A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragments (3)Fe(CO)(4) and (3)Fe(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 (1)Fe(CO)(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 (3)Fe(CO)(4) with H(2) cannot be easily probed in n-heptane since conversion to (1)Fe(CO)(4)(heptane) dominates. scAr doped with H(2) provides a unique environment to monitor this reaction--Ar cannot be added to form (1)Fe(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.

The photochemistry of Fe(CO)5 (5) has been studied in heptane, supercritical (sc) Ar, scXe, and scCH4 using time-resolved infrared spectroscopy (TRIR). 3Fe(CO)4 ((3)4) and Fe(CO)3(solvent) (3) are formed as primary photoproducts within the first few picoseconds. Complex 3 is formed via a single-photon process. In heptane, scCH4, and scXe, (3)4 decays to form (1)4 x L (L = heptane, CH4, or Xe) as well as reacting with 5 to form Fe2(CO)9. In heptane, 3 reacts with CO to form (1)4 x L. The conversion of (3)4 to (1)4 x L has been monitored directly for the first time (L = heptane, kobs = 7.8(+/- 0.3) x 10(7) s(-1); scCH4, 5(+/- 1) x 10(6) s(-1); scXe, 2.1(+/- 0.1) x 10(7) s(-1)). In scAr, (3)4 and 3 react with CO to form 5 and (3)4, respectively. We have determined the rate constant (kCO = 1.2 x 10(7) dm3 mol(-1) s(-1)) for the reaction of (3)4 with CO in scAr, and this is very similar to the value obtained previously in the gas phase. Doping the scAr with either Xe or CH4 resulted in (3)4 reacting with Xe or CH4 to form (1)4 x Xe or (1)4 x CH4. The relative yield, [(3)4]:[3] decreases in the order heptane > scXe > scCH4 >> scAr, and pressure-dependent measurements in scAr and scCH4 indicate an influence of the solvent density on this ratio.