A Combined Theoretical and Experimental Study on the Role of Spin States in the Chemistry of Fe(CO)5 Photoproducts

A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragments Fe-3(CO)(4) and Fe-3(CO)(3) formed upon photolysis of Fe(CO)(5). Experimental data are provided for reactions in liquid n-heptane and in supercritical Xe (scXe) and Ar (scAr). Measurement of the temperature dependence of the rate of decay of 3 Fe(CO)(4) to produce Fe-1(GO)(4)L (L = heptane or Xe) shows that these reactions have significant activation energies of 5.2 (+/- 0.2) and 7.1 (+/- 0.5) kcal mol(-1) respectively. Nonadiabatic transition state theory is used to predict rate constants for ligand addition, based on density functional theory calculations of singlet and triplet potential energy surfaces. On the basis of these results a new mechanism (spin-crossover followed by ligand addition) is proposed for these spin forbidden reactions that gives good agreement with the new experimental results as well as with earlier gas-phase measurements of some addition rate constants. The theoretical work accounts for the different reaction order observed in the gas phase and in some condensed phase experiments. The reaction of Fe-3(CO)(4) with H-2 cannot be easily probed in n-heptane since conversion to Fe-1(CO)(4)(heptane) dominates. scAr doped with H-2 provides a unique environment to monitor this reaction-Ar cannot be added to form Fe-1(CO)(4)Ar, and H-2 addition is observed instead. Again theory accounts for the reactivity and also explains the difference between the very small activation energy measured for H-2 addition in the gas phase (Wang, W. et al J. Am. Chem, Soc, 1996, 118, 8654) and the larger values obtained here for heptane and Xe addition in solution.