LIGAND SUBSTITUTION REACTIONS IN TETRAKIS(BENZYLISOCYANIDE)-BIS(TRI-p-TOLYARSINE OXIDE)COBALT(III) TETRAFLUOROBORATE: AN EXAMPLE OF A LABILE SIX-COORDINATE COBALT(III) COMPLEX

Abstract

The six-coordinate, two-electron paramagnetic (intermediate-spin) cobalt(III) complex, trans-[Co(CNCH2Ph)4{OAs(C6H4-Me-p)3}2](BF4)3, is shown to undergo labile ligand-substitution reactions with trialkylphosphine ligands. The white, diamagnetic Co(III) complexes, trans-[Co(CNCH2Ph)4- (PR3)2] (BF4)3, PR3 = P(C3H7-n)3, P(C4H9-n)3, are prepared in moderate yields (40–50%) by apparently instantaneous reaction at room temperature. The relatively low yields are attributed to the solution instability of the starting material. Analogous reaction with the P(C8H17-n)3 ligand failed to yield a crystalline product. The observation of labile ligand-substitution reactions for intermediate-spin six-coordinate Co(III), a type of cation hitherto not previously investigated, is compatible with the accepted ranking of the inert/labile character of the metal cations and is readily explained in terms of a dissociative reaction mechanism involving a probable five-coordinate square-pyramidal activated complex.