4- AND 5-COORDINATE NICKEL(2) COMPLEXES WITH MONODENTATE PHOSPHINES

Four- and five-coordinate nickel(II) complexes of phenyldimethylphosphine, C6H5P(CH3)2 (L), and pentafluorophenyldimethylphosphine, C8F5P(CH3)2 (LF), have been synthesized and characterized. Whereas L gives both square-planar and five-coordinate complexes, LF gives only square-planar [Ni(LF)2X2] complexes except for the five-coordinate [Ni(LF)3(CN)2]. The intensely red-orange [NiL3(CN)2] and [Ni(LF)3(CN)2] complexes are diamagnetic and nonelectrolytes in nitromethane. Both are assigned a trans-trigonal-bipyramidal structure on the basis of their electronic and vibrational spectra and a single-crystal X-ray study on [NiL3(CN)2], Two other pentacoordinate complexes, [NiL3I2] and [NiL′3(CN)2] (where L′ = (C6H5)2PCH3), were also isolated as solids. In contrast to the [NiL2X2] complexes, none of the [Ni(LF)2X2] (X = Cl, Br, I, NCS) complexes showed any tendency to coordinate a third pentafluorophenyldimethylphosphine molecule in dichloromethane solution. Electronic spectra indicate that the ligand field strengths of C6H5P(CH3)2 and C6F5P(CH3)2 are very similar in analogous complexes. Dissociation constants of [NiL3X2] complexes in dichloromethane solution showed that the stability of the pentacoordinate complexes is influenced by the anion and decreases in the order: CN > I > Br > Cl > N02 > NCS. The tendency to form pentacoordinate nickel(II) complexes is correlated with the electronegativity or polarizability of the anions rather than with their relative positions in the spectrochemical series. The reduced tendency of pentafluorophenyldimethylphosphine to stabilize five-coordinate complexes, as compared with phenyldimethylphosphine, is discussed from the viewpoint of electronic and steric factors. © 1969, American Chemical Society. All rights reserved.