For the species RuHXP2(CO), IrH2XP2, and Cp*RuXP (P = bulky phosphine, X = halide or pseudohalide), both homometallic halide exchange ([M]XP + [M]YP’ reversible [M]YP + [M]XP’) and heterometallic halide exchange ([M]X + [M]’Y reversible [M]Y + [M]’X) are found to be quite rapid. In addition, hydride exchange occurs for RuHCl-(CO)P2 and RuDCl(CO)P’2, as well as for IrH2Cl(PtBu2Ph)2 and IrD2Cl(PtBu2Me)2. Exchange is generally faster for halides than for hydrides yet is much slower for the groups phenoxide, OSiPh3, and C2Ph. These equilibria favor the better donating halide being bonded to the less electron-rich metal center. A variable-temperature H-1 NMR study of the degenerate exchange Cp*Ru(PtBu2Me)Cl + Cp*Ru’(PtBu2Me)Br reversible Cp*Ru(PtBu2Me)Br + Cp*Ru’--(PtBu2Me)Cl establishes a second-order rate law with DELTAH(double-dagger) = 8.6 +/- 0.8 kcal/mol and DELTAS(double-dagger) = -20 +/- 3 cal/(mol K). These results clearly indicate the transient existence in solution of halide- and/or hydride-bridged dimers of monomeric metal complexes.