The quadruply bonded molybdenum dimer [Mo-2{mu-eta(2)-(NPh)(2)CNHPh}(4)] (1), bridged by four triphenylguanidine anion (guanidino) ligands, has been synthesized and structurally characterized. The guanidino ligands greatly stabilize the oxidized forms of this complex, and it is reversibly oxidized to the red monocation and the blue dication at readily accessible potentials (E(1/2) = -0.05 and +0.85 V vs Ag/AgCl, respectively). The [Mo-2{mu-eta(2)-(NPh)(2)CNHPh}(4)](+) cation (2) has also been structurally characterized as its [BF4](-) salt showing that the Mo-Mo distance increases from 2.0839(9) to 2.2902(12) Angstrom on oxidation, an unprecedented increase for such systems and almost 4 times that previously observed for a [Mo-2](4+/5+) pair. The N-Mo-Mo-N torsion angle is also found to increase from 4.5 to 7.6 degrees on oxidation. The crystallographic data are as follows. 1 . 2.2Et(2)O : C84.8H86Mo2N12O2.2, monoclinic, P2/n, a = 13.127(3) Angstrom, b = 14.335(3) Angstrom, c = 22.752(3) Angstrom, beta = 90.434(13)degrees, Z = 2. 2[BF4](-). 3CH(2)Cl(2) : C79H70BCl6F4Mo2N12, monoclinic, P2(1)/c, a = 13.426(3) Angstrom, b = 23.693(7) Angstrom, c = 23.573(7)Angstrom, beta = 92.93(2)degrees, Z = 4.