Capturing the Trans Influence in Low-Spin d(8) Square-Planar Platinum(II) Systems using Molecular Mechanics.

Molecular modeling of coordination complexes continues to present challenges for force field methods. Implicit or explicit treatment of the significant d electron effects is mandatory. Ligand field molecular mechanics is designed for coordination complexes by explicitly including the ligand field stabilization energy (LFSE) and it is applied here to model the trans influence in tetracoordinate Pt(II) complexes of general formulas PtX4, PtX3Y, cis-PtX2Y2, and trans-PtX2Y2, where X and Y are OH2, H(-), Cl(-), Br(-), PR3, SH2, NR3, and pyridine. Parameters have been developed within the Merck molecular force field using DFT structures and energies as reference data. Both geometric changes and relative energies are generally well-reproduced although PH3 and H(-) complexes show deviations. However, for phosphine complexes, replacing PH3 with PMe3 resolves all bar one of these. The LFSE associated with the low-spin d(8) configuration ensures planar coordination and provides an electronic connection between all the ligands, thus enabling a correct description of the trans influence. The parameters developed for NR3 and PR3 with R = H work well for R = Me and Et and, in agreement with experimental and/or DFT structures, display either a tetrahedral distortion or even ligand dissociation.