF] Peña | Earlier speech exposure does not accelerate speech acquisition[END_REF] collected electro-encephalographic (EEG) measures in 9-and 12-month-old Spanish-learning infants using a mismatch paradigm. They computed infants' auditory mismatch response (MMR), an ERP component that reflects the automatic detection of perceptual change in both adults [START_REF] Näätänen | Language-specific phoneme representations revealed by electric and magnetic brain responses[END_REF][START_REF] Näätänen | The mismatch negativity (MMN)-a unique window to disturbed central auditory processing in ageing and different clinical conditions[END_REF] and infants [START_REF] Dehaene-Lambertz | A phonological representation in the infant brain[END_REF][START_REF] Dehaene-Lambertz | Speed and cerebral correlates of syllable discrimination in infants[END_REF]. In each trial, infants listened to a series of three identical syllables ([ɖa]) that were followed by a fourth syllable that was either identical or differed from them in the first consonant. Crucially, the deviant consonant always differed on the same phonetic feature (i.e., place of articulation) but could be either native (i.e., [ba]) or non-native (i.e., [ɖa], with [ɖ] a retroflex consonant that does not occur in Spanish). The authors observed that at 9 months, infants showed a MMR in response to both the native and the non-native deviant consonants; hence, they detected the phonetic change in place of articulation from [ɖa] to both native [ba] and non-native [ɖa]. By contrast, 12-month-olds showed a MMR in response to the change to native [ba], but failed to detect the phonetic change to non-native [ɖa].

Thus, by the age of 12 months, infants have lost the ability to discriminate non-native phonetic contrasts. This type of phonetic learning is often referred to as perceptual attunement (or perceptual narrowing). It should be noted that perceptual attunement likewise occurs in the case of native phonetic contrasts that are not used to distinguish word meaning. For instance, in English, the consonant [t] is usually aspirated when it occurs before a vowel (as in top [t h ɔp]), but it is unaspirated if preceded by [s], (as in stop [stɔp]). Acoustically, this unaspirated [t] is very similar to [d], which never occurs preceded by [s], and native English adults have difficulty discriminating the contrast between unaspirated [t] and [d] despite the fact that they both occur in their language. Importantly, English-learning infants discriminate the contrast at 6-8 months of age, but fail to do so by the time they are 10-months of age, thus showing perceptual attunement [START_REF] Pegg | Adult and infant perception of two English phones[END_REF]. In a similar vein, English-learning infants lose their sensitivity to the contrast between oral and nasal vowels between the ages of 4 and 11 months [START_REF] Seidl | Allophonic and phonemic contrasts in infants' learning of sound patterns[END_REF]. While both types of vowels occur in their language, the contrast between them is never used to distinguish meaning: nasal vowels are phonetic variants of oral vowels, occurring before nasal consonants (cf. beet and bean, whose vowels only differ in nasality).

In this article we focus on yet another type of perceptual attunement, which has hitherto not been investigated. Specifically, depending upon the phonological context in which they appear, even native contrasts that are used to distinguish word meaning can be difficult to discriminate [START_REF] Mitterer | Coping with phonological assimilation in speech perception: evidence for early compensation[END_REF][START_REF] Sun | Complex linguistic rules modulate early auditory brain responses[END_REF]. For instance, [START_REF] Sun | Complex linguistic rules modulate early auditory brain responses[END_REF] collected EEG measures in a mismatch paradigm and showed that French adults do not detect a change from [ofbe] to [ovbe] (i.e., they fail to show a MMR), despite the fact that the distinction between [f] and [v], a voicing2 distinction, is phonemic in their language and hence used to distinguish meaning (e.g., greffe [gʁεf] 'graft'grève [gʁεv] 'strike'); by contrast, they correctly detect a change from [ofne] to [ovne], as evidenced by the presence of a MMR. This result can be explained by the fact that in French, native voicing contrastssuch as the one between [f] and [v]can be neutralized in certain phonological contexts. Specifically, in speech production, voiceless consonants can become voiced when followed by certain voiced consonants, a process called voicing assimilation.3 For instance, the voiceless [f] in oeuf [oef] 'egg' is typically produced as its voiced counterpart [v] in oeuf blanc [oevblɑ] 'white egg', where it is followed by [b]. This process does not apply, however, in oeuf noir [oefnwaʁ] 'black egg', where it is followed by [n]. Specifically, French voicing assimilation applies before voiced obstruents (such as [b]) but not in other contexts (such as before [n]). The findings by [START_REF] Sun | Complex linguistic rules modulate early auditory brain responses[END_REF], then, show that this process affects speech perception: when presented with consonants in an assimilation context (i.e., followed by a voiced obstruent), French adults fail to detect a change in voicing. In other words, the capacity to discriminate a native contrast depends upon the phonological context in which the contrast occurs.

For phonetic learning, this raises the question of whether infants' perceptual attunement is likewise context-dependent, taking into account language-specific phonological processes and the contexts in which they apply. Nine-month-old infants are sensitive to phonological context: they have acquired constraints upon the sequencing of phonemes within syllables and words in their language (i.e., phonotactics), and prefer to listen to phoneme sequences that are phonotactically legal or frequent as opposed to illegal or infrequent [START_REF] Friederici | Phonotactic knowledge of word boundaries and its use in infant speech perception[END_REF][START_REF] Jusczyk | Infants′ sensitivity to the sound patterns of native language words[END_REF][START_REF] Jusczyk | Infants′ sensitivity to phonotactic patterns in the native language[END_REF]. Moreover, 14-month-old infants show some evidence of context-sensitive perceptual attunement: Japanese infants of this age perceive an illusory vowel within consonant clusters that are illegal in their language [START_REF] Mazuka | The development of a phonological illusion: a cross-linguistic study with Japanese and French infants[END_REF], similarly to Japanese adults [START_REF] Dehaene-Lambertz | Electrophysiological correlates of phonological processing: a cross-linguistic study[END_REF][START_REF] Dupoux | Epenthetic vowels in Japanese: a perceptual illusion?[END_REF]. However, whether and how language-specific phonological processes (like French voicing assimilation) influence infants' capacity to discriminate native phoneme contrasts in the context of these processes has not been investigated.

In the present study, we thus tested whether French 14-month-olds, who have already lost the ability to distinguish non-native contrasts,4 are also insensitive to the native voicing contrast when it occurs in an assimilation context. We recorded high-density EEG, using a mismatch trial design as in [START_REF] Peña | Earlier speech exposure does not accelerate speech acquisition[END_REF]. We analyzed the presence or absence of a MMR in response to a voicing change in two types of context, one that allows and one that does not allow for voicing assimilation. The MMR is particularly suited to study native phoneme discrimination, given its sensitivity to the linguistic relevance of the phonetic change in the participant's native language [START_REF] Dehaene-Lambertz | A phonological representation in the infant brain[END_REF][START_REF] Näätänen | Language-specific phoneme representations revealed by electric and magnetic brain responses[END_REF]. If 14-month-olds have already developed sensitivity to the native process of voicing assimilation, they shouldlike French adultsexhibit a MMR in response to a voicing change only in contexts that cannot trigger voicing assimilation in French. Alternatively, if they have not yet developed this sensitivity, they should exhibit a MMR to a voicing change regardless of context.

Material and methods

Participants

Forty-three healthy infants (23 females; mean age: 427 days, range: 396-441 days) raised in a monolingual French-speaking environment participated in the study. All infants were born full-term (37-42 weeks gestation) with normal birthweight (> 2500 g). All parents gave informed consent before the study. According to parental report, infants had normal vision and audition and had no exposure to other languages. Forty additional infants were excluded from the analyses because of an insufficient number of trials (N = 27), parental interference (N = 2), or because they presented less than 13 artifact-free EEG trials per condition (N = 11). We received 29 others infants who did not provide any data because they refused to wear the net.

Design and stimuli

The stimuli were a subset of the ones used by [START_REF] Sun | Complex linguistic rules modulate early auditory brain responses[END_REF]; they all had a vowel-consonant-consonant-vowel structure (

V 1 C 1 C 2 V 2 ).
Given infants' limited attention span, we used two out of the eight original item pairs, i.e. [ofbe]-[ofne] and [ikdo]- [ikmo]. Each of the precursor stimuli was spoken by four female speakers, while the test stimuli was spoken by a male speaker (we used one token per speaker); the use of multiple speakers promotes the detection of phonological rather than purely acoustical changes [START_REF] Dehaene-Lambertz | Cerebral specialization for speech and non-speech stimuli in infants[END_REF][START_REF] Eulitz | Neurobiological evidence for abstract phonological representations in the mental lexicon during speech recognition[END_REF]. All V 1 C 1 C 2 V 2 tokens was created by means of crosssplicing of entirely voiceless C 1 C 2 consonant clusters with entirely voiced ones. Thus, in all stimuli (both precursor and test stimuli), C 1 was always completely voiceless or voiced across all the conditions. For instance, [ofbe] was created by combining [of]-from [ofpe] and -[be] from [ovbe], such that [f] was completely voiceless (see [START_REF] Sun | Complex linguistic rules modulate early auditory brain responses[END_REF]; for more details). The duration of the items was normalized to 500 ms; the duration of the VC 1 part varied (mean: 229 ms; range: 190-279 ms). Similarly, the intensity of the items was normalized to mean RMS amplitude (60 dB).

