A [CoSiH2] Silylene Synthon Provides Modular Access to Homo- and Heterobimetallic [Co=Si=M] (M = Co, Fe) Silicide Complexes

Base-stabilized [BP3iPr](H)2CoSiH2(DMAP) (1, [BP3iPr] = PhB(CH2PiPr2)3–; DMAP = 4-dimethylaminopyridine) is a rare instance of a synthon for the simplest “parent” silylene complex (LM=SiH2). Complex 1 was accessed in high yields via double Si–H bond activation in SiH4 by [BP3iPr]Co(DMAP), and in solution, it undergoes rapid exchange between bound and free DMAP by an associative mechanism (as determined by variable-temperature 1H NMR dynamic studies). The DMAP ligand of 1 is readily displaced by metal-based fragments that bind silicon and cleave the Si–H bonds of the SiH2 moiety to produce bimetallic [Co=Si=M] (M = Co, Fe) molecular silicides. Thus, treatment of 1 with 0.5 equiv of (LCoI)2(μ-N2) (L = a tripodal ligand) resulted in the spontaneous formation of [BP3iPr](H)2Co=Si=Co(H)2L (L = [BP2tBuPz], PhB(CH2PtBu2)2(pyrazolyl)− (3); Tp″, HB(3,5-diisopropylpyrazolyl)3– (4)) with the concomitant release of DMAP. The symmetrical silicide [BP3iPr](H)2Co=Si=Co(H)2[BP3iPr] (5) was prepared by treatment of a mixture of 1 and [BP3iPr]Co(DMAP) with 2 equiv of Ph3B, which in this case is required to sequester DMAP as the elimination product Ph3B-DMAP. A heterobimetallic silicide, [BP3iPr](H)2Co=Si=Fe(H)2[SiP3iPr] (7; [SiP3iPr] = PhSi(CH2PiPr2)3), was obtained via in situ KC8 reduction of [SiP3iPr]FeCl and subsequent addition of 1 and Ph3B. These transformations involving a metal–SiH2 derivative demonstrate a fundamentally new type of reactivity for silylene complexes and provide a unique synthetic method for construction of molecular silicide complexes.


INTRODUCTION
−6 This is especially apparent in the direct process, which is responsible for the global production of chlorosilanes that serve as precursors to the silicone industry. 1,2−11 Silicide phases find additional utility in the production of silicon nanomaterials (e.g., nanowires) using silanes as a source of silicon atoms. 4These applications of solid-state silicides highlight the utility of both silicide (M x Si y ) and silylene (LM�SiRR') reaction centers in promoting useful transformations, and a better understanding of both types of intermediates is expected to enable new silicon-based transformations.−15 Molecular silicide complexes are uncommon since there are very few general, controlled synthetic routes to molecules possessing M x Si y cores.Thus, an understanding of structure− function relationships for silicon atoms ligated to transition metals is severely lacking.Bimetallic silicides, LM�Si�ML, represent prototypical molecules of this class due to their structural simplicity and the high reactivity expected for the two-coordinate μ-silicon center.Access to such complexes was demonstrated through two primary strategies.In one case, metathetical exchange between (SIPr)SiBr 2 (SIPr = 1,3-bis-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene) and 2 equiv of [Tp*Mo(CO) 2 (PMe 3 )] − (Tp* = HB(3,5-dimethylpyrazolyl) 3 − ) produced the unsymmetrical silicide Tp*-(OC) 2 Mo�Si−Mo(CO) 2 (PMe 3 )Tp*. 16In addition, our laboratories reported the formation of [M�Si�M]  tBu Pz] = PhB(CH 2 P t Bu 2 ) 2 (pyrazolyl) − ). 17The [MSiM] cores of these silicides react in unprecedented ways; for example, the μsilicon center of Tp*(OC) 2 Mo�Si−Mo(CO) 2 (PMe 3 )Tp* binds alkynes to generate adducts incorporating planar, tetracoordinate silicon in [(RCCR')SiMo 2 ] units. 18In addition, reaction of [BP 2 tBu Pz] with MeCl produced small quantities of functionalized silane products Me x SiH 4−x (x = 1−3). 