Variable-pressure kinetics studies (to 150 MPa) at various temperatures for chelate ring displacement from cfs-(S2)M(CO)4 complexes (S2 = dithiaalkane, dithiaalkene (2,5-dithiahexane, 3,6-dithiaoctane, and m-2,2,7,7-tetramethyl-3,6-dithiaoct-4-ene); M = Cr, Mo) by L (=Lewis liase (P(OR)3, R = Me, Et, -Pr, Ph)) in chlorobenzene (CB) and 1,2-dichloroethane (DCE) have been carried out. All reactions obey the rate law -d[(S2)M.(CO)4]/d< = fc[(S2)M(CO)4] [L]. However, calculated volumes ofactivation (AV*) and thermal activation parameters (AH*, AS*) indicate that ligand displacement in the Cr complexes takes place via initial, reversible Cr-S bond fission, while for Mo and S2 = 2,5-dithiahexane and 3,6-dithiaoctane an associative pathway involving significant initial L-Mo bond making is operative. For S2 = ti.s-2,2,7,7-tetramethyl-3,6-dithiaoct-4-ene and = Mo, activation parameters (the thermal data were taken over a 70 °C temperature range) are equivocal as to whether the mechanism is best described as an interchange process or in terms of competing dissociative and associative pathways. It is concluded that the mechanistic differences observed in the Cr and Mo systems result from the differing atomic sizes of these metals. The results are also discussed in terms of the properties of the chelating rings.
