Room temperature reaction of the bis(dihydrogen) complex RuH(2)(H(2))(2)(PCy(3))(2) (1) with excess pinacol borane (HBpin) generates the novel complex RuH[(mu-H)(2)Bpin](sigma-HBpin)(PCy(3))(2) (2) by loss of dihydrogen. Complex 2 was characterized spectroscopically and by X-ray crystallography. It contains two pinacolborane moieties coordinated in a different fashion, one as a dihydroborate (B-H distances : 1.58(3) and 1.47(3) A) and the other as a sigma-borane (B-H distance: 1.35(3) A). In addition, reaction of 1 with one equiv of HBpin yields total conversion to a new complex tentatively formulated as RuH[(mu-H)(2)Bpin](H(2))(PCy(3))(2) (3) on the basis of NMR data. In the presence of excess HBpin, 3 is converted to 2. Furthermore, under an atmosphere of dihydrogen, a C(7)D(8) solution of 2 rapidly converts to 3 and finally regenerates 1 over a much longer period. Thus, complex 3 is an intermediate in the formation of 2 from 1. In these processes the borane is eliminated as HBpin later hydrolyzed to BpinOBpin. Selective hydroboration of ethylene (3 bar) into C(2)H(5)Bpin is achieved using 1 or 2 as catalyst precursors in toluene, whereas in THF, competitive formation of the vinylborane C(2)H(3)Bpin (56% under 20 bar of C(2)H(4)) can be favored.