CAN STERIC HINDRANCE CAUSE A GRADUAL CHANGE IN THE RING-CLOSURE MECHANISM OF COMPLEXES OF THE TYPE W(CO)5N-N, WHERE N-N REPRESENTS A SERIES OF SUBSTITUTED BIPYRIDINE LIGANDS

The kinetics of the thermal ring-closure reactions of W(CO)5L, produced during the photolysis of W(CO)6/L, where L = 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, 4,4'-diphenyl-2,2'-bipyridine and 4,4'-di-tert-butyl-2,2'-bipyridine, were studied as a function of pressure. The values of the activation parameter DELTAV(double dagger) are small and negative for these ligands, supporting an associative interchange mechanism (I(a)) for CO extrusion. The DELTAV(double dagger) values exhibit a specific trend toward more positive values on increasing the steric hindrance on the ligand. This trend is interpreted as evidence for a gradual change in mechanism from a more associative to a more dissociative ring-closure process.