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 = OMe_Bz, H_Bz, Br_Bz, Ph) bearing ethylene linkers between a single N and two P donors. The Co^II 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 Co^II 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 Ni^II 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 Ni^II complexes exhibit sharp ¹H and ³¹P spectra in the diamagnetic region. The Co ^II 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 Co^II equilibrium that is dependent on solvent and ligand substituent.