FIVE-COORDINATION WITH “HYBRID” LIGANDS. PART VIII: Cobalt(II) and Nickel(II) Complexes with an Open-Chain Tetradentate Nitrogen-Phosphorus Ligand

The open-chain tetradentate ligand N,N′-bis(2-diphenylphosphinoethyl)-N,N′-dimethylethylenediamine, (PNNP), forms square planar, five-coordinate and octahedral cobalt(II) and nickel(II) complexes. The five-coordinate complexes, having the general formula [M(PNNP)X]Y (X = halogen, pseudohalogen; Y = Br, I, ClO4, BPh4, PF6) are low-spin and square pyramidal in structure, whereas the corresponding cobalt(II) complexes with the tripod ligand having the same N2P2 donor set are high-spin and have a distorted trigonal bipyramidal structure.5b It is suggested that square pyramidal symmetry favours the cobalt(II) low-spin ground state.