17espite these recent advances, molecular silicide chemistry remains largely unexplored, mainly due to the absence of synthetic methods that provide control over stoichiometry and structural features (e.g., symmetry, coordination numbers, metal identities, etc.).The results reported here demonstrate the versatile, modular assembly of complex silicide structures by way of a synthon for the simplest terminal silylene complex, the adduct [BP − ; DMAP = 4-dimethylaminopyridine).n a benzene-d 6 solution, 1 exhibits dynamic behavior as evidenced by its 1 H and 31 P{ 1 H} NMR spectra.At 292 K, the Co−H and Si−H hydrogen nuclei resonate at δ −15.44 ppm (br s, 2H) and 6.36 ppm (pseudo-q, J = 7.5 Hz, 1 J SiH = 159 Hz, 2H), respectively.The 31 P{ 1 H} NMR spectrum displays a single broad resonance at δ 62.6 ppm, indicating interconversion of phosphorus environments in the [BP 3 iPr ] ligand.The silicon nucleus of 1 resonates at δ 33 ppm according to a 29 Si− 1 H HMBC NMR spectrum.Variable-temperature (VT) NMR spectroscopic studies of 1 in a toluene-d 8 solution show that upon cooling, the 31 P{ 1 H} NMR resonance decoalesces, and at 237 K, it is fully resolved into two resonances (1:2 ratio) at δ 61.1 and 62.9 ppm.The solution 29 Si chemical shift of complex 1, coupled with the tetrahedral coordination geometry of silicon in the solid-state structure, indicates that there is minimal double-bond character within the Co−Si linkage, a feature that is consistent with other base-stabilized silylene species. 13he lability of the coordinated DMAP donor in 1 was of interest as an indication of possible reactivity modes for the SiH 2 (DMAP) ligand.For context, note that previous VT-NMR studies of the base-stabilized silylene complex [Cp*-(Me 3 P) 2 RuSiPh 2 (NCMe)] + (Cp* = η 5 -C 5 Me 5 ) demonstrated that exchange of bound and free NCMe occurs by a dissociative mechanism involving a base-free [Ru�SiPh 2 ] silylene intermediate. 21Indeed, the 1 H NMR spectrum of a mixture containing 1 and 1 equiv of added DMAP (292 K, toluene-d 8 ) contains broadened resonances for the uncoordinated DMAP ligand, indicating a facile exchange process.Upon cooling, the resonances corresponding to coordinated and "free" DMAP sharpen.Line-shape analyses of spectra from 215 to 256 K provided an Eyring plot, from which ΔH ‡ (1.8 ± 0.2 kcal −1 mol −1 ) and ΔS ‡ (−40.0 ± 0.7 J mol −1 K −1 ) parameters were extracted (ΔG (298 K) ‡ = 13.7 ± 0.3 kcal −1 mol −1 ).The large negative entropy of activation points to an associative mechanism for DMAP exchange in 1, in contrast to the dissociative mechanism identified for [Cp*-(Me 3 P) 2 RuSiPh 2 (NCMe)] + .

RESULTS AND DISCUSSION
A reasonable pathway for DMAP exchange in 1 involves the hypercoordinate intermediate [BP 3  iPr ](H) 2 CoSiH 2 (DMAP) 2 (Scheme 2a).This exchange pathway was investigated by density functional theory (DFT) studies (ωB97X-D3/def2-TZVP(Co,Si,P),def2-SVP(C,H,B)// CPCM(toluene)) for the model complex The geometry-optimized structure of 1* was obtained following the adaptation of the crystallographic atomic coordinates of 1.A potential energy scan of rotation about the Co−Si bond identified a rotamer, 1*' (Scheme 2b), which is further stabilized by −1.1 kcal −1 mol −1 compared to 1*; the rotamers 1* and 1*' are separated by a small rotational barrier of 5.0 kcal −1 mol −1 (Table S4).Exchange of DMAP with either 1* or 1*' is expected to involve the bis-DMAP adduct [BP 3  Me ](H) 2 CoSiH 2 (DMAP) 2 (Int).A To examine whether DMAP could be removed from 1 by chemical abstraction, the complex was treated with 1 equiv of Ph 3 B in a toluene solution.A darkening of the solution's color was immediately apparent, and 1 H and 31     ), was generated in an analogous fashion by treatment of 1 with 0.5 equiv of (Tp″Co) 2 (μ-N 2 ). 17A color change of the reaction mixture from dark brown to blue was observed upon addition of (Tp″Co) 2 (μ-N 2 ), and multinuclear NMR spectroscopy ( 1 H, 31 P{ 1 H}) indicated formation of a single diamagnetic species (in addition to DMAP) possessing features consistent with [BP 3  iPr ](H) 2 Co�Si�Co(H) 2 Tp″ (4; Scheme 4).Complex 4 was isolated as an analytically pure dark-blue powder, following precipitation from a concentrated diethyl ether solution.However, 4 has yet to be isolated in a crystalline form suitable for X-ray diffraction studies.
