In chloroform, [RuCl 2 (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-Me 4 -NUPHOS) to give the dimer [Ru 2 Cl 3 (η 4 -1,2,3,4-Me 4 -NUPHOS) 2 ]Cl (2a), whereas, in THF [RuCl 2 (1,2,3,4-Me 4 -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 C 1 symmetry and the minor with either C s or C 2 symmetry. A single-crystal X-ray analysis of [Ru 2 Cl 3 (η 4 -1,2,3,4-Me 4 -NUPHOS) 2 ] [SbF 6 ] (2b), the hexafluoroantimonate salt of 2a, revealed that the diphosphine coordinates in an unusual manner, as a η 4 -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 [RuCl 2 (1,2,3,4-Me 4 -NUPHOS)(en)] (4), which rapidly dissociates a chloride ligand in chloroform to give [RuCl(η 4 -1,2,3,4-Me 4 -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 η 2 -interaction and re-form 4. A single-crystal X-ray structure determination of [RuCl(η 4 -1,2,3,4-Me 4 -NUPHOS)(en)]-[ClO 4 ] (5b) confirmed that the diphosphine coordinates in an η 4 -manner as a facial six-electron donor with the η 2 -coordinated double bond occupying the site trans to chloride. The η 4 -bonding mode can be readily identified by the unusually high-field chemical shift associated with the phosphorus atom adjacent to the η 2 -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.