Interplay of ketenyl and nitrile ligands on d6-transition metal centres.: Acetonitrile as an end-on (two-electron) and a side-on (four-electron) ligand

Carbyne carbonyl coupling takes place in acetonitrile solutions of the Fischer-type carbyne complexes [tpb'(CO)(2)Mo=CC6H4Me-4] (4b) (tpb' = tris(3,5-dimethylpyrazolyl) hydroborate) and [(C5Me5) (CO)(2)W=CSiPh3] (5). The product [tpb'(Co)(NCMe)Mo{C(C6H4Me-4)CO}] (7b) has conventional eta(1)-nitrile and eta(2)-ketenyl ligands. In contrast, the product [(C5Me5)(Co)(NCMe)W{C(SiPh3)CO}] (8) displays a highly unusual 'four-electron-donor' eta(2)-nitrile and a eta(1)-ketenyl ligand. The structures of 4b, [tpb'(CO)(2)W=CC6H4Me-4] 6, 7b and 8 were determined by X-ray crystallography. Extended Huckel and density functional theory (DFT) calculations on model compounds show that the observed coordination geometry around W is the electronically preferred for 8. The side-on nitrile in this and other related complexes binds to the metal donating from the two highest occupied orbitals (corresponding to the nitrogen lone pair and the pi(perpendicular to)), in contrast to two-electron donors where only the HOMO is involved in bonding. Back-donation takes place for all the side-on bound nitriles. (C) 1999 Elsevier Science S.A. All rights resented.