Two bicyclic guanidinate ligands consisting of 5,5-membered (tbo) and 5,6-membered (tbn) rings have been used for the preparation of dimolybdenum compounds, such as Mo-2(tbo)(4) and Mo2(tbo)(4)Cl, and species containing Mo-2(tbn)(4)(n+) with n = 0-2. The compounds with quadruply bonded Mo-2(4+) species are strong reducing agents and have potentials of about -1V (vs Ag/AgCl) for the Mo-2(5+/4+) process. The structure of the THF solvate of Mo-2(tbo)(4) shows the longest Mo-Mo bond distance, 2.1453(4) angstrom, for a quadruply bonded species, and this is due to a large divergent angle induced by the geometry of the ligand. This distance increases to 2.2305(8) angstrom upon oxidation by CH2Cl2 to Mo-2(tbo)(4)Cl. For the 5,6- membered-ring ligand tbn, even though the divergent angle is large compared to formamidinate ligands, it is not as large as that in tbo, and the Mo-Mo distance in Mo-2(tbn)(4), 2.082(1) angstrom, is in the normal range for paddlewheel Mo-2(4+) compounds. This distance increases to 2.2233(8) angstrom upon oxidation by O-2 in CH2Cl2, which forms Mo-2(tbn)4Cl(2)(.)CH(2)Cl(2).