Dibromonitrosyl(dihydrogen)rhenium(I) complexes [ReBr2(NO)(PR3)(2)(eta(2)-H-2)] (1; R = iPr, a; Cy, b) and Me2NH center dot BH3 (DMAB) catalyze at either 90 degrees C or ambient temperature under 10 bar of H-2 the hydrogenation of various terminal and cyclic alkenes (1-hexene, 1-octene, cyclooctene, styrene, 1,5-cyclooctadiene, 1,7-octadiene, a-methylstyrene). Maximum turnover frequency (TOF) values of 3.6 x 10(4) h(-1) at 90 degrees C and 1.7 x 10(4) h(-1) at 23 degrees C were achieved in the hydrogenation of 1-hexene. The extraordinary catalytic performance of the 1/DMAB system is attributed to the formation of five-coordinate rhenium(I) hydride complexes (Re(Br)(H)(NO)(PR3)(2)] (2; R = iPr, a; Cy, b) and the action of the Lewis acid BH3 originating from DMAB. The related 2/BH3 center dot THF catalytic system also exhibits under the same conditions high activity in the hydrogenation of various alkenes with a maximum turnover number (TON) of 1.2 x 10(4) and a maximum TOF of 4.0 x 10(4) h(-1). For the hydrogenations of 1-hexene with 2a and 2b, the effect of the strength of the boron Lewis acid was studied, the acidity being in the following order: BCI3 > BH3 > BEt3 approximate to BF3 > B(C6F5)(3) > BPh3 >> B(OMe)(3). The order in catalytic activity was found to be B(C6F5)(3) > BEt3 approximate to BH3 center dot THF > BPh3 >> BF3 center dot OEt2 > B(OMe)(3) >> BCI3. The stability of the catalytic systems was checked via TON vs time plots, which revealed the boron Lewis acids to cause an approximate inverse order with the Lewis acid strength: BPh3 > BEt3 approximate to BH3 center dot THF > B(C6F5)(3). For the 2a/BPh3 system a maximum TON of 3.1 x 10(4) and for the 2a/B(C6F5)(3) system a maximum TOF of 5.6 x 10(4) h(-1) were obtained in the hydrogenation of 1-hexene. On the basis of kinetic isotope effect determinations, H-2/D-2 scrambling, halide exchange experiments, Lewis acid variations, and isomerization of terminal alkenes, an Osborn-type catalytic cycle is proposed with olefin before H-2 addition. The active rhenium(I) monohydride species is assumed to be formed via reversible bromide abstraction with the "cocatalytic" Lewis acid. Homogeneity of the hydrogenations was tested with filtration and mercury poisoning experiments. These "rhenium(I) hydride/boron Lewis acid" systems demonstrate catalytic activities comparable to those of Wilkinson- or Schrock-Osborn-type hydrogenations accomplished with precious metal catalysts.