Exchange of the olefin ligand in Cr(CO)5(eta2-(Z)-cyclooctene) by 1,4-diisopropyl-1,4-diazabutadiene (iprop-DAB) yields Cr(CO)5(iprop-DAB) (1), where the potentially bidentate DAB ligand coordinates in a monodentate fashion. Complex 1 is isolated as red crystals and fully characterized (elemental analysis, IR, UV-vis, H-1 NMR and C-13 NMR spectra). In hydrocarbon solution at ambient temperature it decays via two competitive routes involving (a) chelate ring closure with CO extrusion to form Cr(CO)4(iprop-DAB) (2) and (b) loss of the iprop-DAB ligand and takeup of CO to form Cr(CO)6, as monitored by means of NMR, UV-vis, and IR spectroscopy. Favorable conditions for the chelate ring closure leading to 2 are the high concentration of 1, the presence of added iprop-DAB, and the absence of CO, while the opposite is true for the formation of Cr(CO)6. The decay of 1 is retarded in the presence of increasing amounts of added iprop-DAB. It essentially follows pseudo-first-order kinetics with k(obs) approaching a lower limiting value of 2.7 x 10(-5) s-1 under Ar at 23-degrees-C, whereby Cr(CO)4(iprop-DAB) (2) and Cr(CO)6 are formed in a ca. 20:1 ratio. Experiments at variable temperature yield DELTAH(double dagger) = 48 +/- 6 kJ mol-1 and DELTAS(double dagger) = -170 +/- 18 J K-1 mol-1. In the presence of added CO or (E)-cyclooctene (ECO) the decay of 1 is accelerated, whereby additional Cr(CO)6 or Cr(CO)5(eta2-ECO) are formed at the expense of Cr(CO)4(iprop-DAB) (2) production. Complementary studies involving continuous irradiation of Cr(CO)6 and iprop-DAB indicate that photogenerated 1 subsequently undergoes photolytic CO dissociation with formation of 2 in addition to the thermal chelate ring closure.