A Rare Thermochromic Zwitterionic Nickel(II) Complex of the Bulky Alpha Ligand 1,2-Bis-(di-tbutylphosphinomethyl)benzene

The reaction of the bulky ligand 1,2-bis-(di-tert-butylphosphinomethyl)benzene with [Ni(DME)Cl2] affords the blue Zwitterionic complex containing a phosphonium group and an anionic nickel trichloride. The neutral and stable complex is of interest as it shows an apparently Zwitterionic structure, one counter-anion to the Ni(II) being delocalised into the backbone of the ligand. This complex decomposes in alcohols such as methanol and turns yellow. A discussion of the mechanism leading to the observed product is presented. In dimethylformamide, however, the complex exhibits thermochromic properties: an ambient temperature pale blue solution changes colour reversibly to yellow on cooling. The structures of the previously prepared molybdenum complex, [1,2-(C6H4-CH2PtBu2)2Mo(CO)4] and the diphosphine sulfide 1,2-(C6H4-CH2P(S)tBu2)2 are described for comparative purposes.