Surprising diversity of non-classical silicon-hydrogen interactions in half-sandwich complexes of Nb and Ta: M-H···Si-Cl interligand hypervalent interaction (IHI) versus stretched and unstretched β-Si-H···M agostic bonding

Reaction of the niobium diphosphine compound [NbCp(NAr)(PMe3)2] (Ar = 2,6-C6H3Pri2) with HSiMe2Cl gives the formally d2 silylamido derivative [NbCp{η3-N(Ar)SiMe2-H}Cl(PMe3)] 6. X-Ray diffraction and NMR studies of this compound show that it has a stretched β-agostic Si–H → Nb interaction. Reaction of the related precursor [NbCp(NAr′)(PMe3)2] (Ar′ = 2,6-C6H3Me2) with HSiMe2Cl gives an isomeric structure [NbCp{η3-N(Ar′)SiMe2-H}(PMe3)(Cl)] 7 differing from 6 in that the phosphine rather than chloride lies trans to the co-ordinated Si–H bond. A preliminary X-ray study and large 1J(Si–H) coupling constant of 116 Hz suggest that this compound is best described as an unstretched β-agostic (Si–H⋯M) d2 silylamide complex. Reaction of the tantalum diphosphine compound [TaCp(NAr)(PMe3)2] with HSiMe2Cl affords the d0 silylhydride derivative [TaCp(NAr)(H)(SiMe2Cl)(PMe3)] 8 which, according to an X-ray diffraction study and NMR data, has an interligand hypervalent interaction (IHI) between the silyl and hydride ligands. Reactions of 6 and 8 with Me3SiX (X = I, OTf) lead to the corresponding iodido and triflate derivatives [NbCp{η3-N(Ar)SiMe2-H}X(PMe3)] (X = OTf 11 or I 12) and [TaCp(NAr)(H)(SiMe2X)(PMe3)] (X = OTf 14 or I 15). Reaction of 8 with AgOTf gives [TaCp(NAr)(PMe3)2Cl]OTf 13, the crystal structure of which has been determined. Density functional theory calculations on models of the compounds 6 and 7 showed that the experimental geometries are only correctly reproduced when the phosphine ligands are adequately modelled. The extent of oxidative addition of the Si–H bond to the metal in 6 mainly depends on the basicity of the phosphine ligand. With PH3 in place of PMe3 the calculated structures are better described as silanimine-hydrido derivatives. The formation of isomeric type 6versus7 is determined by an interplay of the steric and electronic effects of the ligand environment.