Nickel coordination compounds of the cyanamido squarate ligand. The crystal structures of the 2,4-bis(cyanamido)cyclobutane-1,3-dione dianion (2,4-NCNsq2−) and of three polymers with general formula [Ni(Him)x(H2O)4 − x(2,4-NCNsq) · yH2O]n (x = 2–4; y = 1–

Disodium and ditetraphenylphosphonium salts of 2,4-bis(cyanamido)cyclobutane-1,3-dione, Na2(2,4-NCNsq) · 2H2O (1) and (Ph4P)2(2,4-NCNsq) · 4H2O (2) have been synthesized and used to prepare three nickel(II) complexes: [Ni(Him)4(2,4-NCNsq) · 2H2O]n (3), [Ni(Him)3(H2O)(2,4-NCNsq) · H2O]n (4) and [Ni(Him)2(H2O)2(2,4-NCNsq) · 3H2O]n (5). Compounds 1–5 have been synthesized and characterized by electronic absorption and IR spectroscopies and by electrochemical studies. The structures of the salt 2 and of the complexes 3–5 have been determined by X-ray crystallography. The dianion of 2 presents a centro-symmetric square planar structure with two NCN functions deviated from the ring plane by an angle of 20.6°. Complexes 3–5 are polynuclear nickel(II) compounds containing 2,4-NCNsq2− bridging ligands. These ligands are coordinated by the nitrile nitrogen atoms of the cyanamido functions and lie in cis (complex 4) or trans (complexes 3 and 5) positions in the coordination sphere of the nickel(II). The magnetic properties of the three complexes have been investigated in the 2–300 K temperature range (μeff = 2.79–3.34 μB for 3, 2.76–3.25 μB for 4 and 3.14–3.46 μB for 5). Their redox properties are discussed and compared to those of the free 2,4-NCNsq2− dianion.