Standardizing of know-hows of synthesizing polymers, contained an immobilized metal particles of nanosize, with beforehand given properties important for microelectronic requires analysis of the mechanism of born of a new phase. The first stage by synthesis of a composite material containing Fe particles of nanosize using Fe(CO)5 thermolysis at 130-160 ^oC is: Fe(CO)₄ + Fe(CO)₅ --> Fe₂(CO)₉ (*). An ab initio study of its mechanism in the pre-reaction complex approach is reported.The method used is second-order Moller-Pleset theory using canonical orbitals (MP2) in the atomic basis set – Fe: (14s9p5d) / [10s8p3d]; C, O: (11s,6p)/[5s,3p]. We have predicted a new Fe(CO)₄ conformer as the flat square (D_4h) structure in the fundamental singlet electronic state[1], which bond energy BE 96.81 kcal/mol coincides well with the experimental one's 102.3±10.5 kcal/mol. A structure of pre-reaction complex of orbital controlling reaction (*) is discussed.