Dual Coordination Modes of Ethylene-Linked NP2 Ligands in Cobalt(II) and Nickel(II) Iodides

Here we report the syntheses and crystal structures of a series of cobalt(II) and nickel(II) complexes derived from RNP2 ligands (where R = OMeBz, HBz, BrBz, Ph) bearing ethylene linkers between a single N and two P donors. The CoII complexes generally adopt a tetrahedral configuration of general formula [(NP2)Co(I)2], wherein the two phosphorus donors are bound to the metal center but the central N-donor remains unbound. We have found one case of structural isomerism within a single crystal structure. The CoII complex derived from BzNP2 displays dual coordination modes: one in the tetrahedral complex [(BzNP2)Co(I)2]; and the other in a square pyramidal variant, [(BzNP2)Co(I)2]. In contrast, the NiII complexes adopt a square planar geometry in which the P(Et)N(Et)P donors in the ligand backbone are coordinated to the metal center, resulting in cationic species of formula [(RNP2)Ni(I)]+ with iodide as counterion. All NiII complexes exhibit sharp 1H and 31P spectra in the diamagnetic region. The CoII complexes are high-spin (S = 3/2) in the solid state as determined by SQUID measurements from 4 to 300 K. Solution electron paramagnetic resonance (EPR) experiments reveal a high-spin/low-spin CoII equilibrium that is dependent on solvent and ligand substituent.