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)6, 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)6. We have found that, different from
the results published earlier, the metal–CO bond in Sg­(CO)6 should be weaker than that in W­(CO)6. 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)6 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)6, which should serve as guidance for ongoing
experiments.