THERMAL-DISSOCIATION AND BOND-ENERGIES OF IRON CARBONYLS FE(CO)(N) (N=1-5)

The thermal decomposition of Fe(CO)(5) was studied behind incident shock waves using absorption spectrophotometry for monitoring the concentrations of the parent molecule and Fe atoms. A new method based on a joint analysis of the Fe(CO)(5), dissociation and the recombination of unsaturated carbonyls Fe(CO)(n) is proposed. By employing this method, the values of bond dissociation energies for a series of iron carbonyls Fe(CO)(n) (n = 1 - 5) were derived: Delta H degrees(n,0) = 159 +/- 12 (n = 5); 138 +/- 12 (n = 4); 149 +/- 8 (n = 3); 109 +/- 12 (n = 2); 32 +/- 12 (n = 1) kJ/mol. The activation barriers for spin-forbidden reactions of CO addition to Fe(CO)(4) and (FeD)-D-5 were evaluated to be 11 +/- 6 and 21 +/- 12 kJ/mol, respectively. Using the thermochemical data obtained and the molecular parameters from other studies, the rate constants for decomposition of Fe(CO)(n)(n = 1 - 5) in the low- and high-pressure limits were calculated.