Bond Energies and Bonding Interactions in Fe(CO)5-n(N2)n (n = 0−5) and Cr(CO)6-n(N2)n (n = 0−6) Complexes: Density Functional Theory Calculations and Comparisons to Experimental Data

Performance of diffusion Monte Carlo for the first dissociation energies of transition metal carbonyls

Fixed node diffusion Monte Carlo (FNDMC) calculations are carried out for the first ligand dissociation energies of the prototype transition metal carbonyls Cr(CO)6, Fe(CO)5, Ni(CO)4, and Fe(CO)4N2. Since Hartree-Fock theory performs particularly badly for these type of compounds they are difficult to treat with conventional ab initio methods. We find that a Kohn-Sham determinant from a standard density functional provides a balanced description of the fermionic nodal hyper surfaces of all compounds involved in the dissociation reaction. With one exception, the experimental dissociation enthalpies are reproduced by FNDMC within the statistical accuracy of the method.

Experimental determination of the Cr-C(2)Cl(4) bond dissociation enthalpy in Cr(CO)(5)(C(2)Cl(4)): quantifying metal-olefin bonding interactions

The bond dissociation enthalpy for the Cr-C(2)Cl(4) bond in gas-phase Cr(CO)(5)(C(2)Cl(4)) has been determined to be 12.8 +/- 1.6 kcal/mol using transient infrared spectroscopy. The results of a density functional theory-based energy decomposition analysis are used to quantify the metal-olefin bonding interactions in terms of the bonding description provided by the Dewar-Chatt-Duncanson model (sigma donation and back-bonding). The bond energy decomposition analysis reveals that metal-olefin bond strengths can be strongly influenced by the Pauli repulsion energy and by the energy necessary to deform the olefin and the metal-centered moiety from their equilibrium geometries to their geometry in the final complex. Further, a comparison between the metal-olefin bond strengths and the magnitude of the electronic interactions demonstrates that the energy associated with these deformations is the determining factor in the trends in bond enthalpies in the series of complexes Cr(CO)(5)(C(2)X(4)) (X = H, F, Cl). Though deformation of the Cr(CO)(5) moiety contributes to the overall deformation energy, the major contribution involves deformation of the olefin. This occurs as a consequence of rehybridization of the olefin as a result of metal-olefin back-bonding. The results are discussed in terms of the Dewar-Chatt-Duncanson model, which provides the accepted qualitative description of bonding in organometallic olefin complexes.