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 <sub><b>R</b></sub><b>NP2</b> ligands (where <b>R</b> = OMe<sub>Bz</sub>, H<sub>Bz</sub>, Br<sub>Bz</sub>, Ph) bearing ethylene linkers between a single N and two P donors. The Co<sup>II</sup> complexes generally adopt a tetrahedral configuration of general formula [(N<u>P2</u>)Co(I)<sub>2</sub>], 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<sup>II</sup> complex derived from <sub><b>Bz</b></sub><b>NP2</b> displays dual coordination modes: one in the tetrahedral complex [(<sub>Bz</sub>N<u>P2</u>)Co(I)<sub>2</sub>]; and the other in a square pyramidal variant, [(<sub>Bz</sub><u>NP2</u>)Co(I)<sub>2</sub>]. In contrast, the Ni<sup>II</sup> complexes adopt a square planar geometry in which the <u>P</u>(Et)<u>N</u>(Et)<u>P</u> donors in the ligand backbone are coordinated to the metal center, resulting in cationic species of formula [(<sub>R</sub>NP2)Ni(I)]<sup>+</sup> with iodide as counterion. All Ni<sup>II</sup> complexes exhibit sharp <sup>1</sup>H and <sup>31</sup>P spectra in the diamagnetic region. The Co<sup>II</sup> complexes are high-spin (<i>S</i> = 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<sup>II</sup> equilibrium that is dependent on solvent and ligand substituent.