Thermal ring-closure reactions of pentacarbonylmetal complexes with bidentate N−N ligands were studied in detail in the past using photochemical techniques to produce these species in solution prior to the actual reaction. Some of the monodentate pentacarbonylchromium complexes have now been isolated and enable a detailed reinvestigation of the thermal chelation reaction of Cr(CO)5(N−N) (N−N = 1,4-diisopropyl-1,4-diazabutadiene, 4,4‘-dimethyl-2,2‘-bipyridine). The reactions were studied as a function of [N−N], temperature, and pressure. The unexpected ligand concentration dependence, viz. a decrease in observed rate constant with increasing [N−N], can be accounted for in terms of competitive 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 between +13 and +18 cm3 mol-1. These strongly support the operation of a dissociatively activated chelation mechanism.