The isolation of 4 is surprising as direct treatment of (Tp″Co) 2 (μ-N 2 ) with SiH 4 or PhSiH 3 has been shown to result in mixtures of the μ-hydrides (Tp″Co) 2 (μ-H) x (x = 1, 2). 17  iPr ]Co(DMAP) (eq 1).Monitoring the reaction by 1 H NMR spectroscopy showed that prior to the addition of Ph 3 B, no reaction of the starting complexes was evident.Addition of Ph 3 B was accompanied by a rapid color change from dark orange-brown to red, and 1 H and 31 P{ 1 H} NMR spectroscopic analyses of the crude reaction mixture indicated that 5 and Ph 3 B�DMAP were generated as the exclusive products in quantitative yields.Complex 5 was isolated as an analytically pure crystalline solid by crystallization from a concentrated neat THF solution.
In contrast to the reactions that produce 3 and 4, the reaction of 1 and [BP 3 iPr ]Co(DMAP) to generate 5 did not spontaneously liberate DMAP.Thus, coordinatively labile starting materials are not strictly necessary for silicide formation, and Ph 3 B is a potential trigger for such coupling processes.By analogy with the reactions depicted in Scheme 4, another potentially suitable starting material for the preparation of 5 seemed to be the μ-dinitrogen complex ([BP 3 iPr ] - Co) 2 (μ-N 2 ) (6).Complex 6 was independently generated by the addition of Ph 3 B to [BP 3 iPr ]Co(DMAP) in a benzene-d 6 solution in high yield (Scheme 5a).Complex 6 is thermally sensitive and decomposes to an intractable brown-colored material upon exposure to vacuum although small quantities of crystalline 6 were obtained following storage of a (Me 3 Si) 2 O/ benzene-d 6 mixture at −35 °C, allowing for structural determination by single-crystal X-ray diffraction analysis (see the Supporting Information page S16).
Surprisingly, 1 H and 31 P{ 1 H} NMR spectroscopic monitoring of the reaction of in situ generated 6 (0.5 equiv) and 1 (1 equiv) revealed the formation of a mixture containing 0.5 equiv of [BP 3 iPr ]Co(DMAP), 0.5 equiv of 1, and 0.5 equiv of 5 rather than full conversion to 5 (Scheme 5b).Further addition of Ph 3 B (1 equiv) resulted in the near-quantitative formation of 5 and Ph 3 B�DMAP (by integration of the 1 H NMR spectrum against an internal standard).These observations are consistent with a dynamic system in which 6 serves both to activate Si−H bonds in 1 and to abstract DMAP.The latter event results in sequestration of 0. ]FeCl with 1 equiv of KC 8 in a THF solution was accompanied by a color change from clear green to dark red.Although the darkred intermediate has yet to be characterized, closely related tridentate tris-phosphine iron(0) systems have been described and isolated as LFe(N 2 ) 2 adducts. 24,25No conversion to a silicide was apparent upon addition of 1 to the dark-red solution, according to 1 H and 31 P{ 1 H} NMR spectroscopies.However, the addition of Ph 3 B to the reaction mixture resulted in rapid conversion to a new diamagnetic species, formulated iPr ](H) 2 Co�Si�Fe(H) 2 [SiP 3 iPr ] (7; eq 2).Complex 7 was authenticated by single-crystal X-ray diffraction analysis (Figure 2a, right), elemental analysis, and multinuclear NMR spectroscopy (vide infra).

