The diimine platinum(II) ethylene hydride complex [((NN)-N-and)Pt(H)(ethylene)][BAr'(4)] (1, (NN)-N-and = [(2,6-Me2C6H3)NC(An)-C(An)N(2,6-Me2C6H3)], An = 1,8-naphthalenediyl, Ar' = 3,5-(CF3)(2)C6H3) was prepared by protonation of the diethyl complex ((NN)-N-and)PtEt2 with [H(OEt2)(2)][BAr'(4)]. The energy barrier to interchange of the platinum hydride with the olefinic hydrogens in 1 was determined to be 19.2 kcal/mol by spin saturation transfer experiments. Complex 1 initiates ethylene dimerization; the ethyl ethylene complex ((NN)-N-and)Pt(Et)(ethylene)(+) (2) has been identified as the catalyst resting state. Trapping of 1 by ethylene to yield 2 is a second-order process; kinetic studies suggest this occurs via trapping of a reversibly formed beta-agostic ethyl complex. Complex 2 has been isolated and characterized by X-ray crystallography. The barrier to migratory insertion of 2, the turnover-limiting step in catalysis, was determined to be 29.8 kcal/mol. The 1-butene hydride complex, ((NN)-N-and)Pt(H)(1-butene)(+) (3), is a key intermediate in the dimerization cycle and has also been isolated and characterized. Surprisingly rapid rates of degenerate associative exchange of free ethylene with bound ethylene in complexes 1 and 2 as well as the rate of degenerate exchange of free nitrile with bound nitrile in ((NN)-N-and)Pt(Et)(CH3CN)(+) are reported.