FIRST BOND-DISSOCIATION ENERGY OF M(CO)(6) (M=CR, MO, W) REVISITED - THE PERFORMANCE OF DENSITY-FUNCTIONAL THEORY AND THE INFLUENCE OF RELATIVISTIC EFFECTS

Density functional (DF) calculations have been carried out on the M-CO bond lengths as well as the first bond dissociation energy (FBDE) for the M(CO)(6) hexacarbonyls with M = Cr, Mo, and W. The main objective of the investigation has been to examine how well DF theory can predict the observed bond dissociation energies and M-CO bond distances. It is shown that the calculated M-CO bond lengths are within 0.01 Angstrom of experiment after the inclusion of nonlocal corrections and relativistic effects, NL-SCF+QR. Relativity contracts the W-CO bond by 0.07 Angstrom, and bond distances calculated without nonlocal corrections are too short by 0.04 Angstrom. The FBDEs calculated by the NL-SCF+QR scheme were 46.2 kcal/mol (Cr), 39.7 kcal/mol (Mo), and 43.7 kcal/mol (W), whereas the corresponding experimental values are given by 36.8 +/- 2 kcal/mol (Cr), 40.5 +/- 2 kcal/mol (Mo), and 46.0 +/- 2 kcal/mol (W). The agreement with experiment is good for M = Mo and W, where relativity increase the bond strength with 1.5 and 4.9 kcal/mol, respectively. The origin of the relativistic effects are analyzed by an energy decomposition scheme. The NL-SCF+QR value of 46.2 kcal/mol for M = Cr agrees well with recent ab initio estimates. However, it is at variance with the experimental value of 36.8 +/- 2 kcal/mol. It is suggested that the experimental value should be adjusted upward. Nonlocal corrections are essential for a quantitative estimate of the FBDEs.