Spectroscopic and Structural Properties of
Silicides.The 1 H, 31 P{ 1 H}, and 29 Si{ 1 H} DEPT NMR spectra of 3, 4, 5, and 7 display features consistent with silicide structures of this class. 17,19Owing to its high molecular symmetry, complex 5 displays the most straightforward NMR spectroscopic features.In benzene-d 6 solution, the hydride ligands of 5 appear as a broad singlet (δ −13.16 ppm, 4H) in the 1 H NMR spectrum, and only one line is displayed in the 31  Pz]) in a ca.3:2 ratio, respectively.For 4 and 7, two distinct 2:2 resonances are apparent in the high-field ("hydride") regions of their 1 H NMR spectra.For 3, 4, and 5, H−Si and H−P coupling were not observable for the hydride resonances, presumably due to the presence of quadrupolar 59 Co (I = 7/2) nuclei. 26The heterobimetallic silicide 7 is unique in this regard as fine structure of the Fe−H resonance is present (δ −14.49ppm, 2 J HP = 16.9Hz); notably, 29 Si satellites for this resonance were not detectable in either the 1 H or 1 H{ 31 P} NMR spectra of 7, implying minimal coupling to the μ-silicon nucleus.
A notable feature of these silicides is the low-field 29 Si resonances in their 29 Si{ 1 H} DEPT NMR spectra (δ 264−358 ppm; Table 1).These 29 Si chemical shifts span a well-defined range that is diagnostic for this family of 3d-metal silicides and is consistent with structurally related, previously reported silicides. 17,19Notably, the 4d-and 5d-metal silicides in the Tp*(OC) 2 M�Si−M(CO) 2 (L)Tp* class (M = Mo, W) generally possess lower-field 29 Si resonances (δ 396− 439). 16,18,27omparison of the solid-state molecular structures of 3, 5, and 7 reveal similar geometries for the [(H) 2 M�Si�M′(H) 2 ] cores (M = Co, M′ = Co, Fe).The metric parameters for the    Journal of the American Chemical Society solid-state structures are summarized in Table 1.In each case, the single-crystal X-ray diffraction data are of sufficient quality for location of the hydride ligands in the difference maps; accordingly, these hydrogen atoms were refined isotropically.For 3, 5, and 7, the hydride ligands do not form a tetrahedral array about the μ-silicon atom, which would be expected for [M(μ-SiH 4 )M′] systems in which Si−H activation is minimal. 28,29Instead, the hydride ligands are oriented to accommodate a pseudo-octahedral coordination geometry for the associated metal centers (Figure 2b).Additionally, in 3, 5, and 7, the hydride ligands of the neighboring [L(H) 2 M] units are canted toward one hemisphere of the molecule, leaving a face of the μ-silicon atom exposed.These features are consistent across 3, 5, and 7, as shown in Figure 2b, which presents perspective views of the silicide solid-state structures along the Co•••M′ axes.Coupled with the solution-state multinuclear NMR data for these species, the data strongly point toward assignment of these silicides as [(H) 2 M�Si� M′(H) 2 ] rather than [M(μ-SiH 4 )M′] structures, in which significant residual Si•••H interactions are present. 17,19,28,29he asymmetric unit in the crystal structure of iPr ] ligands is apparent from the electron density map, and the site is wellmodeled as being occupied by 0.5 B and 0.5 Si atoms.Significant distortions of the atomic thermal parameters occurred when the site was modeled as either fully occupied boron or silicon.