The stimuli were presented in series of five, with an ISI of 400 ms and a final 2100 ms of silence. In each series, four identical precursor stimuli (e.g., [ofbe] or [ofne] four consecutive times) were followed by a fifth stimulus. In the No Change condition, the last item was identical to the precursors; in the Voicing Change condition, it differed in the voicing of C 1 (e.g., [ovbe] or [ovne], respectively). Given the voicing assimilation process in French, a voicing change is expected to be perceived in 1). We also used a control Place Change condition in which the fifth stimulus differed from the previous ones in its place of articulation of C 1 (e.g., [osbe] or [osne], respectively). Given that these place of articulation changes are phonemic in French and do not reflect any assimilation process, they are expected to be perceived in both the Assimilation context (e.g., [ofbe]-[osbe]) and the No Assimilation context (e.g., [ofne]-[osne]); specifically, see Dehaene-Lambertz and Baillet (1998) for evidence that 3-to 4month-old French infants are sensitive to a change in place of articulation.

[ofne]-[ovne] (No Assimilation context) , but not in [ofbe]- [ovbe] (Assimilation context, cf. Table

Procedure

Infants were seated on their parent's lap in a sound-proof room. To limit body movement, they were presented with silent animated cartoons on a screen, and/or passively observed the experimenter making soap bubbles, and/or played with a small silent toy during testing. They heard 240 trials of 5 items, 40 in each of the six conditions (two types of Contexts, i.e., Assimilation and No Assimilation; by three types of test stimuli, i.e., No Change, Voicing Change and Place Change). The order of the female voices for the precursor stimuli was counterbalanced across trials. Trials were presented in a pseudorandom order, such that no more than two consecutive trials used the same precursor. The experiment was run using the MATLAB Psychophysics Toolbox extensions [START_REF] Brainard | The psychophysics toolbox[END_REF]. The maximum duration was approximately 20 minutes, but the experiment was stopped earlier if the infant became restless.

EEG recording and preprocessing

EEG was recorded using the Net Station 4.3 (EGI software, net amp 200 system) and the EGI (Electrical Geodesics Incorporated, USA) 128channel HCGSN sensor net adapted to infants' cranial perimeter. During the recording, the EEG signal was referenced to the vertex (Cz) and digitized at a sampling rate of 250 Hz.

After acquisition, data were preprocessed using MATLAB and the EEGLab toolbox [START_REF] Delorme | EEGLAB: an open source toolbox for analysis of single-trial EEG dynamics including independent component analysis[END_REF]. Recordings were digitally band-pass filtered (0.3-20 Hz) and segmented into epochs starting 500 ms prior to the onset of the first consonant of each test stimulus and ending 600 ms after it. For each epoch, channels contaminated by eye or motion artifacts (defined as local deviations higher than 80 μV) were automatically excluded, and trials with more than 40% contaminated channels were rejected. Channels containing fewer than 50% good trials were also automatically rejected for the entire recording. The artifact-free epochs (total mean number: 169, total range: 100-239, mean number per condition: 28, range per condition: 13-40) were then averaged for each participant in each of the six conditions (Context: Assimilation, No Assimilation x Phonological Change: No Change, Voicing Change, Place Change), detrended, baseline-corrected (from 500 ms before the onset of the critical consonant C 1 ), and finally transformed into reference-independent values using the average reference method [START_REF] Luck | An introduction to the event-related potential technique[END_REF].

EEG data analysis

Due to the multiple dimensions that can potentially be considered, ERP data analyses are especially sensitive to Type I errors (false alarms), notably when selecting the time window and cluster of electrodes to include in the analysis. To discard this multiple comparison problem, we used our control condition (Place Change) as a localizer to determine the time window and the topography of the MMR in our study. Specifically, we compared the recorded activity in the Place Change condition relative to the No Change condition, collapsing the data of the Assimilation and the No Assimilation contexts in order to maximize statistical power and avoid a selection biased towards one particular context. Indeed, a change in place of articulation should be detected in both the Assimilation and the No Assimilation contexts. We visually selected the time windows and electrode clusters featuring the difference between these two conditions, and averaged the voltage over the selected electrodes and time windows for each participant (Fig. 1). We then used the same time windows and electrode clusters to analyze our contrasts of interest in a 2 × 2 ANOVA with Context (Assimilation vs. No Assimilation) and Phonological Change (No Change vs. Voicing Change) as within-participant factors.

Results

Determination of the channel-clusters and time windows of interest with the place change condition

The visual inspection of the two-dimensional reconstructions of the difference between the Place Change and the No Change conditions (averaged across both Assimilation and No Assimilation contexts) revealed two distinct negativity peaks around the right centro-frontal electrodes, including C4 and T8 synchronous with a positivity on the posterior electrodes. The first negative response lasted from 0 to 200 ms after the onset of the critical consonant (C 1 ); the second negativity response lasted from 250 until 400 ms after the onset of the critical consonant (cf. Fig. 1).

We infer that the first negativity response is automatically generated by the detection of the acoustic differences of C 1 in the first syllable of the precursor and test items (e.g., [of], [ov] and [os] for the [ofbe]-[ofne] pair). Its bilateral topography and latency (0-200 ms) are compatible with the early auditory response usually observed in infants of the same age (Rivera-Gaxiola et al., 2007b;Rivera-Gaxiola et al., 2005a,b). The second response consisted of a right anterior negative pole and a reverse of polarity over the left temporal region. This topography and latency is compatible with a mismatch response, which is interpreted as an error signal between the predictions based on the repetition of the precursors and the actual perception of the target stimulus [START_REF] Basirat | A hierarchy of cortical responses to sequence violations in three-month-old infants[END_REF][START_REF] Bendixen | Probability dependence and functional separation of the object-related and mismatch negativity event-related potential components[END_REF][START_REF] Lieder | A neurocomputational model of the mismatch negativity[END_REF][START_REF] Wacongne | A neuronal model of predictive coding accounting for the mismatch negativity[END_REF][START_REF] Winkler | Interpreting the mismatch negativity[END_REF]. Given the nature of the predicted effects (failure to detect a voicing change in a languagespecific assimilation context), we thus focused our analysis of the conditions of interest on this second response. We selected the corresponding electrodes cluster featuring the largest difference (p < 0.005 for each channel) between the No Change and the Place Change conditions. These clusters contained 17 channels (including F4, C4, P4 and T8; t(42) = 3.25, p < 0.005, see the black triangles on Fig. 1). We used the same time window and electrode clusters extracted from this localizer to test for the presence of a mismatch response in each condition of interest.

Analysis of the conditions of interest

The 2 × 2 ANOVA with context (Assimilation vs. No Assimilation) and phonological change (No Change vs. Voicing Change) conducted on the MMR response [250-400 ms] (cf. Fig. 2) yielded no main effect of Context (F < 1) but a marginal effect of Phonological Change (F(1.42) = 2.85 p = 0.09), as well as an interaction between these two factors (F(1, 42) = 4.75, p < 0.05). This interaction was due to an effect of Phonological Change in the No Assimilation context (t(42) = 6.82, p < 0.05) but not in the Assimilation context (t < 1). Thus, infants detected the voicing change in the No Assimilation context but not in the Assimilation context.

Discussion

We showed that 14 month-old French-learning infants' capacity to discriminate a native voicing contrast is context-dependent. That is, similarly to French adults [START_REF] Sun | Complex linguistic rules modulate early auditory brain responses[END_REF], they exhibit a MMR to a voicing change when the following sound is a nasal (No Assimilation context) but not when the following sound is an obstruent (Assimilation context). This, then, is evidence for a novel type of perceptual attunement. More specifically, the present results show that at 14 months of age, French-learning infants have not only constructed their native phoneme inventory, they have also acquired some knowledge about language-specific interactions among phonemes. Indeed, French voicing assimilationa phonological process that changes the voicing of a consonantapplies before obstruents but not before nasals; infants have thus acquired this context-dependent neutralization of the voicing contrast.

A straightforward interpretation of our results is that infants compensate for voicing changes in assimilation contexts. That is, given that [ofbe] is usually pronounced as [ovbe], they automatically retrieve the form [ofbe] upon hearing [ovbe]. Direct evidence for such compensation at an older age has been shown with a word recognition task [START_REF] Skoruppa | Early word recognition in sentence context: French and English 24-month-olds' sensitivity to sentence-medial mispronunciations and assimilations[END_REF]. In this study, 24-month-old French toddlers were tested in a looking-while-listening paradigm. Target pictures of familiar objects whose name ends in a consonant that can undergo assimilation (e.g., an egg -French: oeuf [oef]) were presented side-by-side with distracter pictures of unfamiliar objects. In the probe sentences, the target name was pronounced with a final voicing change either in an assimilation context, i.e. before an obstruent (e.g., Regarde l'oeuv devant toi. 'Look at the egg in front of you.') or in a noassimilation context, i.e. before a nasal (e.g., Regarde l'oeuv maintenant. 'Look at the egg now.'). Toddlers looked longer at the target pictures in the assimilation than in the no-assimilation context. Thus, they more often considered the assimilated form of the target words as a possible instance of these words when the context allowed for voicing assimilation than when it did not. In other words, they showed context-specific compensation for their native process of voicing assimilation (see Darcy 2009), for a similar study with French adults). As shown by the EEG study with adults reported in [START_REF] Sun | Complex linguistic rules modulate early auditory brain responses[END_REF], compensation for assimilation influences not only word recognition but also phonetic discrimination. Using a subset of [START_REF] Sun | Complex linguistic rules modulate early auditory brain responses[END_REF]'s stimuli and an adapted paradigm, we provide evidence that this type of contextdependent perceptual attunement is already in place at 14 months of age.

