TRICARBONYL(CARBABORANE) COMPLEXES OF THE GROUP-6 METALS AS REAGENTS FOR PREPARING HETERONUCLEAR TRIMETAL COMPOUNDS INVOLVING RHODIUM

The reaction between the salt [Rh(NCMe)3(eta-C5Me5)][BF4]2 and the rea gents Na2[M(CO)3(eta5-7,8-C2B9H9R2')] (M = Mo or W, R' = H or Me) in t etrahydrofuran affords [Rh(CO)2(eta-C5Me5)] and the trimetal complexes CO)(mu-sigma:eta5-7,8-CB9H8R2')(CO)2(eta-C5Me5)2]. An X-ray diffracti on study of the compound with M = W and R' = Me revealed a structure b ased on a triangulated metal core [Rh(1)-Rh(2) 2.859(1), W-Rh(1) 2.736 (1), W-Rh(2) 2.822(1) angstrom] with the Rh-Rh bond bridged by a hydri do ligand. Two of the carbonyl groups are terminally attached to the t ungsten atom, while the third [W-C-O 154(1)-degrees] semi-bridges a W- Rh bond. The open pentagonal CB3 face of the carbaborane framework is eta5 co-ordinated to the tungsten, but the cage also bridges the metal triangle through exopolyhedral B-Rh and B-H half arrow pointing right Rh bonds, using boron atoms in the C2B3 ring attached to the tungsten . The B-Rh bond bridges the W-Rh vector which is asymmetrically bridge d by the CO group, and involves the boron atom alpha to the carbons in the CCBBB ring. The B-H half arrow pointing right Rh three-centre two -electron bond utilises the boron atom which is beta to the carbons in the CCBBB ring. Examination of the NMR spectra (H-1, C-13-{H-1}, B-11 -{H-1}, B-11) of the trimetal complexes revealed that the structures o f these molecules at -60-degrees-C were similar to that established in the solid state for M = W and R' = Me. However, at ambient temperatur es three of the complexes (M = Mo, R' = H or Me; M = W, R' = Me) isome rised to a structure sigma:sigma':eta5-7,8-C2B9H7R2')(CO)2(eta-C5Me5)2 ] in which the carbaborane cage bridged the metal triangle via two B-R h bonds.