Geometry optimizations (DFT) of 3, 5, and 7 were initiated from the crystallographic atomic coordinates at the ωB97X-D3/def2-TZVP(Co,Fe,Si,P),def2-SVP(C,H,B,N) level of theory.Across the series, the natural charge 30 (q) distribution in the [CoSiM′] cores of (nontruncated) 3*, 5*, and 7* remains broadly consistent, with modest positive charge accumulation at the μ-silicon atom (Table 1).The small but generally slightly negative charges of the metal centers in 3*, 5*, and 7* indicate that the metal centers are not welldescribed as Co V and Fe IV in the oxidation state formalism, which is often ambiguous in cases of atoms engaged in highly covalent bonding. 31,32nalysis of the Wiberg bond indices 33 (WBIs) of 3*, 5*, and 7* provided further insight into the bonding situation within these molecules (Table 1 19 the value of 1 J Si−H (8 Hz) was found to be consistent with minimal Si−H bonding.

CONCLUSIONS
Base-stabilized silylene (SiH 2 ) complex 1 has enabled facile, modular access to a series of symmetrical, unsymmetrical, and heterobimetallic silicides via Si•••H bond activations.In certain cases, 1 allowed access to silicides that are not obtainable by direct L(H) 2 M�Si�M(H) 2 L formation from SiH 4 and a (LM) 2 (μ-N 2 ) precursor.For example, reaction of (Tp″Co) 2 (μ-N 2 ) with SiH 4 was found to generate mixtures containing (Tp″Co) 2 (μ-H) 1,2 as opposed to a silicide product. 17This result implies that the presence of a bulky [BP  19 or a mixture of 2 and 5 (both in ∼20% yield) 39 is produced as the major siliconcontaining product, respectively.Thus, 1 avoids degradation pathways involving the [BP 3 iPr ] ligand, such as −CH 2 P i Pr 2 side-arm migration and coupling to generate [H 2 SiCH 2 P i Pr 2 ] fragments, as seen in 2. This feature is likely derived from the base stabilization in 1, which tempers the high reactivity expected for a base-free terminal SiH 2 complex.
The extension of silicide chemistry to heterobimetallic metal combinations, as in complex 7, presents a new metal−silicon structural type and potentially new avenues for cooperative substrate activation and silicon atom functionalization that exploit diverse electronic properties of the metal centers.Significantly, precursor silylene complex 1 demonstrates a novel type of reactivity for transition-metal silylene complexes (i.e., metal-mediated Si−H bond activation of a coordinated silylene) and highlights the utility of [SiH 2 ] synthons for accessing unusual metal−silicon bonding arrangements.
Experimental methods, detailed syntheses, details of VT-NMR spectroscopy, details of crystallography, details of calculations, NMR spectra, computed atomic coordinates (PDF) Journal of the American Chemical Society

Figure 1 .
Figure 1.(a) Direct process reaction of copper silicide with MeCl to produce Me 2 SiCl 2 via a silylene intermediate.(b) Conversion of a molecular SiH 2 complex (or synthon) to a molecular silicide.

2. 1 .
Access to a Base-Stabilized SiH 2 Complex.The base-stabilized silylene complex [BP 3 iPr ](H) 2 CoSiH 2 (DMAP) (1) was obtained by reaction of [BP 3 iPr ]Co(DMAP) 19 with SiH 4 (1 equiv, 15% in nitrogen) in toluene (Scheme 1a), which resulted in a color change from dark brown to bright orange.Complex 1 was isolated in high yield as an analytically pure solid following crystallization by diffusion of pentane into a saturated 1,2-difluorobenzene solution at −35 °C.The solidstate molecular structure of 1 consists of two unique molecules in the asymmetric unit, one of which exhibits considerable disorder of the [BP 3 iPr ] ligand; the structure of the nondisordered complex is shown in Scheme 1b.The Co−Si bond length in this molecule (2.135(2) Å) is modestly contracted in

Scheme 1 .
