o-Phosphinophenols 1 (P⌒OH ) react with equimolar amounts of Ni(PMe 3 ) 4 at low temperatures to give yellow Ni(0) complexes such as [(HO∼P)Ni(PMe 3 ) 3 ] 2a with only P coordination of the P⌒OH ligand. Oxidation of solutions of 1 and Ni(PMe 3 ) 4 by dioxygen leads to brown bis(o-phosphinophenolato-P⌒O)nickel chelate complexes 3a-d. Structure elucidation by NMR is consistent with a cis-square planar geometry for 3a-c and a trans-square planar solution structure of the tert-butylphenylphosphino derivative 3d. The geometric isomers were distinguished by different ranges of phosphorus coordination shifts and 31 P- 13 C-2 and 31 P- 13 C-1 coupling constants. In the solid state, 3d adopts also a cis-square planar geometry. The steric stress of the substituents causes anti-orientation of the tert-butyl groups at phosphorus (R,R and S,S diastereoisomers) and a significant distorsion (22°) of the planes of the two five-membered rings. With less bulky substituents the R,S and S,R diastereoisomers are preferred as in the cis-square planar complex 3c with syn-orientation of the two isopropyl and phenyl groups, respectively.
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Heinicke et al. (1999) studied this question.