Studies of Fe2(CO)9 Formation in the Thermolysis of Fe(CO)5 Using Ab Initio Calculations of Fe(CO)5 and Fe(CO)4

Ab initio calculations using the GAMESS program package in the atomic basis TZV (Fe: (14s, 11p, 6d)/10s, 8p, 3d]; C, O: (11s, 6p)/5s, 3p]) were performed with account taken of the correlation with the second-order Möller–Plesset (MP2) perturbation theory to predict a new conformer Fe(CO)₄ (with D _4h symmetry). This conformer has a square planar configuration in the ground singlet electronic state and is a mild electrophile produced by dissociation of Fe(CO)₅ along the axial Fe–C bond. The process of nucleation of iron nanoparticles Fe(CO)₅ + Fe(CO)₄ Fe₂(CO)₉ is supposed to occur in two stages. The first stage is an orbital-controlled reaction which should be monitored as an increase in medium polarity and temperature. It should proceed with participation of only one of the stable conformers of the nucleophile Fe(CO)₅, namely, a mild conformer with square-pyramidal structure (C _4v ) rather than a hard but energetically more advantageous conformer with trigonal–bipyramidal structure (D _3h ). The structure of a prereaction complex was discussed.