The series of tetrakis(mu-N,N’-diarylformamidinato)dinickel(II), where the aryl is the phenyl modified with various substituent (X), is synthesized and thoroughly characterized. While both the coordination geometry and the UV-vis spectra remain essentially independent of X across the series, the one-electron oxidation potential, E(1/2)(Ni-2(5+)/Ni-2(4+)), correlates with the Hammett constant (sigma(x)) of substituents according to the equation : E(1/2)(X) - E(1/2)(H) = 114(8 sigma(x) (mV).