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 SERIESOF SUBSTITUTED BIPYRIDINE LIGANDS

The kinetics of the thermal ring-closure reactions of W(CO)5L, produce d during the photolysis of W(CO)6/L, where L = 2,2'-bipyridine, 4,4'-d imethyl-2,2'-bipyridine, 4,4'-diphenyl-2,2'-bipyridine and 4,4'-di-ter t-butyl-2,2'-bipyridine, were studied as a function of pressure. The v alues of the activation parameter DELTAV(double dagger) are small and negative for these ligands, supporting an associative interchange mech anism (I(a)) for CO extrusion. The DELTAV(double dagger) values exhibi t a specific trend toward more positive values on increasing the steri c hindrance on the ligand. This trend is interpreted as evidence for a gradual change in mechanism from a more associative to a more dissoci ative ring-closure process.