A series of ruthenium complexes [ Ru( OAc)( dioxolene)( terpy)] having various substituents on the dioxolene ligand ( dioxolene) 3,5-t-Bu2C6H2O2 ( 1), 4-t-BuC6H3O2 ( 2), 4-ClC6H3O2 ( 3), 3,5-Cl2C6H2O2 ( 4), Cl4C6O2 ( 5); terpy) 2,2': 6'2 ''-terpyridine) were prepared. EPR spectra of these complexes in glassy frozen solutions ( CH2Cl2: MeOH = 95: 5, v/v) at 20 K showed anisotropic signals with g tensor components 2.242 > g(1) > 2.104, 2.097 > g(2) > 2.042, and 1.951 > g(3) > 1.846. An anisotropic value, < g > = g(1) - g(3), and an isotropic g value, < g > [( g(1)(2) + g(2)(2) + g(3)(2))/3](1/2), increase in the order 1 < 2 < 3 < 4 < 5. The resonance between the Ru-II( sq) ( sq = semiquinone) and RuIII( cat) ( cat) catecholato) frameworks shifts to the latter with an increase of the number of electron-withdrawing substituents on the dioxolene ligand. DFT calculations of 1, 2, 3, and 5 also support the increase of the Ru spin density ( RuIII character) with an increase of the number of Cl atoms on the dioxolene ligand. The singly occupied molecular orbitals ( SOMOs) of 1 and 5 are very similar to each other and stretch out the Ru - dioxolene frameworks, whereas the lowest unoccupied molecular orbital ( LUMO) of 5 is localized on Ru and two oxygen atoms of dioxolene in comparison with that of 1. Electron-withdrawing groups decrease the energy levels of both the SOMO and LUMO. In other words, an increase in the number of Cl atoms in the dioxolene ligand results in an increase of the positive charge on Ru. Successive shifts in the electronic structure between the RuII( sq) and RuIII( cat) frameworks caused by the variation of the substituents are compatible with the experimental data.