Hexacarbonyls of Mo, W, and Sg: Metal-CO Bonding Revisited.

Calculations of the first bond dissociation energies (FBDEs) and other molecular properties of M(CO), where M = Mo, W, and Sg, have been performed using a variety of nonrelativistic and relativistic methods, such as ZORA-DFT, X2c+AMFI-CCSD(T), and Dirac-Coulomb density functional theory. The aim of the study is to assist experiments on the measurements of the FBDE of Sg(CO). We have found that, different from the results published earlier, the metal-CO bond in Sg(CO) should be weaker than that in W(CO). A comparison of the relativistic and nonrelativistic FBDE values, as well as molecular orbital and vibrational frequency analyses within both the nonrelativistic and relativistic approaches, have shown that this is a relativistic, predominantly scalar, effect causing weaker d(M) → π(CO) back-bonding in Sg(CO) than in the lighter homologues. Good agreement between the calculated FBDEs in this work and the experimental FBDEs for the Mo and W compounds gives credit to the present FBDE of Sg(CO), which should serve as guidance for ongoing experiments.