The molecular and electronic structures of the members of the following electron transfer series have been determined by single crystal X-ray crystallography, temperature dependent magnetic susceptibility measurements, and UV-vis-NIR and electron paramagnetic resonance spectroscopy and verified by density functional theory calculations (DFT B3LYP): [Re((Me)bpy)(3)](n), [Re(tpy)(2)](n), [Re(Tp)(bpy)Cl](n) (n = 2+, 1+, 0, 1), and [Re(bpy)(CO)(3)](1+,0,1-) ((Me)bpy = 4, 4'-dirnethyl-2,2'-bipyricline; Tp- = tris-pyrazolylborate, tpy = 2, 21:6, 2"terpyridine). For each series we show that the average C-py-C-py bond length and the average C-N-chel bond distance vary in a linear fashion with the charge n of the N,N'-coordinated (bpy)(n) and N,N',N"-coordinated (tpy)" ligand. Consequently, the difference Delta between these two bond lengths varies also linearly with n. Delta is shown to be a useful single marker for the oxidation level of these two heterocyclic ligands (neutral, pi-radical anion, and dianion). In addition, we have synthesized and structurally as well as spectroscopically characterized the following complexes: [((cy)DAB(center dot))(ReCl3)-Cl-IV(PPh3)] [Re-III)Cl(PPh3)(2)] Cl 2, [Re-III(tpy(0))(2)Cl] (OTf)(2).center dot Et2O 8. There are no structurally significant (experimentally detectable) pi-back-bond effects of the neutral bpy or tpy ligands irrespective of the d(N) configuration (N = 0-7) of the central Re atom.