The Rh(III) species Rh(PCy3)(2)H2Cl is an effective catalyst (2 mol %, 298 K) for the dehydrogenation of H3B center dot NMe2H (0.072 M in 1,2-F2C6H4 solvent) to ultimately afford the dimeric aminoborane [H2BNMe2](2). Mechanistic studies on the early stages in the consumption of H3B center dot NMe2H, using initial rate and H/D exchange experiments, indicate possible dehydrogenation mechanisms that invoke turnover-limiting N-H activation, which either precedes or follows B-H activation, to form H2B=NMe2, which then dimerizes to give [H2BNMe2](2). An additional detail is that the active catalyst Rh(PCy3)(2)H2Cl is in rapid equilibrium with an inactive dimeric species, [Rh(PCy3)H2Cl](2). The reaction of Rh(PCy3)(2)H2Cl with [Rh(PCy3)H-2(H-2)(2)][BAr4F] forms the halide-bridged adduct [Rh(PCy3)(2)H-2(mu-Cl)H-2(PCy3)(2)Rh][BAr4F] (Ar-F = 3,5-(CF3)(2)C6H3), which has been crystallographically characterized. This clinuclear cation dissociates on addition of H3B center dot NMe2H to re-form Rh(PCy3)(2)H2Cl and generate [Rh(PCy3)(2)H-2(eta(2)-H3B center dot NMe2H)][BAr4F]. The fate of the catalyst at low catalyst loadings (0.5 mol %) is also addressed, with the formation of an inactive borohydride species, Rh(PCy3)(2)H-2(eta(2)-H2BH2), observed. On addition of H3B center dot NMe2H to Ir(PCy3)(2)H2Cl, the Ir congener Ir(PCy3)(2)H-2(eta(2)-H2BH2) is formed, with concomitant generation of the salt [H2B(NMe2H)(2)]Cl.