RELATIONSHIP BETWEEN THE BITE SIZE OF DIPHOSPHINE LIGANDS AND TETRAHEDRAL DISTORTIONS OF SQUARE-PLANAR NICKEL(II) COMPLEXES - STABILIZATION OF NICKEL(I) AND PALLADIUM(I) COMPLEXES USING DIPHOSPHINE LIGANDS WITH LARGE BITES

Nickel and palladium complexes of the type [M(L2)2](BF4)2 and [M(L2)(L2')](BF4)2 (where L2 and L2' are diphosphine ligands) have been synthesized. The lowest energy electronic absorption band for the nickel complexes decreases in energy as the bite size of the diphosphine ligand increases. Similarly, the half-wave potentials for the Ni(II/I) and Pd(II/0) couples become more positive as the bite size increases. Structural studies of [Ni(dppm)2](BF4)2 (where dppm is bis(diphenylphosphino)methane) and [Ni(dppb)2](PF6)2 (where dppb is 1,2-bis(diphenylphosphino)benzene) show that increasing the bite size of the diphosphine ligands results in larger tetrahedral distortions. [Ni(dppm)2](BF4)2 (C50H44B2F8NiP4) crystallizes in the monoclinic space group P2(1/n) with a = 11.158 (10) angstrom, b = 17.43 (2) angstrom, c = 12.849 (11) angstrom, beta = 111.38 (7)-degrees, V = 3228 angstrom3, and Z = 2. The structure was refined to R = 0.155 and R(W) = 0.201 for 1005 independent reflections with F(o) > 6-sigma-(F(o)). [Ni(dppb)2](PF6)2 (C74H64F12NiP6) crystallizes in space group P1BAR with a = 11.359 (6) angstrom, b = 13.960 (5) angstrom, c = 21.297 (7) angstrom, alpha = 84.69 (3)-degrees, beta = 84.79 (4)-degrees, gamma = 78.96 (3)-degrees, V = 3298 (2) angstrom3, and Z = 2. The structure was refined to R = 0.072 and R(W) = 0.085 for 2333 independent reflections with F(o) > 6-sigma-(F(o)). The tetrahedral distortion observed for [Ni(dppb)2](PF6)2 was modeled by using extended Huckel calculations. These calculations indicate that the observed distortion may have an electronic as well as a steric component. The calculations also allow rationalization of the electronic absorption spectra, electrochemical data, and the stability of the Ni(I) and Pd(I) complexes [Ni(dppp)2](BF4) (where dppp is 1,3-bis(diphenylphosphino)propane) and [Pd(dppx)2](BF4) (where dppx is alpha,alpha'-bis(diphenyl-phosphino)-o-xylene). Complexes containing the ligand dppm have a marked tendency to become five-coordinate, as indicated by the structural determination of [Ni(dppm)2(CH3CN)](PF6)2. The latter complex (C59H55F12NNiP6) crystallizes in space group P4/m with a = 19.363 (4) angstrom, c = 15.526 (4) angstrom, V = 5822 (2) angstrom3, and Z = 4. The structure was refined to R = 0.048 and R(W) = 0.062 for 3470 independent reflections with F(o) > 6-sigma-(F(o)).