Bond-Dissociation Energies to Probe Pyridine Electronic Effects on Organoplatinum(II) Complexes and Their Organogold(III) Analogues: Decoupling Bond Length, Bond Strength, and Trans Influence

Abstract Understanding how metal centers modulate ligand electronic effects is essential for rationalizing the reactivity of organometallic complexes. Herein, we report a systematic investigation of pyridine electronic effects in cationic cyclometalated platinum(II) complexes, [(N^C^N)Pt(pyrR)]PF6, bearing para-substituted pyridines spanning a broad range of donor abilities. Metal–ligand bond strengths were quantified experimentally through gas-phase critical energies (E0) obtained by tandem mass spectrometry and RRKM kinetic modeling and compared with DFT-calculated bond-dissociation energies (BDEs). While Pt–NpyrR bond lengths and coordination-induced 1H NMR shifts proved largely insensitive to ligand electronics, E0 values correlated closely with the pKa of the pyridinium ions, establishing E0 measurements as a sensitive probe of ligand donor strength. Energy decomposition analysis revealed that Pt–NpyrR bonding is dominated by electrostatic interactions and σ-donation, with only a minor contribution from π-backdonation due to sterically enforced pyridine twisting. Comparison with the isoelectronic Au(III) series [(C^C)Au(NHC)(pyrR)]PF6 showed similar M–NpyrR interaction energy despite distinct bonding characters, with increased Pauli repulsion offsetting the greater covalency of Au–NpyrR. QTAIM and IBSI analyses demonstrated that the Au–Ctrans bond is intrinsically weaker than its Pt analogue. These results highlight the value of combining experimental thermochemistry with theoretical descriptors to disentangle metal and ligand electronic effects.