These results leave open two questions concerning how and when infants acquire native phonological processes and hence become insensitive to native sound contrasts in particular contexts. Let us start by considering possible learning mechanisms. For the classic case of perceptual attunement to native sound categories, distributional learning theories [START_REF] Kuhl | Linguistic experience alters phonetic perception in infants by 6 months of age[END_REF][START_REF] Maye | Statistical phonetic learning in infants: facilitation and feature generalization[END_REF][START_REF] Maye | Infant sensitivity to distributional information can affect phonetic discrimination[END_REF] postulate that infants attend to the distribution of speech sounds in the acoustic space. As tokens of a given sound category cluster together, this would allow them to discover the speech sound categories used in their language. By taking into account the contexts in which the sounds of their language occur, infants could similarly infer which contrasts are meaningful, and hence distinguish words [START_REF] Peperkamp | The acquisition of allophonic rules: statistical learning with linguistic constraints[END_REF]. For instance, before knowing any words English-learning infants could infer that the contrast between oral and nasal vowels is not meaningful in their language by observing that nasal vowels occur before a nasal consonant while oral vowels occur in all other contexts. Noticing this complementary distribution, then, would result in loss of sensitivity to the oralnasal contrast. Distributional learning mechanisms have been argued to be supplemented by top-down information. For instance, infants could benefit from consistent pairing of speech with visible objects [START_REF] Yeung | Referential labeling can facilitate phonetic learning in infancy[END_REF][START_REF] Yeung | Learning words' sounds before learning how words sound: 9-month-olds use distinct objects as cues to categorize speech information[END_REF], or they might use their knowledge about (real or even approximate) word forms (Feldman et al., 2013a,b;[START_REF] Martin | Learning phonemes with a proto-Lexicon[END_REF][START_REF] Swingley | Contributions of infant word learning to language development[END_REF]. Could distribu-tional learning, with or without top-down influences, also account for the acquisition of native phonological processes and hence for the context-dependent perceptual attunement evidenced by our results? Note that unlike the English oral and nasal vowels mentioned above, French voiced and voiceless obstruents do not have complementary distributions. Indeed, they can both occur before, say, nasal consonants, where the voicing assimilation never occurs. Due to voicing assimilation, however, voiceless obstruents tend not to occur before voiced obstruents. 5 Observing this distributional fact might be useful in the path towards the acquisition of voicing assimilation, but it is not enough. Indeed, in order for infants to infer that, say, [fb] is rare because it is most often changed into [vb] rather than into something else, they need more information. There are at least two (mutually nonexclusive) sources of top-down information they could use to infer the assimilation process.

On the one hand, infants might rely on word meaning. For instance, knowing that the phrases oeuf blanc, [oevblɑ] 'white egg' and oeuf noir [oefnwar] 'black egg' refer to the same type of object (i.e., eggs) would allow French-learning infants to deduce that [oev] and [oef] are different instances of the same word, and hence that, more generally, a difference in voicing before voiced obstruents does not signal a difference in word meaning. According to this rationale, this specific form of perceptual attunement would depend upon infants' ability to associate auditory labels to a corresponding meaning. While words that can potentially undergo assimilation may be frequent in the infant's environment (approximately 28% of the words contained in the French version of the Communicative Development Inventory, CDI (Fenson are not yet known at the age of 14 months: in vocabulary data for a sample of 103 monolingual 14-month-olds (mean number of known words: 74, SD = 64) tested in the same lab as those in the present study, only 13% of the assimilable words contained in the French CDI are reported to be known by more than a quarter of the infants, and only 2% by more than half of them. Thus, an account based on infants' knowledge of word meanings appears unlikely.

On the other hand, infants may rely on a natural clustering of tokens of assimilable words: if both assimilated and non-assimilated forms of the same word often appear within a short period of time (for example, within a short conversation), it might be possible for them to consider them as two instances for the same word. Importantly, this mechanism is independent on infants' knowledge of word meanings. Some support for this hypothesis is provided by data from an orthographically transcribed corpus of spontaneous conversations in the presence of young children, taken from the CHILDES database [START_REF] Macwhinney | The CHILDES project: tools for analyzing talk (third edition): volume I: transcription format and programs, volume II: the database[END_REF]. In this corpus, 7% of the word tokens end in a consonant that can undergo assimilation and of these, 10.2% occur in an assimilation context. Crucially, pairs of tokens of the same word, one occurring in an assimilation and one in a no-assimilation context (within a single recording session), tend to cluster together. That is, nearly 40% of these co-occurrences are found within an estimated two-minute stretch of speech.7 Moreover, co-occurring tokens of distinct words that differ only in their final consonant (e.g. mousse [mus] 'foam' vs. mouche [muʃ] 'fly') are three times less likely to appear within the same time frame. The clustering of assimilated and unassimilated tokens, then, might be a powerful cue for infants to infer that word-final voicing differences do not signal differences in word meaning, but, rather, reflect systematic variation induced by voicing assimilation.

Finally, as to the question of when infants acquire native phonological processes and hence show context-sensitive perceptual attunement, the scenario sketched above relies on word segmentation and sensitivity to the clustering of assimilated and non-assimilated word forms within short stretches of speech. Word segmentation abilities emerge between 6 and 12 months of age (for a review, see Nazzi et al. (2013); we therefore cannot exclude that the acquisition of native phonetic categories and of phonological processes proceed simultaneously, such that context-dependent perceptual attunement evidenced here arises at the same age as 'standard' perceptual attunement, i.e. before the end of the first year of life. Alternatively, the acquisition of phonological processes might lag behind the acquisition of native phonetic categories. We deem this to be more likely, as it takes time not only to fully develop word segmentation abilities but also to gather enough data on clustered tokens of assimilated and non-assimilated word forms. We therefore predict that 12-month-old French-learning infants behave differently from the 14-month-old ones that we tested, and discriminate native voicing contrasts regardless of the context in which they appear.

Conclusion

To conclude, this research provides evidence for a novel type of context-dependent perceptual attunement. Specifically, we demonstrated that by the age of 14 months, infants' native phoneme categorization does not only rely on the processing of individual sounds, but also takes into account the phonological contexts in which these sounds occur, in a language-specific manner. Further research is needed to determine the precise age range at which this contextdependent perceptual attunement emerges, and to understand the underlying learning mechanism.

Fig. 1 .

 1 Fig. 1. MMR obtained for the localizer comparison between No Change minus Place Change, averaged across the Assimilation and the No-assimilation Contexts, for the second negative response (250-400 ms). The left side displays a 2-D map of the corresponding z-scores (A) while the right side displays the time course of the voltage averaged over the respective electrode clusters for the No Change vs. Place Change conditions (B). In these maps, triangles mark the location of the electrodes selected for statistical analysis. The onset (cf. black arrow) of the MMR was measured from the onset of the first consonant C 1 as shown in the example for the test stimulus ofbe.

Fig. 2 .

 2 Fig. 2. MMR obtained for the comparison No Change minus Voicing Change, in the Assimilation Context (legal context for voicing assimilation, A and B) and the No-assimilation Context (illegal context for voicing assimilation, A' and B'), for the time window 250-400 ms. In each panel, the left side displays a 2-D map of the corresponding z-scores (A and A'). In these maps, triangles mark the location of the electrodes selected for statistical analysis. The central columns display the time course of the voltage averaged over the selected electrodes for each of the conditions of interest (B and B'). The rightmost part of the figure displays the graphical representation of this activity averaged for the whole duration of the response (C). Error bars represent standard error of the mean.

Table 1

 1 Design.

	Pair	Context	Precursor (x4)	Test stimuli
	ofbe-ofne			No Change: [ɔfbe]
		Assimilation	[ɔfbe]	Voicing Change: [ɔvbe]
				Place Change localizer:
				[ɔsbe]
				No Change: [ɔfne]
		No-assimilation	[ɔfne]	Voicing Change: [ɔvne]
				Place Change localizer: [ɔsne]
	ikdo-ikmo			No Change: [ikdo]
		Assimilation	[ikdo]	Voicing Change: [igdo]
				Place Change localizer: [ipdo]
				No Change: [ikmo]
		No-assimilation	[ikmo]	Voicing Change: [igmo]
				Place Change localizer: [ipmo]

Voicing is the phonetic feature that refers to the vibration of the vocal cords during the production of a given speech sound. Sounds that are produced with vibration of the vocal cords are called voiced, and those without vibration are called voiceless.

Similarly, certain voiced consonants can become voiceless when followed by a voiceless consonant. In this article, however, we are only concerned with the change from voiceless to voiced.

To our knowledge, there is no ERP study investigating perceptual attunement for phoneme categories in French-learning infants, but given that it has been documented at 12 months of age for infants learning a number of languages, including English, Spanish, and Japanese, we have every reason to assume that French 14-month-olds have likewise acquired their native phoneme categories.

They sometimes occur in this position, because voicing assimilation is not obligatory (Hallé and Adda-Decker, 2011).

For adjectives, we considered both masculine and feminine forms, and for verbs, all forms of the indicative mode. For the following analyses, we used an augmented version of the corpus analyzed by[START_REF] Ngon | Non)words, (non)words, (non)words: evidence for a protolexicon during the first year of life[END_REF], containing over 425,000 word tokens, and considered the full extent of the French voicing assimilation process, including both voicing and devoicing changes (see note 1).

Based on[START_REF] Hart | 42 American families[END_REF]; we considered that two minutes of child-directed speech contains on average 42 word tokens.

Acknowledgements

This study was funded by grants from the Agence Nationale de la Recherche (ANR-13-APPR-0012; ANR-10-LABX-0087 IEC; ANR-10-IDEX-0001-02 PSL*) and the Fondation de France. We thank Ghislaine Deheane-Lambertz for comments on analyses and previous versions of this manuscript. We are grateful to the parents and infants who participated in this study, as well as to Anne-Caroline Fievet and Luce Legros for their help in recruiting and testing participants and Michel Dutat for technical support.


