Niobocene silyl hydride complexes with nonclassical interligand hypervalent interactions

Reaction of [Cp2NbBH4] with tertiary silanes HSiR3 (R-3 =(OEt)(3), Me2Ph, M e2Cl) in the presence of an amine affords monosilyl complexes [Cp2NbH2SiR3] (R-3 = (OEt)(3) (1), Me2Ph (2), Me2Cl (3)). Complex 3 was obtained as a mi xture of two isomers. with the silyl ligand in the lateral (3b) and central (3a) positions. The more sterically strained isomer 3b was obtained in a s lightly greater amount than 3a, signifying the presence of an additional st abilizing electronic factor. Complex 3b was shown to be a thermodynamic pro duct of the reaction. Thermal reaction of [Cp2NbH3] with HSiMe2Cl at 50 deg rees C gives initially a mixture of 3a and 3b, but under more severe condit ions (at 90 degrees C) unprecedented dihydrogen elimination occurs to yield thebis(silyl) complex [Cp2NbH(SiMe2Cl)(2)]. Complexes [Cp2NbH(SiMe2X)(2)] (X = Cl (4), Ph (6)) were obtained by reaction of [Cp2NbH(C2H3Ph)] with the corresponding silanes HSiMe2X. Reduction of complex 4 by LiAlH4 gives a hi gh yield of [Cp2NbH(SiMe2H)(2)], which reacts with [Ph3C]PF6 and Br-2.dioxa ne to afford bis(silyl) complexes [Cp2NbH(Si-Me2F)(2)] (8) and [Cp2NbH(SiMe 2X)(2)] (9), respectively. Compounds 3b, 4, 6, 8 and 9 were studied by NMR and IR spectroscopy and X-ray diffraction analysis. The X-ray data for 3b, 4, 8 and 9, together with some reactivity trends, suggest a nonclassical na ture of these complexes due to the presence of interligand interaction betw een the silyl and hydride ligands. The observed trends in structural parame ters are different from those predicted for three-center, two-electron (3c- 2e) interactions of silanes with transition metals. This new type of noncla ssical bonding was rationalizedas a hypervalent interaction at the silicon centre due to the overlap of the Nb-H bonding orbital and the Si-CI antibon ding orbital. The main structural trends as well as the occurrence of an Nb -H-->Si-Cl* electron density transfer were supported by natural bond orbita l (NBO) analysis. The calculation results show stronger interligand interac tions in the chlorosilyl complexes than in the corresponding SiH3 complexes . However, a correlation between the structural and NMR parameters for the niobocene silyl hydrides was not established because of the unfavourable in fluence of the quadrupolar niobium nucleus.