Electronic structure factors of silicon-hydrogen bond activation by transition metals. Valence photoelectron spectra of (.eta.5-C5H4CH3)Mn(CO)(PMe3)HSiCl3 and (.eta.5-C5H4CH3)Mn(CO)(PMe3)HSiHPh2 (Me = CH3, Ph = C6H5)

Abstract

between atoms in bonds.32 Orbital u3 has nodes between atom pairs 1 and 2 and pairs 3 and 4, and the 2-3 interaction is bonding, again different from the 5c scheme. Thus we would expect the 4c hypervalent bond to be significantly weaker than the 3c or 5c hypervalent bonds. In the 4c bond, nodes occur between atoms in the occupied orbitals whereas, in the 3c and 5c bond, nodes occur at atoms in these orbitals. The above simple model assumes that the atoms are of equal electronegativity. However, in [ FKrNCH]+ the atoms are clearly not of equal electronegativity. Furthermore, the C-H bond is of the right energy for mixing, which complicates the analysis. The presence of a triple bond between C and N is also a complicating factor, as this increases the s component (sp hybridization) in the C-N u bond, which would be part of the 4c hypervalent bond. Examination of Figure 1 shows that the orbitals do not delocalize as extensively as suggested by our model for the 4c hypervalent bond due to the large electronegativity differences. Furthermore, the s and p orbitals are clearly mixed. The highest occupied u orbital, u6, is similar to orbital u2/ of the 4c bond with the largest population on N and the next highest population on F. Because of the large s character in the C N u bond, this orbital is very low in energy and the C-H u bond mixes with the Kr-F u bond instead of mixing with the C-N u bond as one would expect from the 4c hypervalent bond. The C N u bond is mixing, instead, with the (32) Arduengo, A. J.; Burgess, E. M. J . Am. Chem. SOC. 1977, 99, 2376.Kr valence s orbital. Thus the bonding of the u,' orbital of the 4c bond is dispersed over a range of CT orbitals, Another model for this bonding would involve considering both the C-N and C-H bonds as "spectators" and dealing only with a normal 3c,4e hypervalent bond centered at Kr based on the Kr-F bond and the lone pair on N. In this case, u6 of [FKrNCH]' is like u2" of the 3c,4e bond. There is essentially a node at Kr, and the population is not equally distributed between N and F. The u4 orbital of the complex then corresponds to u," of the 3c,4e bond. A complication for this model is that the 3c,4e bond preferentially places the least electronegative atom in the center. Clearly, "Kr+" is not the least electronegative atom. In order for it to lower its electronegativity in order to participate in the hypervalent bond, charge transfer occurs from the C-H region to the Kr to lower its positive charge and hence its electronegativity. This is actually observed in the population analysis and is consistent with the resonance structure shown above.All of our results are consistent with some type of covalent bonding interaction in [FKrNCH]+ being present. However, as the above discussion points out, there is no clear choice between the 3c and 4c hypervalent bond models. Rather, the bonding description for the interaction of [FKr]+ with N C H has components of both models present. The results do show that the bonding between Kr and N is not a simple covalent u bond betwe...

Reactions of Hydrosilanes with Transition-Metal Complexes: Formation of Stable Transition-Metal Silyl Compounds

“…Electron-donating groups on silicon decrease the binding interaction of HSi with the Mn center and give products with a 3c-2e bond. In the manganese system, data from the solid state (Table 5) and UV photoelectron spectra 476,480,498 both support a silane-metal interaction that approaches complete oxidative addition with electron-withdrawing chlorine substituents on the silicon. Earlier calculations supported π-back-bonding from the Mn unit to the Si-H σ*-orbital (although the authors preferred a description that did not involve the term oxidative addition).…”

“…Examples of TM(η 2 -HSi) interactions are summarized in Table − and the available crystallographic data are presented in Table . The early work on η 2 -coordinated HSi bonds has been reviewed by Schubert 1c,d,5a and has also been included in general reviews of σ-complexes. 7a,b A brief summary of investigations primarily from 1990 to 1996 has also been published…”

“…Photoelectron spectroscopy (PES) has also been utilized as a probe for Mn−Si−H interactions. ,,, Data for the manganese complexes 7 − 17 , 7 − 18 , 7 − 21a , 7 − 22 , 7 − 24 to 7 − 26 as well as Cp‘Mn(CO) 2 (η 2 -HSiCl 3 ) indicated a d 6 -electron count for the metal, thus Mn(I). The Si−H interaction with manganese is in the initial stages of oxidative addition to the metal.…”

Electron Distribution, Bonding, andJ(Si−H) NMR Coupling Constant in (η5-C5H5)(CO)2MnHSiCl3: The Molecular Orbital View

“…Among the reactions of hydrosilanes with transition-metal complexes, the oxidative addition of HSiCl 3 to the Mn center in Cp(CO) 2 MnHSiCl 3 has been particularly interesting. , A key question concerns whether the bonding tends more toward the σ-bond complex 1 or has proceeded substantially to the classical oxidative addition product 2 . The magnitude of the J (Si−H) NMR coupling constant in comparison to that of other metal silyls seems to indicate substantial retention of Si−H bonding as in 1 , whereas the frontier orbital model and the characters obtained from variable photon energy photoelectron spectroscopy clearly indicate nearly complete oxidation of the Mn center to a formal d 4 configuration most consistent with structure 2, with some residual Si−H interaction remaining, as might be represented by contributions from valence bond drawings 3 or 4 . Nikonov offers a different interpretation of the electronic structure of this molecule.…”

“…Localized orbital 15 is a well-formed Mn−hydride bond with some polarization toward the Si center. This polarization was not observed in the original Fenske−Hall calculations, which do not include polarization functions on H. The frontier orbitals 8 and 9 of [Cp(CO) 2 Mn] available for forming these bonds are primarily the d z 2 and d yz orbitals, and the hybrid (d z 2 + d yz ) and (d z 2 − d yz ) orbitals are evident in 14 and 15 . Interestingly, the angle between pure (d z 2 + d yz ) and (d z 2 − d yz ) hybrid orbitals is 49.1°, and the experimental Si−Mn−H angle is 49.9° .…”