Peter M Chapple

Julien Cartron

Ghanem Hamdoun

Marie Cordier

Samia Kahlal

Hassan Oulyadi

Jean-François Carpentier

Jean-Yves Y Saillard
email: jean-yves.saillard@univ-rennes1.fr

Yann Sarazin
email: yann.sarazin@univ-rennes1.fr

J F Orcid

Carpentier

Bonding analysis in ytterbium(II) distannyl and related tetryls

Keywords: detailed synthetic procedures, spectroscopic data, XRD diffraction analysis for Yb-Sn

The syntheses of the ytterbium(II) distannyl [Yb{Sn(SiMe3)3}2.(thf)4] (Yb-Sn) and of its digermyl analogue [Yb{Ge(SiMe3)3}2.(thf)3] (Yb-Ge) are presented. The compounds were characterised by multinuclear high-resolution solution NMR spectroscopy, including 171 Yb NMR, and by X-ray diffraction crystallography. The bonding and electronic properties of the two complexes, along with those of the known ytterbium(II) disilyl derivative [Yb{Si(SiMe3)3}2.(thf)3] (Yb-Si) and those of the congeneric calcium distannyl [Ca{Sn(SiMe3)3}2.(thf)4] (Ca-Sn), were investigated in detail by DFT calculations. This analysis points at a primarily ionic Yb-tetrel bonding, with a small covalent contribution, attributed principally to the 5d(Yb) participation. This weak covalent character is found to be larger for the distannyl Yb-Sn than for its lighter Si-and Ge-derivatives. The covalent component is also found to be greater in Yb-Sn than in Ca-Sn, due to the availability of the 5d(Yb) orbitals for bonding.

Introduction

Salts and soluble complexes of the classical divalent lanthanides ytterbium, europium and samarium have long attracted interest as one-electron reducing agents, for the activation of small molecules [START_REF]Lanthanides: Chemistry and Use in Organic Synthesis[END_REF][START_REF] Steel | [END_REF][3][4][5][6] and, more recently, for their magnetic properties [7][8] or as potent catalysts. [9][10][11][12][13][14][15] Compared to its congeners, the smallest of these elements, Yb(II), offers the advantage of being diamagnetic due to its closed-shell [Xe] 4f 14 6s 2 electronic configuration. The 171 Yb isotope, with a spin number of ½, a natural abundancy of 14.31% and a reasonable receptivity (4.4 vs carbon), is a nucleus well suited to NMR spectroscopy. 171 Yb chemical shifts measured so far are in the range +2500 to -500 ppm. 171 Yb NMR spectroscopy has provided useful information on the structural and bonding parameters in a variety of ytterbium(II) complexes [START_REF] Keates | 171 Yb NMR spectroscopy[END_REF] since this spectroscopic method was first used on a range of amido and cyclopentadienyl species. [START_REF] Avent | [END_REF] We have recently prepared two series of alkaline-earth distannyls [Ae(SnR3)2.(thf)x], for Ae = Ca, Sr and Ba and where R = Ph or SiMe3. 18 We have also reported heteroleptic Ae-stannyls and Ae-silyl [{L}AeE(SiMe3)2.(thf)x] for Ae = Ca and Ba and E = Si or Sn, and where L is a sterically demanding, tridentate bis(imino)carbabazolate. [19][20] Ytterbium(II) is often compared to calcium due to their similar physico-chemical properties, notably their ionic radii (Ca 2+ : 1.00 Å; Yb 2+ : 1.02 Å), Pauling electronegativity (Ca, 1.00; Yb, 1.1) and oxophilicity. We have hence turned our attention to the synthesis of ytterbium(II) tetryls, on the grounds that the characterisation and structural analysis of complete homologous suites of such complexes have not been performed to date. The first ytterbium(II) stannyl, the tetrahedral [Yb{Sn(CH2 t Bu)3}2.(thf)2], was reported and crystallographically characterised in 1991; 21 multinuclear NMR data recorded in toluene-d8 showed diagnostic resonances at δ 171 Yb 725 ppm and δ 119 Sn -95 ppm in the 171 Yb and 119 Sn NMR spectra. Bochkarev and coworkers described the molecular solid-state structures of the octahedral [Yb(SnPh3)2.(thf)4] 22 and [Yb{Sn(SnMe3)3}2.(thf)4], 23 and that of the heteroleptic [(thf)2.(Ph3Sn)Yb(μ-Ph)3Yb.(thf)3], 24 but 171 Yb NMR data was not provided.

The same group also disclosed the only two structurally identified ytterbium(II) germyls known to date, [Yb(GePh3)2.(thf)4] and the metallacycle [cyclo-(Ph2Ge)4Yb.(thf)4], where the geometry about the Yb(II) atoms is octahedral. [25][26] They also mentioned the first structurally characterised ytterbium(II)silyl, [Yb(SiPh3)2.(thf)4], said to be isostructural with the germyl congener, 26 but the X-ray structure was not provided and is currently not available. [START_REF]The CIF file available from the CCDC database, code YEDYEL, does[END_REF] Baumgartner and coworkers demonstrated the detailed syntheses and structures of the hypersilylated trisolvate [Yb{Si(SiMe3)3}2.(thf)3], which adopts a distorted trigonal pyramidal geometry, and those of the metallacycle [{Me2Si(Me3Si)2Si}2Yb.(thf)4]; [START_REF] Zitz | [END_REF] unfortunately, the 171 Yb NMR data was not given, although [START_REF]Lanthanides: Chemistry and Use in Organic Synthesis[END_REF] J 29Si-171Yb and 2 J 29Si-171Yb scalar coupling were visible in the 29 Si NMR spectra for these complexes. Over ten other X-ray structures of Yb(II)silyls are available in the CCDC database, e.g. the seminal [(C5Me5)Yb{Si(SiMe3)3}.(thf)2] (δ 171 solvent benzene-d6). 30 Finally, the organoytterbium(II) [Yb{C(SiMe3)3}2], which exhibits a very bent geometry (C-Sn-C' = 104.8(5)°) with secondary Yb•••H3C interactions in the molecular solid-state, has long been known; 31 it displays a resonance at 812 ppm in the 171 Yb NMR spectrum recorded in toluene-d8. Ytterbium(II) plumbyls are unknown to date. All relevant existing complexes were surveyed in a recent review. 32 We report here on the synthesis, structural determination and multi-nuclear high-resolution NMR characterisation of two new ytterbium(II) tetryls, [Yb{Ge(SiMe3)3}2.(thf)3] (Yb-Ge) and its heavier analogue [Yb{Sn(SiMe3)3}2.(thf)4] (Yb-Sn). Their structural and electronic parameters, as well as those of the known silyl derivative [Yb{Si(SiMe3)3}2.(thf)3] (Yb-Si), [START_REF] Zitz | [END_REF] have been analysed by the means of DFT calculations to inform ourselves about the bonding properties in this family of complexes (Figure 1). The results of this analysis, along with a comparison of the bonding patterns between Yb-Sn and its calcium congener [Ca{Sn(SiMe3)3}2.(thf)4] (Ca-Sn), 18 are described herein. [Yb{Si(SiMe3)3}2.(thf)3] (Yb-Si, δ 29Si -5.2 ppm). [START_REF] Zitz | [END_REF] The large variation in the chemical shift compared to Yb-Sn is most likely due to the change in the coordination environment about the central metal from a 6-coordinate to a 5-coordinate complex, while the different nature of the bonding may also be invoked (see DFT discussion below) as a possible contribution. Electronic factors depending on the nature of the anion for E = Si, Ge or Sn may also be partly responsible for the observed variation of chemical shifts.

Results and Discussion

Synthesis and characterisation

Large

The | 2 J 29Si-171Yb | coupling constant of 20.2 Hz for Yb-Ge is significantly smaller than that in Yb-Sn (30.5 Hz). The 171 Yb NMR displays a slightly broad singlet at δ 171Yb 816 ppm (Δν1/2 = 160 Hz), shifted downfield from that of Yb-Sn.

Although the complex Yb-Si has already been reported, [START_REF] Zitz | [END_REF] its 171 Yb NMR spectrum was not included in its characterisation. Hence, we remade this complex in order to gather its full suite of NMR spectra, and to make comparisons with Yb-Sn and Yb-Ge. Complex Yb-Si was prepared as described, though the NMR data we recorded does have a few subtle differences from the literature data, most importantly the difference in the | 2 J 29Si-171Yb | coupling constant of 8.6 Hz (reported previously to be 41

Hz, a constant that we propose is actually from | 1 J 29Si-13C | scalar coupling). [START_REF] Zitz | [END_REF] The 171 Yb NMR spectrum of Yb-Si has a broad resonance similar to that of Yb-Ge. This signal (δ 171Yb 948 ppm) is shifted downfield relative to Yb-Sn (δ 171Yb 535 ppm) and Yb-Ge (δ 171Yb 816 ppm). Hence, the resonance becomes more deshielded for the lighter tetrels in group 14. For comparison, the resonance in the NMR spectrum of the unsolvated [Yb{C(SiMe3)3}2] was found at δ 171Yb 812 ppm. 31 However, direct comparison between the compounds is ill-advised due to their different degree of solvation by thf molecules. A summary of the NMR data for compounds Yb-Si, Yb-Ge and Yb-Sn is collated in Table 1.

