In chloroform, [RuCl2(nbd)(py)2] (1) (nbd = norbornadiene; py = pyridine) reacts with 1,4-bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-butadiene (1,2,3,4-Me4-NUPHOS) to give the dimer [Ru2Cl3(η4-1,2,3,4-Me4-NUPHOS) 2]Cl (2a), whereas, in THF [RuCl2(1,2,3,4-Me4-NUPHOS)(py)2] (3) is isolated as the sole product of reaction. Compound 2 exists as a 4:1 mixture of two noninterconverting isomers, the major with C1 symmetry and the minor with either Cs or C2 symmetry. A single-crystal X-ray analysis of [Ru2Cl3(η4-1,2,3,4-Me4-NUPHOS) 2] [SbF6] (2b), the hexafluoroantimonate salt of 2a, revealed that the diphosphine coordinates in an unusual manner, as a η4-six-electron donor, bonded through both P atoms and one of the double bonds of the butadiene tether. Compounds 2a and 3 react with 1,2-ethylenediamine (en) in THF to afford [RuCl2(1,2,3,4-Me4-NUPHOS)(en)] (4), which rapidly dissociates a chloride ligand in chloroform to give [RuCl(η4-1,2,3,4-Me4-NUPHOS)(en)] [Cl] (5a). Complexes 4 and 5a cleanly and quantitatively interconvert in a solvent-dependent equilibrium, and in THF 5a readily adds chloride to displace the η2-interaction and re-form 4. A single-crystal X-ray structure determination of [RuCl(η4-1,2,3,4-Me4-NUPHOS)(en)]-[ClO4] (5b) confirmed that the diphosphine coordinates in an η4-manner as a facial six-electron donor with the η2-coordinated double bond occupying the site trans to chloride. The η4-bonding mode can be readily identified by the unusually high-field chemical shift associated with the phosphorus atom adjacent to the η2-coordinated double bond. Complexes 2a, 2b, 4, and 5a form catalysts that are active for transfer hydrogenation of a range of ketones. In all cases, catalysts formed from precursors 2a and 2b are markedly more active than those formed from 4 and 5a.