Metal fragment isomerisation upon grafting a d² ML<inf>4</inf> perhydrocarbyl Os complex on a silica surface: Origin and consequence

Grafting of Os bisalkylidene complexes, [Os(═CHtBu)2(CH2 tBu)2], on a silica partially dehydroxylated at 700 °C selectively yields an alkylidyne complex [(≡SiO)Os(≡CtBu)(CH2 tBu)2] according to mass balance analysis, IR and solid state NMR spectroscopies, but 70% of the silanols remains unreacted. Grafting corresponds to a replacement of one alkyl by a siloxy ligand and induces the isomerisation of the metal fragment from a bis-alkylidene to an alkyl alkylidyne. Molecular (B3PW91) and periodic DFT-calculations show that the bis-alkylidene is the energetically favoured isomer for the perhydrocarbyl complex, while the alkyl alkylidyne isomer is more stable when one of the alkyl ligands is replaced by a siloxy ligand. The change of the nature of the ligand is accompanied with a change of geometry: from a distorted tetrahedral structure for [Os(═CHtBu)2(CH2 tBu)2] to a butterfly-geometry with apical siloxy and alkylidyne ligands for [(≡SiO)Os(≡CtBu)(CH2 tBu)2]. Finally, DFT calculations show that grafting occurs via a σ-bond metathesis between the silanol and a metal–alkyl bond and not through the typical addition of the silanol onto the alkylidene ligand. © 2009 The Royal Society of Chemistry.