The data for [Ca{Sn(SiMe3)3}2.(thf)4] (Ca-Sn), that is, the direct calcium analogue of Yb-Sn recently prepared, has also been added. Compared to Ca-Sn, the 119 Sn resonance for Yb-Sn is slightly deshielded (-857 vs -757 ppm, respectively) whereas remarkably, the absolute value of 2 J 119Sn-117Sn for the scalar coupling of the tin isotopes through the central metal is more than doubled (455 and 932 Hz, respectively). This is presumed to reflect the presence of the 4f 14 electrons and the slightly more covalent bonding in Yb-Sn, although we will refrain from making further claims at this stage. We note that the The germyl Yb-Ge forms a five-coordinate distorted square pyramidal geometry (τ5 = 0.46), with two thf ligands (corresponding to O1 and O2; O1-Yb1-O2 = 152.75(16)°) occupying the apical positions (Figure 6). The four atoms Yb1, Ge1, Ge2 and O3 are nearly coplanar, and the Ge1-Yb1-Ge2, O3-Yb- to involve in the formation of hybrid orbitals compared to the tin atoms in Yb-Sn, due to the lower energy gap between s and p orbitals in the case of germanium. Finally, the structure of Yb-Ge resembles very closely that of its calcium analogue, the five-coordinate [Ca{Sn(SiMe3)3}2}.(thf)3] (Ca-Ge). 35 The key interatomic distances in this calcium digermyl, Ca-Ge (3.022(2)-3.067(2) Ǻ), Ca-O (2.364(7)-2.385(7) Ǻ) and Ge-Si (average 2.388 Ǻ).

[ppm] | n J X-Y | / [Hz] δ 119 Sn / [ppm] | n J X-Y | / [Hz] δ 29 Si / [ppm] | n J X-Y | / [Hz] δ 13 C [ppm] | n J X-Y | / [Hz]
For comparative purposes, a summary of the main metric parameters for the new complexes Yb-Ge and Yb-Sn, as well as those for the already reported ytterbium disilyl [Yb{Si(SiMe3)3}2}.(thf)3] (Yb-Si) [START_REF] Zitz | [END_REF] and for their calcium analogues [Ca{Si(SiMe3)3}2}.(thf)3] (Ca-Si), 36 [Ca{Ge(SiMe3)3}2}.(thf)3] (Ca-Ge) 35 and [Ca{Sn(SiMe3)3}2}.(thf)3] (Ca-Sn), 18 is provided in Table 2. For a given tetrel element (E = Si/Ge/Sn), there is remarkably little difference across the Yb(II) and Ca series. The main variation consists in the slightly shorter Ca-Othf interatomic distances compared to the Yb-Othf ones. It results, in part, from the smaller size calcium (rionic: Ca 2+ : 1.00 Å; Yb 2+ : 1.02 Å), although the difference in the respective ionic radii is too small to account for the observed contraction. The geometry about the tetrel element is hardly affected by the nature of the central metal it is bound to. For a given metal (Met = Ca/ Yb(II)), the Met-Sn interatomic distance is much greater than the other Met-E bonds for E = Si or Ge, consistently with the much larger size on tin. However, it is interesting to note that the Yb-Ge (resp. the Ca-Ge) interatomic distance is not longer than the Yb-Si (resp. the Ca-Si) one, despite the greater

Bonding analysis

The electronic structure of the complexes discussed above was investigated by the means of Density Functional theory (DFT) calculations at the TZP/PBE0-D3-ZORA level (see Computational Details).

All the geometries were fully optimised (gas-phase considered), starting from their structures established by X-ray diffraction analysis when available, or otherwise from that of their closest parent. We first discuss the six-coordinate series of tetrasolvated complexes [Yb{E(SiMe3)2.(thf)4] (E = Si, Ge, Sn), of which only the Sn derivative (Yb-Sn) has been so far isolated (see above; only 3 thf molecules are found in Yb-Si and Yb-Ge). The calcium relative [Ca{Sn(SiMe3)2.(thf)4] (Ca-Sn) 18 is added for comparison.

Selected computed data are provided in Tables 345. The optimised geometries of Yb-Sn and Ca-Sn are in an overall good agreement with their X-ray structures, with, however, shorter Met-Sn distances. Such underestimation of this specific type of bonds has been observed previously, and is a tendency intrinsic to the method of calculations that is already documented. 18,24 Within the set of tetrasolvated Yb complexes, the HOMO-LUMO gap decreases when descending group 14 from Si to Sn, whereas the increase of the Wiberg bond indices (WBI) and the variation of the Yb NAO charges are consistent with an increase in Yb-E covalency (Table 3). This apparent contradiction between the variations of the HOMO-LUMO gap and covalency is rationalised by the fact that the frontier orbitals of the complexes are not associated with Yb-ligand bonding. The HOMOs are the seven (non-bonding) 4f orbitals and the LUMOs are better seen as describing diffuse Rydberg states loosely bounds to the methyl hydrogens. The two orbitals associated with the Yb-E bonding electron pairs are located below the seven nearly-degenerated 4f HOMOs. They can be viewed as the out-ofphase and in-phase combinations of localised Yb-E -bonding orbitals, respectively (see Figure 7 for the illustrative example of Yb-Sn). They are largely E-polarised, with, from Si to Ge and Sn, E(%)/Yb(%) composition of 84/15, 82/15 and 67/27 for the HOMO-7 and 92/3, 82/14 and 81/13 for the HOMO-8, respectively. These values are indicative of the strong ionic character of Yb-E bonding. The (weak) covalency which increases when descending the tetrel column can be related to a stronger interaction of the valence np AO of E with the 6s AO of Yb.

Comparison of the computed data in Table 3 for [Yb{Sn(SiMe3)3}2.(thf)4] (Yb-Sn) with those of [Ca{Sn(SiMe3)3}2.(thf)4] (Ca-Sn) shows that the Ca-Sn bond is tangibly more ionic than its Yb-Sn congener. The larger covalency of the Yb-Sn bond is not related to the 4f(Yb) orbitals, which remain as expected fully non-bonding, but rather to the comparatively greater participation to the bonding of its 5d AOs, as compared to the 3d(Ca) orbitals, compare the NAO 5d and 3d populations of 0.43 and 0.08, respectively.

Another point-of-view on the nature of the Met-E bonding in the [Met{E(SiMe3)2.(thf)4] series displayed in Table 3 can be obtained by performing an energy decomposition analysis (EDA) of the interaction between two frozen molecular fragments, according to the Morokuma-Ziegler procedure. 37- HOMO-7 HOMO-8 TBE is expressed as the sum of four components: the Pauli repulsion (EPauli), the electrostatic interaction energy (Eelstat), the orbital interaction energy (Eorb) and the component associated with the dispersion forces (Edisp). The four compounds have similar TBEs. Their moderate variation indicates that the Yb-E bond strength increases in the order Sn < Si < Ge. The Ca-to-Sn bond in Ca-Sn is weaker than that of Yb-Sn. In any case, these variations are rather weak. For all the computed compounds, TBE is dominated by its electrostatic component, indicating a prevailing ionic bonding. The Eorb component, which is associated with covalency, is about 4 times smaller, and the observed trend for Eorb within the suite is not fully consistent with that of the Met-E WBI values given in Table 3. Such apparent contradiction with the monotonous variation of the WBIs within the series is not uncommon 18 when the differences between values are small, as in the present scenario. It can be, inter alia, related to the arbitrary choice of fragmentation upon which our EDA analysis is built. 40 It should be however mentioned that the alternative chemically intuitive fragmentation [Yb{E(SiMe3)3}(thf)4] + + [E(SiMe3)3] - provides similar trends across the investigated series.

A different angle on the Met-E bonding analysis can also be provided by the Quantum Theory of Atoms in Molecules (QTAIM) approach. [START_REF] Bader | Atoms in Molecules -A Quantum Theory[END_REF][START_REF] Popelier | The Chemical Bond[END_REF] Selected QTAIM data are collated in Table 5. The AIM atomic charges, delocalisation index and electron densities at the bond critical points indicate similar trends as the NAO charges and WBI indices, that is, a gradual (and relative) increase of the covalent nature of the Met-E bond following the order Ca-Sn < Yb-Si < Yb-Ge < Yb-Sn. It is also of note that all the Met-E bond critical point (bcp) indicators have small absolute values. The positive sign of the Laplacian density, the negative sign of the (very small) energy density and the V/ G ratio significantly larger than 1 are indicative of a bonding mode dominated by an ionic interaction with some very polar covalent character.

For comparison, selected computed data for the Yb trisolvated series [Yb{E(SiMe3)3}2.(thf)3] (E = Si, Yb-Si; Ge, Yb-Ge; Sn, not isolated) are provided in Tables 678. The optimised geometries of the experimentally isolated Yb-Si and Yb-Si complexes are in good agreement with their X-ray structures. The data collected in Table 6 indicate a significant Yb-E covalent character in the case of E = Sn, due to a stronger interaction between the orbitals 6s(Yb) and 5p(Sn). The EDA analysis based on a fragmentation between the "solvated" cation [Yb(thf)3] 2+ and the [(SiMe3)E...E(SiMe3)] 2-dimeric fragment (Table 7) and the QTAIM analysis (Table 8) overall provide similar tendencies.

Unsurprisingly, the Yb-E bonding is found to be stronger in the case of the trisolvated complexes with respect to their tetrasolvated analogues, both in terms of covalency and ionicity.

