Cationic nickel(II) complexes with azine diphosphines—structural and electrochemical study

Nickel(II) complexes of azine diphosphine ligands, PR2CH2C(But)NNC(But)CH2PR2 (R=Ph, But, Pri, C6H11), were prepared for the first time by reactions of anhydrous NiX2 (X=Cl, Br, I) with ligands. Complexes are cationic, the counterion being either simple halide anion or, in two cases, 1/2[NiCl4]2−. The azine diphosphines are coordinated terdentately in (E,Z) configuration forming thereby a bicyclic ligand frame (five and six-membered ring) with diphosphine bite angles 160–165°. A balance of stereoelectronic factors governing square planar versus tetrahedral coordination of the ligands is suggested. Structures of four of the complexes with bulky ligands (R=But, X−=Cl−, Br−, I−; R=C6H11, X−=1/2[NiCl]2−) were determined by X-ray diffraction and qualitative Hückel MO calculations on model undistorted square planar [trans-NiX(NH2)(PH3)2]+ were carried out to elucidate the contributions of steric and electronic factors. Two reduction and two oxidation processes depending on the substituent R and the halide were identified from cyclic voltammetry data on the complexes.