Fe(CO)4 flexible as a two‐level system avoided conical intersection

This article reports new square‐planar Fe(CO)₄ D_4h structures that are optimized, using the Hartree–Fock (HF) approach, and multiconfiguration self‐consistent field (MCSCF) theory in active space 2b_2g2e_ga_1ga_2u]⁸, and which energy increased in sequence: ³B_2g TS < ¹A_1g TS < ¹A_1g GS. A triple ζ valence basis set supplemented with 4f for Fe and 3d for C and O polarization shells TZV (DF)] was used. At the HF/TZV (DF) level, ¹A_1g TS and ³B_2g TS (³B_2g TS energetically more favorable), there are transition states of tetrahedral inversion (defining stereochemical flexibility of Fe(CO)₄) between known equivalent ¹A₁ and ³B₂ Jahn–Teller distorted tetrahedron C_2v structures with activation energy at ～0.96 kcal/mol according to the experimental data. ¹A_1g TS differs from ¹A_1g GS in electronic configuration by occupation of a_1g and a_2u MOs. At the MCSCF/ TZV (DF) level, ¹A_1g TS and ¹A_1g GS are optimized as near‐pure states in different potential energy surfaces (PES) avoided conical intersection with near‐equal interatomic distances, and define electronic flexibility of Fe(CO)₄. Estimation of the energy separation in a two‐level system that avoids a conical intersection from vibrational spectrum is based on the effective Hamiltonian of the perturbation theory. The energy gap between two square‐planar Fe(CO)₄ D_4h ¹A_1g TS < ¹A_1g GS is 0.27 kcal/mol. The energy gap between ¹A_1g GS and ¹A₁ is 1.28 kcal/mol. It is possible to observe ³B₂, ¹A₁ and ¹A_1g GS separately in the course of the experiment. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006