Variational
energy decomposition analyses have been presented to
quantify the σ-dative, ligand-to-metal forward charge transfer
(CT) and the π-conjugative, metal-to-ligand backward charge
delocalization on a series of isolelectronic transition-metal carbonyl
complexes M(CO)6, including M = Ti2–,
V–, Cr, Mn+, and Fe2+. Although
the qualitative features of these energy terms are understood, well-defined
quantitative studies have been scarce. Consistent with early findings,
electrostatic and Pauli exchange effects play a key role in σ-donation,
resulting in blue shifts in ligand vibrational frequency in the complex
geometries. Excluding chemical bonding interactions between the CO
ligand and the metal fragments in the energy decomposition analysis,
we found that loosely bound electrostatic complexes can be formed
at a longer metal-to-ligand distance due to the exponential decay
of Pauli exchange. In all complexes, the overall binding stabilization
can be attributed to CT effects, with opposing trends between σ-donation
and π-back bonding that follows an order of Ti2– (4.4) > V1– (2.6) > Cr (1.5) > Mn1+ (1.1) > Fe2+ (0.5) in π-to-σ CT
ratio. These
electronic and energetic features are mirrored in the vibrational
frequency shifts induced by different factors. The present investigation
may help stimulate the use of energy decomposition techniques to understand
the structure and activity of metallocatalysts using density functional
theory.