THERMAL RING-CLOSURE REACTIONS OF PENTACARBONYLCHROMIUM COMPLEXES WITH BIDENTATE N-N LIGANDS REVISITED - ELIMINATION OF SIDE REACTIONS AND ELUCIDATION OF THE INTIMATE CHELATION MECHANISM

Thermal ring-closure reactions of pentacarbonylmetal complexes with bi dentate N-N ligands were studied in detail in the past using photochem ical techniques to produce these species in solution prior to the actu al reaction. Some of the monodentate pentacarbonylchromium complexes h ave now been isolated and enable a detailed reinvestigation of the the rmal chelation reaction of Cr(CO)(5)(N-N) (N-N = 1,4-diisopropyl-1,4-d iazabutadiene, 4,4'-dimethyl-2,2'-bipyridine). The reactions were stud ied as a function of[N-N], temperature, and pressure. The unexpected l igand concentration dependence, viz. a decrease in observed rate const ant with increasing [N-N], can be accounted for in terms of competitiv e reactions leading to the formation of Cr(CO)(6) and chelated Cr(CO)( 4)(N-N). At high [N-N] the limiting rate constant represents only the chelation reaction, for which the observed activation volumes are betw een +13 and +18 cm(3) mol(-1). These strongly support the operation of a dissociatively activated chelation mechanism.