The reaction of [ReBr(2)(MeCN)(NO)(P boolean AND P)] (P boolean AND P = 1,1'-bisdiphenylphosphinoferrocene (dppfc) (la), 1,1'-bisriiisopropylphosphinopherrocene (diprpfc) (1b), 2,2'-bis(diphenylphosphino)diphenyi ether (dpephos) (1c), 10,11-dihydro-4,5bis(diphenylphosphino)dibenzo[bf]oxepine (homoxantphos) (1d) and 4,6-bis(diphenylphosphino)-10,10-dimethylphenoxasilin (Sixantphos) (1e)) led in the presence of HSiEt(3) and ethylene to formation of the ethylene hydride complexes [ReBrH(eta(2)-C(2)H(4))(NO)(P boolean AND P)] (3a,b,d), the MeCN ethyl complex [ReBr(Et)(MeCN)(NO)(dpephos)] (5c) and two ortho-metalated stereoisomers [ReBr(eta(2)-C(2)H(4))(NO)(eta(3)-o-C(6)H(4)-Sixantphos)] 8e(up) and 8e(down). The complexes 3a,b,d, and 5c and the isomers of 8e showed high catalytic activity (TOFs ranging from 22 to 4870 h(-1), TONs up to 24000) in the hydrogenation of monosubstituted olefins. For 8e(down) and 8e(up) a remarkable functional group tolerance and catalyst stability were noticed. Kinetic experiments revealed k(obs) to be first order in c(cat) and c(H(2)) and zeroth order in c(olefin). Mechanistic studies and DFT calculations suggest the catalysis to proceed along an Osborn-type catalytic cycle with olefin before H2 addition. The unsaturated key intermediates [ReBrH(NO)(P boolean AND P)] (2a-e) could be intercepted with MeCN as [ReBrH(MeCN)(NO)(P boolean AND P)] (10a-d) complexes or isolated as dimeric mu(2)-(H)(2) complexes [{ReBr(mu(2)-H)(NO)(P boolean AND P)}(2)] (9b and 9e). Variation of the bidentate ligand demonstrated a crucial influence of the (large)-bite-angle on the catalytic performance and reactivity of 3a,b,d, 5c, and 8e.