Finally, the question of the isolation of Yb-Sn as a tetrasolvated species, whereas the related Yb-Ge (this work) and Yb-Si 28 complexes were obtained as trisolvated species, has been explored. Part of the answer is provided by the computed energy differences

E = E[Yb{E(SiMe3)3}2.(thf)4] - E[Yb{E(SiMe3)3}2.(thf)3] -E(thf).
The three values are negative (-15.4, -15.9 and -21.7 kcal/mol for E = Si, Ge and Sn, respectively), indicating that for the whole suite, tetrasolvated species are more stable in vacuum. In solution in thf, explicit solvent effect should be considered, [START_REF]When solvent effect is implicitly considered via the COSMO model[END_REF] which would require a computational effort that is far beyond the scope of this investigation. It can however be noted that the value of E corresponding to E = Sn is far more negative than for its Si and Ge congeners, highlighting the enhanced stability of the tetrasolvated species with respect to the trisolvated one in the specific case of Sn. Energy decomposition analysis indicates that with Sn, the major contribution responsible for the lower value of E is the Pauli repulsion, which can be in that particular case approximated to steric repulsion. It thus appears that in the investigated ytterbium ditetryl complexes, tin offers greater flexibility and room around the central ytterbium than silicon and germanium do, with the consequence that the hosting of a fourth thf molecule in the coordination sphere of ytterbium becomes favourable. This is consistent with accepted wisdom, since the sterically shielding Ph groups on the tetrel elements will be located further away from ytterbium as the size of the tetrel increases.

Conclusions

We have prepared the ytterbium(II) distannylide [Yb{Sn(SiMe3)3}2.(thf)4] (Yb-Sn), which we have fully characterised, including by X-ray diffraction analysis and high-resolution NMR spectroscopy in benzene-d6. The octahedral tetrasolvated complex, with stannyl groups in axial position, exhibits in particular well-defined resonances at δ 119Sn -757 ppm and δ 171Yb 5570 ppm in the 119 Sn and 171 Yb NMR spectra, respectively. It is a rare case where both XRD and multinuclear NMR data have been collated

for an Yb(II) stannylide, and it adds to the limited set of such complexes documented in the literature.

The NMR and structural data for the stannylide Yb-Sn have been compared to those obtained for its derivatives incorporating lighter tetrels, namely, the known [START_REF] Zitz | [END_REF] DFT calculations, completed with NBO, EDA and QTAIM analyses, have shed light on the nature of the metal-to-tetrel bond in these three ytterbium complexes and in the related calcium distannylide [Ca{Sn(SiMe3)3}2.(thf)4] (Ca-Sn). In all computed compounds, the metal-to-tetrel bonding is found to be of prevailing ionic character, with only a weak covalent component. This is likely the reason why no relationship could be found between the NMR coupling constants and the degree of covalency in these compounds. Nevertheless, this weak covalency is stronger in the case of the heavier tetrel element considered, tin. It is also found to be more important in Yb-Sn than in Ca-Sn, due to the availability of the 5d(Yb) orbitals for bonding. All values in a.u.

Experimental section

General considerations

All manipulations were performed under an inert atmosphere by using standard Schlenk techniques or in a dry, solvent-free glovebox (Jacomex; O2<1 ppm, H2O<3 ppm). Hexanes (petroleum ether), toluene, dichloromethane, and Et2O were collected from MBraun SPS-800 purification alumina columns and thoroughly degassed with argon before being stored on 4 Å molecular sieves; thf was distilled under argon from Na/benzophenone prior to use. Deuterated solvents (Eurisotop, Saclay, France) were stored in sealed ampoules over activated 4 Å molecular sieves and degassed by a minimum of three freezethaw cycles. Standard NMR spectra (see below for high-resolution data) were recorded with Bruker AM-400 or AM-500 spectrometers. All 1 H and 13 C chemical shifts (δ, reported in ppm) were determined relative to the residual signal of the deuterated solvent. Assignment of the signals was assisted by 1D

( 1 H, 13 C) NMR experiments. [K(Si(SiMe3)3], [START_REF] Marschner | [END_REF] [Ge(SiMe3)4], 45 [Sn(SiMe3)4], [46][47] YbI2.(thf)2 48 and [Yb{Si(SiMe3)3}.(thf)3] 28 were prepared following literature procedures. All other chemicals were provided by commercial suppliers and used as received. Satisfactory combustion analysis of the complexes could not be achieved, likely due to their extreme air-and moisture-sensitivity.

Synthesis of [Yb{Sn(SiMe3)3}2}.(thf)4] (Yb-Sn).

To a Schlenk flask was added Sn(SiMe3)4 (500 mg, 1.22 mmol) and KO t Bu (137 mg, 1.22 mmol), and the contents were dissolved in thf (7 mL). The pale-yellow solution was left to stir for 1 h, before it was added dropwise to a suspension of YbI2.(thf)2 (240 mg, 0.82 mmol) in thf (15 mL) at room temperature. The suspension was left to stir overnight. The thf was removed under reduced pressure and the residue extracted into hexanes (30 mL). The solution was isolated via cannula filtration, concentrated to 5 mL and transferred to a -30 °C freezer to give the compound as pale yellow crystals, which were isolated by filtration. Further concentration of the mother liquor gave a second crop of crystals suitable for an X-ray diffraction study. Yield: 252 mg (41%)

Synthesis of [Yb{Ge(SiMe3)3}2}.(thf)3] (Yb-Ge).

To a Schlenk flask was added Ge(SiMe3)4 (390 mg, 1.07 mmol) and KO t Bu (120 mg, 1.07 mmol), and the contents were dissolved in thf (15 mL). The paleyellow solution was left to stir for 1 h, before it was added dropwise to a suspension of YbI2.(thf)2 (300 mg, 0.53 mmol) in thf (15 mL) at room temperature. The suspension was left to stir overnight. The thf was removed under reduced pressure and the residue was extracted into hexanes (30 mL). The solution was isolated via cannula filtration, concentrated to 5 mL and transferred to a -30 °C freezer to give the compound as slightly dark yellow/orange crystals, which were isolated by filtration and dried under reduced pressure. Further concentration of the mother liquor gave a second crop of crystals suitable for an X-ray diffraction study. Yield: 254 mg (32%) MHz for 29 Si. 1 H and 13 C NMR chemical shifts are reported in ppm using the residual peak of benzene-d6 (7.16 ppm and 128.0 ppm) as internal standard. 29 Si, 119 Sn and 171 Yb NMR spectra were referenced (δ = 0.0 ppm) to calculated 29 Si, 119 Sn and 171 Yb frequencies in Me4Si, Me4Sn and Yb(-C5Me5)2(THF)2 references compounds, respectively. 49

1D 1 H. One-dimensional 1 H experiments were acquired with standard Bruker "zg" program. The spectra were acquired with sweep width of 10000 Hz and 59998 data points, using 256 scans and the relaxation delay (D1) was 30 s. Exponential window function (LB=0.3) was applied before Fourier transformation.

1D 13 C. One-dimensional 13 C NMR spectra were recorded with standard Bruker "zgig" program with broadband proton decoupling in order to remove 13 C -1 H coupling. The spectra were acquired with sweep width of 50505 Hz and 202014 data points, using 1340 scans and the relaxation delay (D1) was 30 s. Exponential window function (LB=1.5) was applied before Fourier Transformation.

1D 29 Si. One-dimensional 29 Si experiment were acquired with standard Bruker "dept45" program with broadband proton decoupling in order to remove 29 Si -1 H coupling. The spectra were acquired width sweep with of 3894 and 36012 data points, using 1024 scans and the recycling delay was 5 s. Exponential window function (LB=0.2) was applied before Fourier transformation.

DFT computational details

Density functional theory (DFT) calculations were performed using the Amsterdam Density Functional ADF2019 code. [50][51] Scalar relativistic effects were addressed through the Zeroth Order Regular Approximation (ZORA). 52 A triple-zeta basis set augmented with a polarisation function (STO-TZP) 53 was used together with the PBE0 functional [54][55][56] and Grimme's empirical DFT-D3 (BJDAMP) corrections. 57 for geometry optimisation and analytical vibrational frequency computations. Natural atomic orbital (NAO) populations and Wiberg bond indices were computed with the NBO6.0 program. 58 Interaction energy between fragments were computed according to the Morokuma-Ziegler energy decomposition analysis (EDA) formalism [37][38][39] implemented in the ADF2019 program. The QTAIM analysis [START_REF] Bader | Atoms in Molecules -A Quantum Theory[END_REF] was performed as implemented in the ADF2019 suite. [59][60]

  Yb 421 ppm and | 1 J 171Yb-29Si | = 829 Hz in toluene-d8), 29 and the three-coordinate [KYb{Si(SiMe3)3}{N(SiMe3)2}2] (δ 171 Yb 1057 ppm, | 1 J 171Yb-29Si | = 716 Hz, | 2 J 171Yb-29Si | = 5.0-8.9 Hz;

Figure 1 .

 1 Figure 1. Ytterbium(II) tetryls (Yb-Si, Yb-Ge and Yb-Sn) investigated here, and their known Yb-C hydrocarbyl and Ca-Sn derivatives.

Scheme 1 .

 1 Scheme 1. One-pot synthesis of [Yb{Ge(SiMe3)3}2.(thf)3] (Yb-Ge) and [Yb{Ge(SiMe3)3}2.(thf)4] (Yb-Sn).

Figure 2 .| 1 | 1 | 2 | 3

 21123 Figure 2. 29 Si{ 1 H} DEPT NMR spectrum (99.36 MHz, benzene-d6, 300 K) of [Yb{Sn(SiMe3)3}2.(thf)4] (Yb-Sn).

  potassium stannylide [KSn(SiMe3)3], either as a 18-c-6 crown ether adduct in benzene-d6 (δ 119Sn -892.2 ppm, δ 29Si -12.7 ppm, | 1 J 29Si-119Sn | = 271 Hz, | 1 J 29Si-117Sn | = 259 Hz) or as a solution in dimethoxyethane (δ 119Sn -896.8 ppm, δ 29Si -12.9 ppm, | 1 J 29Si-119Sn | = 295 Hz, | 1 J 29Si-117Sn | = 282 Hz) display chemical shifts that are very similar to those of Yb-Sn and Ca-Sn, although the tin-to-silicon coupling constants are much larger for the alkali salt.33

Figure 3 .

