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 _(Bz)NP2 displays dual coordination modes: one in the tetrahedral complex [(_(Bz)NP2)Co(I)_2]; and the other in a square pyramidal variant, [(_(Bz)NP2)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 ^(31)P 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.