FUNDAMENTAL METAL-CARBONYL EQUILIBRIA, V-1 - REINVESTIGATION OF THE EQUILIBRIUM BETWEEN DICOBALT OCTACARBONYL AND COBALT TETRACARBONYL HYDRIDE UNDER HYDROGEN PRESSURE

The equilibrium reaction of dicobalt octacarbonyl with dihydrogen is r egarded as one of the key steps in the activation of hydrogen by cobal t carbonyls. This has been reconfirmed in recent years by important ne w observations regarding the versatile chemistry of the cobalt hydride complex. Although the nature of the molecule responsible for the spli tting of the transiently formed RCO.Co(CO)(3) in the hydroformylation reaction is still the subject of discussions, the essential importance of the cobalt tetracarbonyl hydride is evident and undisputed. This p aper presents a critical review of the existing data on the equilibriu m reaction of dicobalt octacarbonyl with dihydrogen, along with the re sults of new infrared spectroscopic measurements under semi in-situ co nditions. The equilibrium constant, K-p = [HCo(CO)(4)](2)/[Co-2(CO)(8) ].pH(2) (mol.l(-1).bar(-1)), obtained from these latter measurements i n hexane as solvent, depends on temperature in the 50.7-117.0 degrees C range according to the equation log K-p = - (0.670 +/- 0.106)-(886 /- 38)/T. The van't Hoff plot of these data yielded the thermodynamic parameters Delta H = 4.054 +/- 0.175 kcal.mol(-1) and Delta S = -3.067 +/- 0.488 cal.mol(-1).K-1. (C) 1998 Elsevier Science S.A. All rights reserved.