First-principles calculations of structural, electronic, optical, and catalytic properties in gallium nitride-substituted iron carbonyl complexes: [Fe(CO)4(GaN)] and [Fe(CO)3(GaN)2].

Density functional theory (DFT) calculations at the B3LYP with LANL2DZ, SDD and 6-31G+(d) basis set provide comprehensive insights into the structural dynamics, bonding interactions, bond orders and nonlinear optical (NLO) response of novel iron-gallium carbonyl complexes [Fe(CO)4(GaN)] (C1) and [Fe(CO)3(GaN)2] (C2). Geometry optimizations and bond orders confirm distorted pseudo-octahedral Fe centers with contracted Fe-CO bonds (1.75-1.81 Å) and typical Fe-Ga (2.31-2.36 Å) and Ga-N (1.81-1.85 Å) metrics, accompanied by near-linear Fe-C-O angles favouring optimal π-backdonation. Vibrational analysis reveals characteristic CO stretches that blue shifted upon bis(GaN) substitution, coupled Ga-N modes, and softened Fe-Ga scaffold vibrations indicative of enhanced mechanical coupling. Frontier molecular orbital (FMO) analysis shows a wider HOMO-LUMO gap in C1 (1.43 eV) versus C2 (1.03 eV), with delocalized π-type manifolds dominated by Fe. The t2g (d)-like HOMOs and antibonding π∗ LUMOs incorporating Ga-N contributions, correlating with lower chemical hardness (η = 0.515 eV) and higher electrophilicity (ω = 4.245 eV) for the softer, more reactive C2. Natural bond orbital (NBO) dissection quantifies strong donor-acceptor delocalizations (E2 > 20 kcal mol-1) across Fe-CO, Fe-Ga, and Ga-N linkages, with bis (GaN) coordination amplifying intramolecular charge transfer and modulating back bonding. Notably, finite-field hyperpolarizability calculations yield β0 values of 3.124 × 10-30 esu (1) and 4.769 × 10-30 esu (2), surpassing urea and highlighting 2 as a superior second-order NLO pore due to extended conjugation and push-pull asymmetry. These findings establish Ga-N fragments as isolobal CO mimics that systematically tune stability, reactivity, and NLO efficiency in low-valent iron carbonyls, paving the way for rational design of organometallic materials in optoelectronics and catalysis.