A series of five bicyclic guanidinate compounds containing various combinations of five- and six- membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six- membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo-2(4+) units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo-2(TEhpp)(4) complex which has a broad first ionization band centered around 4.27 +/- 0.03 eV and an ionization onset at the very low energy of 3.93 +/- 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo - Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (eta(5)-C9Me7)(2)Co and (eta(5)-C5Me5)(2)Cr.