Cy3PCuMe (1) undergoes reversible ligand redistribution at low temperature in solution to form the tight ion pair [Cu(PCy3)(2)][CuMe2] (3). The structure of 3 was assigned on the basis of (i) the stoichiometry of the 1 = 3 equilibrium, (ii) the observation of a triplet for the PCy3 C1 C-13 NMR resonance due to virtual coupling to two P-31 nuclei, and (iii) reverse synthesis of 1 by combining separately generated Cu(PCy3)(2)(+) and CuMe2- ions. Complex 1 and [Cu(PCy3)(2)][PF6] (5) coordinate additional PCy3 to form (Cy3P)(2)CuMe and [Cu(PCy3)(3)][PF6], respectively, while 3 does not. Complex 1, free PCy3, and (bipy)(2)FeEt2 (2) each initiate the polymerization of acrylonitrile. In each case, the polyacrylonitrile contains branches that are characteristic of an anionic polymerization mechanism. The major initiator in acrylonitrile polymerization by 1 is PCy3, which is liberated from 1. A transient iron hydride complex is proposed to initiate acrylonitrile polymerization by 2.