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 regarded as one of the key steps in the activation of hydrogen by cobalt carbonyls. This has been reconfirmed in recent years by important new observations regarding the versatile chemistry of the cobalt hydride complex. Although the nature of the molecule responsible for the splitting 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 paper presents a critical review of the existing data on the equilibrium reaction of dicobalt octacarbonyl with dihydrogen, along with the results of new infrared spectroscopic measurements under semi in-situ conditions. The equilibrium constant, K_p=[HCo(CO)₄]²/[Co₂(CO) ₈]·pH₂ (mol·l^-1·bar^-1), obtained from these latter measurements in hexane as solvent, depends on temperature in the 50.7-117.0°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 ΔH=4.054±0.175 kcal·mol^-1 and ΔS=-3.067±0.488 cal·mol^-1·K^-1.