Scheme 1.(a) Synthesis of 1.(b) Solid-State Molecular Structure of 1 with 50% Probability Thermal Ellipsoids Drawn.Most Hydrogen Atoms Are Omitted for Clarity P{ 1 H} NMR spectra of the reaction mixture indicated clean conversion to Ph 3 B− DMAP and a new diamagnetic species, which was identified by X-ray crystallography and solution-state multinuclear NMR spectroscopy as [PhB(CH 2 P i Pr 2 ) 2 ](H) 2 Co[κ 2 -Si,P-H 2 SiCH 2 P i Pr 2 ] (2; Scheme 3).Complex 2 results from coupling of a [BP 3 iPr ] ligand −CH 2 P i Pr 2 side arm with the SiH 2 moiety, suggesting the possible intermediacy of the basefree species [BP 3 iPr ](H) 2 CoSiH 2 .However, the rapid course of the reaction (complete in <5 min) indicates that any such base-free silylene cannot be readily isolated under these conditions.Nevertheless, addition of Ph It therefore seemed that ([BP 2 tBu Pz]Co) 2 (μ-N 2 ), serving as a source of [BP 2 tBu Pz]Co I , might react with 1 to generate the unsymmetrical silicide [BP 3 iPr ](H) 2 Co�Si�Co(H) 2 [BP 2 tBu Pz] (3).Indeed, the addition of a clear red-brown toluene solution of ([BP 2 tBu Pz]Co) 2 (μ-N 2 ) to a toluene solution of 1 resulted in a rapid color change to deep blue.Multinuclear 1 H and 31 P{ 1 H} NMR spectroscopic analyses of the crude reaction mixture indicated full consumption of the starting materials to form a new diamagnetic species whose features are consistent with 3, in high (93%) yield, according to integration against an internal standard of (Me 3 Si) 2 O (Scheme 4).Introduction of Ph 3 B to the reaction mixture resulted in near-quantitative capture of the released DMAP to form Ph 3 B−DMAP (97%, via 1 H NMR spectroscopy).The adduct Ph 3 B−DMAP was conveniently removed from the reaction mixture via crystallization by diffusion of pentane into a dilute tetrahydrofuran (THF) solution at −35 °C.Complex 3, enriched in the supernatant solution, was crystallized by diffusion of (Me 3 Si) 2 O into a saturated THF solution cooled to −35 °C and was isolated in an analytically pure form in good yield (74%).

Scheme 2 .
Scheme 2. (a) Proposed Exchange Pathway for the DMAP Ligand of 1.(b) Reaction Coordinate Diagram for the DFT-Computed DMAP Exchange Mechanism of 1*'.The Stereochemistry of 1*' Is as Presented in the Scheme This hydrogen-atom abstraction was favored over silicide formation, possibly due to the inability of the Tp″ ligands to enforce sufficiently long Co•••Co separations to stabilize a linear [CoSiCo] core. 17Thus, the accessibility of 4 is likely related to beneficial steric properties for the [BP 3 iPr ] ligand.Using this synthetic approach, complex 1 was converted to the symmetrical homobimetallic silicide [BP 3 iPr ](H) 2 Co� Si�Co(H) 2 [BP 3 iPr ] (5; eq 1).Complex 5 has remained elusive despite previous attempts to access this structure by treatment of Na(THF) 6 ([BP 3 iPr ]CoI) with 0.5 equiv of SiH 4 . 19In that case, the silicide [BP 3 iPr ](H) 2 Co�Si�Co(H) 2 (SiH 3 )-[( i Pr 2 PCH 2 ) 2 BPh] was generated via a process involving the degradation of a [BP 3 iPr ] ligand.With complex 1 in hand, 5 was synthesized by addition of 2 equiv of Ph 3 B to a solution containing 1 and [BP 3 5 equiv of [BP 3 iPr ]Co I units supplied by 6 as [BP 3 iPr ]Co(DMAP), necessitating an additional equiv of Ph 3 B for complete conversion to 5. The differing behavior of 6 toward 1 in comparison to the congeneric [BP 2 tBu Pz]-and Tp″-complexes indicates that DMAP is the most strongly bound to the [BP 3 iPr ]Co I fragment.The utility of 1 for preparation of heterobimetallic silicides (i.e., [MSiM′]) was also evaluated.For this purpose, the Fe I starting material [SiP 3 iPr ]FeCl ([SiP 3 iPr ] = PhSi(CH 2 P i Pr 2 ) 3 ), previously reported by this laboratory, 23 was employed.It seemed that the 14-electron [SiP 3 iPr ]Fe 0 fragment, which is electronically similar to [BP 3 iPr ]Co I , would be well suited for activation of the Si−H bonds of 1. Treatment of [SiP 3 iPr

a
Recorded in benzene-d 6 solution (119 MHz).b Co A is the [BP 3 iPr ]-ligated cobalt atom; M′ refers to the second metal atom in each complex (Co or Fe).c Generated from symmetry equivalent fragments.d Recorded in THF-d 8 solution (119 MHz).e Asterisks denote DFT-computed molecules.

