D_3d Ground-State Structure of V(CO)₆:  A Combined Matrix Isolation and ab Initio Study of the Jahn−Teller Effect

Abstract

Structures, relative energies, vibrational spectra, and ESR parameters of the 17emolecule V(CO) 6 in O h , D 3d , D 4h , and D 2h symmetry have been computed with density functional and high-level ab initio methods. At all theoretical levels applied, the same energetic order is obtained, D 3d < D 2h < D 4h < O h , with the D 2h structure as a transition state connecting equivalent D 3d species. At the RCCSD(T)/AE2 level using UBP86/ AE2 geometries, the energies of the D 2h , D 4h and O h species relative to that of D 3d minimum are predicted to be 210, 535, and 731 cm -1 , respectively. According to molecular dynamics simulations on the UBP86/AE1 potential energy surface, the D 3d minimum is preserved at very low temperatures (around 16 K), whereas at 300 K the molecule is highly fluxional with an averaged structure indistinguishable from that of [V(CO) 6 ]with its O h symmetric ground state. Nearly complete IR and Raman spectra of V(CO) 6 and V( 13 CO) 6 have been recorded at 300 K for the first time in the gas phase, in solution, and at cryogenic temperatures in Ne and Ar matrices. The spectra show a pronounced temperature dependence, especially for the Jahn-Teller active modes, E g and T 2g . The observed infrared matrix spectra generally agree well with the calculated spectrum (BP86/AE2 level) for the D 3d structure and much less with that of the D 4h isomer. The A 1g modes in the Raman spectra are reasonably well reproduced computationally in the harmonic approximation, whereas this approximation breaks down for the E g and T 2g bands, as expected. Further evidence for the D 3d symmetry of V(CO) 6 is obtained from a reanalysis of the experimental ESR spectrum that is reported in the literature. The observed ordering of the hyperfine coupling constants A | and A ⊥ is reproduced theoretically only when distortion to D 3d symmetry is assumed. In addition, the bonding properties of V(CO) 6 are compared to those of [V(CO) 6 ]and Cr(CO) 6 .

Intrinsic Dinitrogen Activation at Bare Metal Atoms

“…These experiments give evidence for the complex [V(N 2 ) 6 ]. The EPR spectra that were measured for [V(N 2 ) 6 ] and also [Nb(N 2 ) 6 ] in solid N 2 indicate a tetragonal axial elongation as a consequence of a Jahn-Teller effect, so that they exhibit not O h but only D 4h symmetry (in disagreement with earlier reports, [112] but in agreement with the geometry of the corresponding vanadium hexacarbonyl [113] ). The molecules adopt a 2 B 2g electronic ground state.…”

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“…Yet, the more symmetric structure, that is, octahedral quartet isomer O h ( 4 A 1g ), lies 24.3 kcal/mol higher in energy. By contrast, Ti(CO) 6 , 5 V(CO) 6, 32 and Cr(CO) 6 12, 21, 31 either have a slightly distorted or perfect octahedral structures.…”

Infrared spectroscopy of mass-selected metal carbonyl cations

“…The VðCOÞ þ 6 resonance at 2096 cm À1 can be compared to that of neutral V(CO) 6 , which is a well-known stable carbonyl radical having 17 electrons. Its CO stretch resonance is a Jahn-Teller doublet at 1985/1991 cm À1 [79]. In these vanadium complexes, then, both the cation and the neutral have resonances red-shifted from the free-CO frequency, but again the cation has a significantly smaller red-shift than the neutral.…”