Ligand-dependent electronic structure, bonding, and optical properties of M(L)(CO)₂ complexes (L = acac, Cp, Cp; M =...

Transition-metal carbonyl complexes display rich electronic structures governed by the interplay of metal d orbitals and ligand π systems. Here, a systematic DFT and TD-DFT study was performed on 27 complexes of the type M(L)(CO)₂ (M = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt; L = acac, Cp, Cp*). Geometry optimizations, Mayer bond orders, ADCH charges, frontier molecular orbital energies, and global reactivity descriptors were evaluated at the MN12-L/def2-TZVP level. Scalar relativistic effects for the 4d (Ru, Rh, Pd) and 5d (Os, Ir, Pt) metals are implicitly included through the Stuttgart-Cologne small-core relativistic effective core potentials (ECP28MWB and ECP60MWB) that accompany def2-TZVP. Electronic absorption spectra were simulated in cyclohexane via TD-DFT with Gaussian convolution (σ = 20 nm) to obtain realistic λ_max values. The results reveal a recurrent ligand-dependent pattern: more ionic acac ligands consistently induce a more positive metal charge, longer M-C(CO) bond lengths (1.77-1.99 Å) and lower Mayer bond orders relative to those of the covalent Cp and Cp* analogs. The HOMO-LUMO gap is wider for acac than for the corresponding Cp/Cp* derivative in seven of the nine metals studied; the three d⁸ M(I) cases (Co, Rh, Ir) are clear exceptions in which Cp/Cp* widens the gap further owing to strong covalent donation. The computed MLCT energies, ADCH-charge orderings and CO stretching frequencies for the Ir benchmark series are in quantitative agreement with recently reported HERFD-XAS, VtC-RIXS and infrared data. This work provides a comprehensive understanding of how ancillary-ligand ionicity modulates the metal-ligand bonding, charge distribution, and optical properties of organometallic carbonyl complexes.