3
contains one nondisordered molecule of [BP 3 iPr ](H) 2 Co�Si�Co-(H) 2 [BP 2 tBu Pz], displaying close Co−Si contacts (d([BP 3 iPr ] - Co−Si) = 2.086(2) Å, d([BP 2 tBu Pz]Co−Si) = 2.084(2) Å), and a nearly linear Co−Si−Co linkage (∠= 169.00(5)°).For crystals of 5 and 7 grown from neat THF solutions at −35 °C, the asymmetric units of their crystal structures (in C2/c) contain half of a molecule; the full molecules are generated by a symmetry operation.For complex 7, this feature of the solidstate molecular structure precludes the distinction of the M−Si bond lengths.The presence of a half-occupied silicon atom at the bridgehead site of the overlapping [BP 3 iPr ]/[SiP 3 17SiHPh(DMAP), d(Co−Si) = 2.1428(5) Å17), reflecting a lower steric demand of the SiH 2 (DMAP) "silylene" unit.Complex 1 appears to be only the second example of a LMSiH 2 (L′) complex, since Radius et al. reported that reaction of trans-Mes 2 (Me 2 Im) 2 Fe with 2 equiv of PhSiH 3 3B to 1 represents a potential strategy for activating the [SiH 2 ] unit toward further reactivity in the presence of a suitable substrate (vide infra).2.2.Homo-and Heterobimetallic Silicides.Possible transformations of LMSiH 2 and LMSiH 2 (L′) species involve further activation of the Si−H bonds by an exogenous metal reagent to afford silicide complexes with a [MSiM′] core.

Table 1 .
Tabulated Experimental and Computed Metrical Parameters and Solution 29 Si NMR Data for Silicide Complexes 3, 4, 5, and 7 34 2 PCH 2 ) 2 BPh] (1.45, 1.44).19Inheterobimetallicsilicide7, the differing Co••• Si and Fe•••Si WBIs potentially indicate a greater extent of Fe••• Si multiple bonding.This effect may be related to a greater donation from iron, which can be rationalized by considering isolated [BP 3 iPr ]Co I and [SiP 3 iPr ]Fe 0 fragments.Also, the borate ligand of [BP 3 iPr ]Co I introduces a formal positive charge at the metal center for the overall neutral fragment, perhaps contracting the valence metal-based orbitals.In contrast, the neutral [SiP 3 iPr ]Fe 0 fragment formally possesses a zerovalent metal center, which is expected to more effectively mediate the cleavage of Si−H bonds.Atoms in molecules (AIM)34analyses of 3, 5, and 7 showed that the [H 2 Co�Si�MH 2 ] (M = Fe, Co) cores in these silicides possess few or no bond paths/bond critical points connecting the μ-silicon centers and the flanking hydride ligands (FigureS21).One bond path connecting the hydride ligand oriented cis with respect to the N-donor atom of the [BP 2 SiH 2 Ph)[( i Pr 2 PCH 2 ) 2 BPh], 3 iPr ]Co unit stabilizes silicide structures that are inaccessible with other ligand combinations, as demonstrated by [BP 3 iPr ]Co/Tp″Co silicide 4, which is isolable.Similarly, the symmetrical silicide 5 is inaccessible by direct treatment of Na(THF) 6 ([BP 3 iPr ]CoI) or 6 with SiH 4 ; in these cases, [BP 3 iPr ](H) 2 Co�Si� Co(H) 2 (SiH 3 )[( i Pr 2 PCH 2 ) 2 BPh]