 3 Figure 3. 119 Sn{ 1 H} NMR spectrum (186.36 MHz, benzene-d6, 300 K) of [Yb{Sn(SiMe3)3}2.(thf)4] (Yb-Sn).

Figure 4 .| 1 | 1

 411 Figure 4. 171 Yb{ 1 H} NMR spectrum (87.57 MHz, benzene-d6, 300 K) of [Yb{Sn(SiMe3)3}2.(thf)4] (Yb-Sn).

Figure 5 .

 5 Figure 5. ORTEP representation of [Yb{Sn(SiMe3)3}2}.(thf)4] (Yb-Sn). Ellipsoids at the 30% probability level. Only the major component of the disordered thf molecules shown. Hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (°): Yb1-Sn1 = 3.3308(4), Yb1-Sn2 = 3.3285(4), Yb1-O1 = 2.440(3), Yb1-O2 = 2.437(3), Yb1-O3 = 2.425(3), Yb1-O4 = 2.420(3), Sn1-Si1 = 2.6009(15), Sn1-Si2 = 2.5923(14), Sn1-Si3 = 2.5968(14), Sn2-Si4 = 2.6048(14), Sn2-Si5 = 2.5929(14), Sn2-Si6 = 2.5885(14); Sn1-Yb1-Sn2 = 178.452(10), O1-Yb1-O2 = 92.23(12), O1-Yb1-O3 = 173.26(11), O1-Yb1-O4 = 88.52(12), O2-Yb1-O3 = 94.27(12), O2-Yb1-

  covalent radius of germanium compared to silicon (1.20 vs 1.11 Ǻ). The Yb-Othf and Ca-Othf bond lengths for the six-coordinate complexes Yb-Sn and Ca-Sn are not longer than those in the fivecoordinate complexes Yb-Si/Yb-Ge and Ca-Si/Ca-Ge. This is probably the outcome of the release of steric pressure within the distannyls due to the greater size of tin. It may also be the expression of a more ACCEPTED MANUSCRIPT -CLEAN COPY 12 ionic Met-E bond, however note that the tabulated electronegativity of tetrel elements do not vary linearly upon ascending group 14 (Pauling electronegativity: Pb, 2.33; Sn, 1.96; Ge, 2.01; Si, 1.9; C, 2.55).

Figure 6 .

 6 Figure 6. ORTEP representation of [Yb{Ge(SiMe3)3}2}.(thf)3] (Yb-Ge). Ellipsoids at the 30% probability level. Only the major component of the disordered thf molecules shown. Hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (°): Yb1-Ge1 = 3.0356(6), Yb1-Ge2 = 3.0738(6), Yb1-O1 = 2.409(4), Yb1-O2 = 2.404(4), Yb1-O3 = 2.440(4), Ge1-Si1 = 2.3863(16), Ge1-Si2 = 2.3801(18), Ge1-Si3 = 2.3874(16), Ge2-Si4 = 2.3905(17), Ge2-Si5 = 2.3862(19), Ge2-Si6 = 2.3885(16); Ge1-Yb1-Ge2 = 125.048(17), O1-Yb1-O2 = 152.75(16), O1-Yb1-O3 = 75.94(16), O2-Yb1-O3 = 77.05(15), O1-Yb1-Ge1 = 100.03(11), O1-Yb1-Ge2 = 94.70(10), O2-Yb1-Ge1 = 93.41(9), O2-Yb1-Ge2 = 96.80(10), O3-Yb1-Ge1 = 112.20(9), O3-Yb1-Ge2 = 122.73(10), Si1-Ge1-Si2 = 104.70(6), Si1-Ge1-Si3 = 100.75(5), Si2-Ge1-Si3 = 99.93(7), Si4-Ge2-Si5 = 100.57(7), Si4-Ge2-Si6 = 101.51(6), Si5-Ge2-Si6 = 100.79(7).

Figure 7 .

 7 Figure 7. The two occupied Kohn-Sham orbitals associated with the Yb-Sn bonds in the ytterbium distannylide [Yb{Sn(SiMe3)2.(thf)4] (Yb-Sn).

  five-coordinate [Yb{Si(SiMe3)3}2.(thf)3] (Yb-Si) and the new [Yb{Ge(SiMe3)3}2.(thf)3] (Yb-Ge), which have also been comprehensively ACCEPTED MANUSCRIPT -CLEAN COPY 17 characterised. Variations in the chemical shifts and coupling constants are detectable across the complexes, and they can be tentatively linked to the variation of coordination numbers and bonding in these complexes.

13 C{ 1 (| 1 J 29 Si{ 1

 1311291 H} NMR (100.65 MHz, benzene-d6, 300 K): δ 68.60 (2,5-CH2-thf), 25.22 (3,4-CH2-thf), 6.75 13C-29Si | = 37.5 Hz, | 2 J 13C-117/119Sn | = 6.7 Hz, SiCH3) ppm. H} NMR (99.36 MHz, benzene-d6, 300 K): δ -10.9 (| 1 J 29Si-119Sn | = 66.2 Hz, | 1 J 29Si-117Sn | = 63.1 Hz, | 1 J 29Si-13C | = 37.5 Hz, | 2 J 29Si-171Yb | = 30.5 Hz, | 3 J 29Si-117/119Sn | = 5.4 Hz) ppm. 119 Sn{ 1 H} NMR (186.36 MHz, benzene-d6, 300 K): δ -757 (| 1 J 119Sn-171Yb | = 5570 Hz, | 2 J 119Sn-117Sn | = 932 Hz) ppm. 171 Yb NMR (87.57 MHz, benzene-d6, 300 K): δ 535 (| 1 J 171Yb-119Sn | = 5570 Hz) ppm.

1 H 13 C{ 1 32 (| 1 J 29 Si{ 1

 1131321291 NMR (500.13 MHz, benzene-d6, 300 K): δ 3.69 (m, 12H, 2,5-CH2-thf), 1.40 (m, 12H, 3,4-CH2-thf), 0.53 (br s, 54H, SiCH3) ppm. H} NMR (125.77 MHz, benzene-d6, 300 K): δ 68.73 (2,5-CH2-thf), 25.19 (3,4-CH2-thf), 7.13C-29Si | = 37.5 Hz, | 3 J 13C-171Yb | = 6.8 Hz, SiCH3) ppm. H} NMR (99.36 MHz, benzene-d6, 300 K): δ -2.4 (| 1 J 29Si-13C | = 37.5 Hz, | 2 J 29Si-171Yb | = 20.2 Hz) ppm. 171 Yb NMR (87.57 MHz, benzene-d6, 300 K): δ 816 ppm.High-resolution 1D NMR for compounds Yb-Sn, Yb-Ge and Yb-Si. NMR experiments were carriedout by using Bruker AVIII 500 spectrometers (Bruker, Wissembourg, France) equipped with a 10 A gradient amplifier giving a maximum gradient of 50 G.cm -1 and a 5 mm BBFO probe including shielded z-gradients. Measuring frequencies were 500 MHz for 1 H, 125 MHz for13 C, 186 MHz for 119 Sn and 99

1D 119

 119 Sn. One-dimensional119 Sn experiments were acquired with standard Bruker "zgig" program. The spectra were acquired with sweep width of 64935 Hz and 194798 data points, using 3072 scans and the recycling delay was 18 s. Exponential window function (LB=10) was applied before Fourier transformation.1D 171 Yb. One-dimensional171 Yb experiments were acquired with standard Bruker "zgig" program. The spectra were acquired with sweep width of 19840 Hz and 59520 data points, using 16384 scans and the relaxation delay (D1) was 2 s. Exponential window function (LB=100) was applied before Fourier transformation.

Table 1 .

 1 Summary of NMR spectroscopic data for complexes [Met{E(SiMe3)3}2.

	a
	171 Yb NMR	119 Sn{ 1 H} NMR	29 Si{ 1 H} NMR	13 C{ 1 H} NMR
	δ 171 Yb
	/

(thf)n] (Met = Yb: E = Sn, n = 4, Yb-Sn; Ge, n = 3, Yb-Ge; Si, n = 3, Yb-Si; Met = Ca, E = Sn, n = 4, Ca-Sn).

Table 2 .

 2 Summary of crystallographic data for complexes [Met{E(SiMe3)3}2}.(thf)n] with Met = Yb/Ca, E = Si/Ge/Sn, and n = 3 or 4.

	Compound		Met-E [Ǻ]	Met-Othf [Ǻ]	E-Si [Ǻ] (av.)	E-Met-E [°]	Si-E-Si [°] (av.)	Reference
	[Yb{Si(SiMe3)3}2}.(thf)3]	Yb-Si	3.0644(6)	2.4210(13)-2.435(2)	2.348	124.51(2)	102.20	28
	[Yb{Ge(SiMe3)3}2}.(thf)3]	Yb-Ge	3.0356(6)-3.0738(6)	2.404(4)-2.440(4)	2.3865	125.048(17)	101.37	this work
	[Yb{Sn(SiMe3)3}2}.(thf)4]	Yb-Sn	3.3285(4)-3.3308(4)	2.420(3)-2.440(3)	2.5960	178.452(10)	98.81	this work
	[Ca{Si(SiMe3)3}2}.(thf)3]	Ca-Si	3.0421(19)-3.0861(19)	2.348(2)-2.389(2)	2.344	125.53(3)	101.75	36
	[Ca{Ge(SiMe3)3}2}.(thf)3]	Ca-Ge	3.0223(2)-3.0670(2)	2.364(7)-2.385(7)	2.388	125.43(6)	101.16	35
	[Ca{Sn(SiMe3)3}2}.(thf)4]	Ca-Sn	3.3649(7)-3.3653(7)	2.353(2)-2.392(3)	2.6037	178.29(2)	98.47	18

Table 3 .

