1,2,4-triphospholyl nickel complexes: Evidence for a dimerization equilibrium that includes a sigma-pi rearrangement of the triphospholyl ligand

1-Trimethylstannyl-3,5-di(tert-butyl)-1,2,4-triphosphole (2) reacts efficie ntly with [(PPh3)(2)-NiCl2] or [(NO)(PPh3)(2)NiCl] by elimination of Me3SnC l and formation of 3,5-di(tert-butyl)1,2,4-triphospholyl (1) nickel complex es. [(eta (5)-t-Bu2C2P3)(PPh3)NiCl] (5) is a half-sandwich complex that is closely related to its carbacyclic counterpart [eta (5)-Cp(PPh3)NiCl]. The properties of the reaction product of [(NO)(PPh3)(2)NiCl] and 2, however, d iffer significantly from those expected for the proposed target product [(e ta (5)-t-Bu2C2P3)(NO)Ni] (7). In the solid state and at low temperatures in solution, the dimeric sigma -complex [mu,eta (1):eta (1)(t-Bu2C2P3)(NO)(PP h3)Ni](2) (6) is the only observable species and contains two additional PP h3 ligands. On warming the solutions, dimer 6 dissociates PPh3 reversibly t o form a mixture of free PPh3 and the piano-stool compound [(eta (5)-t-Bu2C 2P3)(NO)Ni] (7) as the ambient-temperature species. This temperature-depend ent and fully reversible sigma-pi rearrangement of the triphospholyl ligand 1 is unique for unsaturated P-heterocycles. The low-temperature complex 6 also exhibits an unusual coordination mode, as its topology is much more cl osely related to the N-heterocyclic Cp analogues pyrazolate and triazolate than to other P-heterocycles.