Resonance Effects\nfrom Substituents on L‑Type\nLigands Mediate Synthetic Control of Gold Nanocluster Frontier Orbital\nEnergies

The ligands of metal nanoclusters can be used to control\ntheir\nproperties and reactivity, but a framework guiding their use remains\nelusive. Hammett studies of Au<sub>8</sub>(PPh<sub>3</sub>)<sub>7</sub><sup>2+</sup> and Au<sub>9</sub>(PPh<sub>3</sub>)<sub>8</sub><sup>3+</sup> nanoclusters with <i>para-</i> and <i>meta</i>-methyl and -methoxy groups indicate that resonance effects, not\ninductive effects, yield quantitative shifts of the HOMO–LUMO\ntransitions involving orbitals local to the cluster core. Individual\nligand exchanges reveal that these shifts are caused by only four\nof seven ligands, inconsistent with inductive effects. Quantum chemical\ncalculations predict no trend in Au atom charges with respect to Hammett\nparameter but do predict bond length trends expected for a resonance\nstructure that includes the Au atoms. Computed orbitals show contributions\nfrom specific <i>para-</i>OMe oxygen lone pairs to the HOMO,\nindicating delocalization from the core to specific ligands. These\nresults suggest that resonance structures could be drawn including\nAu and ligands, guiding efforts to modulate nanocluster electronic\nstructure and energy transfer.