 3 Relevant DFT-computed data for the six-coordinate complexes [Met{E(SiMe3)3}2.(thf)4] (Met = Yb, E = Si, Ge, Sn; Met = Ca, E = Sn).

			[Yb{Si(SiMe3)3}2.(thf)4]	[Yb{Ge(SiMe3)3}2.(thf)4]	[Yb{Sn(SiMe3)3}2.(thf)4]	[Ca{Sn(SiMe3)3}2.(thf)4]
			(putative)	(putative)	(Yb-Sn)	(Ca-Sn)
	HOMO-LUMO gap / eV		4.20	4.17	4.00	4.60
	Met-E /Å a [WBI] a,b		3.224 [0.207]	3.160 [0.209]	3.250 [0.270]	3.251 [0.159]
	Met-Othf./Å a [WBI] a,b		2.439 [0.037]	2.436 [0.039]	2.434 [0.045]	2.386 [0.032]
	 E-Met-E'/ °		174	176	175	176
	NAO charges and	Met 1.38 (4f 13.98 5d 0.41 6s 0.17 6p 0.01 )	1.38 (4f 13.98 5d 0.42 6s 0.17 6p 0.01 )	1.33 (4f 13.98 5d 0.43 6s 0.22 6p 0.01 )	1.71 (4s 0.21 4p 0.01 3d 0.08 )
	configurations	E a	-0.86 (3s 1.43 3p 3.40 )	-0.90 (4s 1.50 4p 3.38 )	-0.70 (5s 1.51 5p 3.16 )	-0.75 (5s 1.56 5p 3.18 )
		O a	-0.62	-0.60	-0.61	-0.61

a Averaged values. b WBI = Wiberg bond index given in brackets.

Table 4 .

 4 Morokuma-Ziegler energy decomposition analysis in the six-coordinate complexes [Met{E(SiMe3)3}2.(thf)4] (Met = Yb, E = Si, Ge, Sn; Met = Ca, E = Sn). a

	Fragmentation

a All values in eV. b Total bonding energy (TBE) = EPauli + Eelstat + Eorb + Edisp.

Table 5 .

 5 QTAIM descriptors of the Met-E bonds in the six-coordinate complexes [Met{E(SiMe3)3}2.(thf)4] (Met = Yb, E = Si, Ge, Sn; Met = Ca, E = Sn). a,b  ,  2 , H, V and G are the electron density, Laplacian of  density, energy density, potential energy density and kinetic energy density values at the Met-E bond critical point (bcp), respectively. All values in a.u.

			[Yb{Si(SiMe3)3}2.(thf)4]	[Yb{Ge(SiMe3)3}2.(thf)4]	[Yb{Sn(SiMe3)3}2.(thf)4]	[Ca{Sn(SiMe3)3}2.(thf)4]
			(putative)	(putative)	(Yb-Sn)	(Ca-Sn)
	Atom charges	Met	1.54	1.53	1.51	1.57
		E a	-0.21	-0.62	-0.30	-0.32
		O a	-1.06	-1.06	-1.05	-1.06
	Delocalisation index	 a	0.245	0.255	0.275	0.197
	bcp indicators a		0.023	0.024	0.025	0.021
		 2 	+0.031	+0.035	+0.032	+0.030
		H	-0.003	-0.003	-0.004	-0.002
		V	-0.013	-0.014	-0.015	-0.012
		V/G	1.26	1.25	1.31	1.21

a Averaged values. b

Table 6 .

 6 Relevant DFT-computed data for the five-coordinate complexes [Yb{E(SiMe3)3}2.(thf)3] (E = Si, Ge, Sn).

	[Yb{Si(SiMe3)3}2.(thf)3]	[Yb{Ge(SiMe3)3}2.(thf)3]	[Yb{Sn(SiMe3)3}2.(thf)3]
	(Yb-Si)	(Yb-Ge)	(putative)

a Averaged values. b WBI = Wiberg bond index.

Table 7 .

 7 Morokuma-Ziegler energy decomposition analysis in the five-coordinate complexes All values in eV. b Total bonding energy (TBE) = EPauli + Eelstat + Eorb + Edisp.

	[Yb{E(SiMe3)3}2.(thf)3] (E = Si, Ge, Sn). a
	Fragmentation		[Yb(thf)3] 2+ + [{E(SiMe3)3}2] 2-
		[Yb{Si(SiMe3)3}2.(thf)3]	[Yb{Ge(SiMe3)3}2.(thf)3]	[Yb{Sn(SiMe3)3}2.(thf)3]
		(Yb-Si)	(Yb-Ge)	(putative)
	EPauli	4.53	4.40	4.65
	Eelstat	-14.97	-14.60	-14.94
	Eorb	-4.35	-4.33	-4.49
	Edisp	-0.96	-0.87	-0.98
	TBE b	-15.76	-15.40	-15.76

a

Table 8 .

 8 QTAIM descriptors of the Yb-E bonds in the five-coordinate complexes [Yb{E(SiMe3)3}2.(thf)3] (E = Averaged values. b  ,  2 , H, V and G are the electron density, Laplacian of  density, energy density, potential energy density and kinetic energy density values at the Met-E bond critical point (bcp), respectively.

	Si, Ge, Sn). a,b
			[Yb{Si(SiMe3)3}2.(thf)3]	[Yb{Ge(SiMe3)3}2.(thf)3]	[Yb{Sn(SiMe3)3}2.(thf)3]
			(Yb-Si)	(Yb-Ge)	(putative)
	Atom charges	Met	1.46	1.45	1.42
		E a	-0.29	-0.70	-0.36
		O a	-1.06	-1.04	-1.06
	Delocalisation index	 a	0.369	0.366	0.372
	bcp indicators a 	0.033	0.033	0.030
		 2 	+0.041	+0.047	+0.036
		H	-0.006	-0.006	-0.005
		V	-0.023	-0.023	-0.020
		V/G 1.374	1.332	1.364

a

[Yb{Sn(SiMe3)3}2.(thf)4] 535| 1 J 171Yb-119Sn | = 5570 -757 | 1 J 119Sn-171Yb | = 5570 SiMe3 = -10.9 | 1 J 29Si-119Sn | = 66.2 6.75 | 1 J 13C-29Si | = 37.5 (Yb-Sn) | 2 J 119Sn-117Sn | = 932 | 1 J 29Si-117Sn | = 63.1 | 1 J 29Si-13C | = 37.5 | 2 J 29Si-171Yb | = 30.5 | 2 J 13C-117/119Sn | = 6.7 | 3 J 29Si-117/119Sn | = 5.4 [Yb{Ge(SiMe3)3}2.(thf)3] 816 not detected n/a n/a SiMe3 = -2.4 | 1 J 29Si-13C | = 37.5 7.32 | 1 J 13C-29Si | = 37.5 (Yb-Ge) | 2 J 29Si-171Yb | = 20.2 | 3 J 13C-171Yb | = 6.8 [Yb{Si(SiMe3)3}2.(thf)3] 948 not detected n/a n/a SiMe3 = -5.2 | 1 J 29Si-13C | = 38.8 ±2.1 c 7.89 | 1 J 13C-29Si | = 38.8 ±2.1 b (Yb-Si) 28 | 1 J 29Si-29Si | = 28.2 | 2 J 29Si-171Yb | = 8.6 | 2 J 13C-29Si | = 4.8 | 3 J 13C-171Yb | = 20.8 SiSi = -145.9 | 1 J 29Si-171Yb | = 728 | 1 J 29Si-29Si | = 28.2 [Ca{Sn(SiMe3)3}2.(thf)4] n/a n/a -857 | 1 J 119Sn-29Si | = 82.1 SiMe3 = -12.9 | 1 J 29Si-119Sn | = 82.9 7.76 | 1 J 13C-29Si | = 36.7 (Ca-Sn) 18 | 2 J 119Sn-117Sn | = 455 | 1 J 29Si-117Sn | = 79.0 | 2 J 13C-117/119Sn | = 19.3 | 1 J 29Si-13C | = 36.7 | 3 J 29Si-117/119Sn | = 2.7a NMR data recorded in benzene-d6 from crystalline samples. Chemical shifts given in ppm and coupling constants in Hz.b The exact values of | 1 J 13C-29Si | determined by29 Si{ 1 H} and 13 C{ 1 H} were, respectively, 40.9 and 36.7 Hz.

The decomposition of the total bonding energy (TBE) between the "solvated" cation [Met(thf)4] 2+ and the [(SiMe3)E...E(SiMe3)] 2-dimeric fragment in the complexes is provided in Table4.

J. I. Rodriguez, J. Comput. Chem., 2013, 34, 681.

Acknowledgements P.M.C thank the Agence Nationale de la Recherche for the provision of a research grant (ANR-17-CE07-0017-01). The GENCI (Grand Equipment National de Calcul Intensif) is acknowledged for HPC resources (Project A0050807367). H.O. and G.H. would like to thank the Region Normandie, Rouen University, CNRS, INSA of Rouen, ERDF and the Labex SynOrg (ANR-11-LABX-0029).

Conflicts of interest

There are no conflicts to declare.