Year: 2026
Type: preprint
Abstract: Metal–ligand covalency is central to the electronic structure of first-row transition-metal complexes, but different descriptors do not necessarily measure the same component of bonding. Here, we examine descriptor agreement and disagreement across a matched series of neutral fourcoordinate complexes, MCl2L2, where M = Mn, Fe, Co, Ni, or Cu and L = NH3, pyridine, PMe3, or an N-heterocyclic carbene. Multiple spin states and tetrahedral or square-planar starting geometries were screened using PBE0-D4 and TPSSh-D4, followed by large-basis single-point calculations and QTAIM analysis. Mn, Fe, Co, and Cu show consistent spin and geometry assignments across both functionals, whereas Ni displays a ligand-dependent d8 spin/geometry crossover. Targeted CAS(8,5)-FIC-NEVPT2 calculations indicate near-degenerate singlet and triplet structures for Ni–NH3, favor singlet square-planar structures for Ni–PMe3 and Ni–NHC, and strongly favor the triplet distorted structure for Ni–pyridine. Across the full series, ρBCP, HBCP, delocalization indices, Mayer bond orders, normalized M–L distances, and spin populations reveal related but non-equivalent trends. PMe3 and NHC show comparable basin sharing and normalized bond contraction but differ in how their bonding is represented by local and population-derived descriptors: NHC is emphasized by ρBCP and HBCP, whereas PMe3 is emphasized by Mayer bond order. These results show that first-row metal–ligand covalency is neither a transferable ligand constant nor a single scalar property, but a state- and metal-dependent bonding response better represented by a multidimensional descriptor fingerprint.
Source: ChemRxiv
Authors: Adam Herrera, Dimarco Mikael, Steven M. Maley
Institution: Wilfrid Laurier University
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7220451694