The borohydride complex Re+1H(eta(2)-BH4)(NO)-(PPh3)(2)] (lph) reacts with ethylene to yield Re+1H2(eta(2)-C2H4)(NO)(PPh3)(2)] (2ph) and triethylborane formed by ethylene hydroboration. Subsequent ethylene insertion into the Re-H bond of 2ph and uptake of another 1 equiv of ethylene led to the kinetically stable cis-hydrido-ethyl complex Re+1H(Et)-(eta(2)-C2H4)(NO)(PPh3)(2)] (3ph). 3ph was found to slowly reductively eliminate ethane. The rate of this process was; determined by quantitative NMR spectroscopy in the temperature range from 293 to 338 K, enabling calculation of the activation parameters (Delta H-double dagger = 68.7 kJ mol(-1), Delta S-double dagger = -94 J mol(-1) K-1; half-life time 1.8 h at 303 K). The reaction was found to follow firstorder kinetics in c(3ph) and is zeroth order in c(C2H4) and c(PPh3), ruling out preceding ligand dissociation. The presumptive intermediate Re-1(eta(2)-C2H4)(NO)(PPh3)(2)] could not be traced, since it rapidly reacted further with ethylene, furnishing the stable butadiene complex Re-1(eta(2)-C2H4)(eta(4)-C4H6)(NO)(PPh3)] (4ph) in 88% yield. This transformation of dehydrogenative ethylene coupling is suggested to involve the elementary steps of rhenacyclopentane formation from two coordinated ethylene ligands and then double C-H activation via beta-hydride shifts to generate the butadiene unit and formal H-2 elimination from the rhenium dihydride with concomitant triphenylphosphine elimination. An X-ray crystallographic study confirmed the spectroscopically derived pentacoordinate structure of 4ph.