Nature of M-(η2-H-SiR2) bonds in chromium, molybdenum and tungsten complexes [(η5-C5H5)(dmpe)M(η2-H-SiR2)] and [(η5-C5H5)(CO)2M(η2-H-SiMe2)]: A theoretical study

Abstract

Geometries, electronic structure, and bonding analysis of the coordinated hydrosilyl groups in chromium, molybdenum and tungsten complexes [(η5-C5H5)(dmpe)M(η2-H-SiMe2)] (1Cr, M=Cr; 1Mo, M=Mo; 1W, M=W), [(η5-C5H5)(CO)2M(η2-H-SiMe2)] (2Cr, M=Cr; 2Mo, M=Mo; 2W, M=W), [(η5-C5H5)(dmpe)M(η2-H-SiEt2)] (3Mo) [(η5-C5H5)(dmpe)Mo(η2-H-SiPh2)] (4Mo) and [(η5-C5H5)(dmpe)M(η2-H-Si(H)Ph)] (5Mo) (dmpe=Me2PCH2CH2PMe2) were investigated at the DFT/BP86/TZ2P/ZORA level of theory. The optimized geometry of model complexes for [(η5-C5H5)(dmpe)M(η2-H-SiMe2)], [(η5-C5H5)(dmpe)M(η2-H-SiEt2)] and [(η5-C5H5)(dmpe)M(η2-H-Si(H)Ph)] are in excellent agreement with the experimental values. The M–H1 bond is directed toward the empty p orbital of the Si atom of the silylene ligands. A significant lengthening of the M–H1 and Si–H1 bonds and small Si–M–H1 bond angles (in the range 45.4°–47.5°) indicate the existence of three center two electron M–H–Si bonding. Upon substitution of the dmpe ligand by better π-acceptor CO ligands, the calculated M–Si and M–H1 bond distances slightly increase, while Si–H1 bond distances decrease due to increase in M←SiR2 σ-bonding. The small values of Mayer bond orders of the M–H and H–Si bonds also indicate the hydride as bridging between the M and Si centers. The M–H–Si bonding orbital is polarized toward the H1 atom (the H1 atom contributes nearly 50% to the M–H–Si bonding orbital). Absolute values of various energy terms increase upon substitution of dmpe ligand by better π-acidic CO ligands. The nature of substituent at Si atom in complexes 1Mo, 3Mo–5Mo has a small effect on the orbital interaction, ΔEorb.