We previously showed that the optical HOMO-LUMO gap of Au 8 (PPh 3 ) 2+ 7 and Au 9 (PPh 3 ) 3+ 8 could be systematically shifted by varying the substituent in para-position of the PPh 3 ligands, and that the magnitude of this shift was strongly correlated to the electron donating strength of substituent as measured by the Hammett para-parameter σ p . Furthermore, the shift was apparently magnified by the higher charge of Au 9 (PPh 3 ) 3+ 8 , which we hypothesized to indicate its higher electron withdrawing potential. Subsequent quantum chemical studies broadly reproduced the directions of these trends, but suggested a complex picture. These studies provided no insight into the underlying chemical principles mediating this control, however. The Hammett equation describes a linear free energy relationship that is defined in the context of the acidity of the carboxylic acid group of benzoic acid, 14 but it has been observed to correlate to a number of commonly encountered chemical observables.