Contrasting Nonclassical Silicon−Hydrogen Interactions in Niobium and Tantalum Half-Sandwich Complexes: Si−H···M Agostic versus M−H···Si−Cl Interligand Hypervalent Interactions

Reaction of [CpM(NAr)(PMe3)2] (M = Nb, Ta; Ar = 2,6-C6H3iPr2) with HSiClMe2 gives two remarkably different nonclassical Si···H···M products depending only on the identity of M; [CpTa(NAr)(H)(SiMe2Cl)(PMe3)] possesses an unusual electron-rich M−H···Si interligand hypervalent interaction while [CpNb{η3-N(Ar)SiMe2−H}Cl(PMe3)] is the first example of a β-agostic silylamine Si−H···M interaction showing a “stretched” Si−H bond.