Steric distortion of planar NiP2N2 chromophores: a spectral, cyclic voltammetric and single crystal X-ray structural study

The complexes trans-[Ni(4-MP)(2)(NCS)(2)]center dot MeCN (1) and trans-[Ni(3-MP)(2)(NCS)(2)] (2) (4-MP = tri(4-methylphenyl)phosphine, 3-MP = tri(3-methylphenyl)phosphine) were prepared and characterized by IR, UV-visible, NMR spectra, CV, TGA and single crystal X-ray crystallography. Both the complexes have planar geometry and are diamagnetic. The Ni-P distances in both complexes are relatively short as a result of strong back donation from nickel to phosphorus. The phenyl rings in the 3-MP analogue (2) show increased pitching with reference to the plane formed by the ipso carbons due to increased steric effects. For complex (2), the N-Ni-N and P-Ni-P angles are significantly lower than the almost linear N-Ni-N and N-Ni-P angles observed for both complex (1) and trans-[Ni(PPh3)(2)(NCS)(2)]. This observation indicates that the 3-methylphosphine ligand forces complex (2) to distort towards a tetrahedral geometry. IR spectra of both complexes show strong bands around 2,090 cm(-1) due to N-coordinated thiocyanate, while the electronic spectra contain d-d transitions around 452 nm. Cyclic voltammograms show that the irreversible one-electron reduction potentials increase in the following order: trans- [Ni(PPh3)(2)(NCS)(2)] < trans- [Ni(3-MP)(2)(NCS)(2)] < trans-[Ni(4-MP)(2)(NCS)(2)], revealing the electron releasing effect of the methyl groups. The planar complexes exhibit interallogony in coordinating solvents.