An indication of spin-transition accompanied by an order-disorder structural transformation in [Ni(phpyNO)(NCS)] (phpyNO = -butyl 5-phenyl-2-pyridyl nitroxide).

Reaction of nickel(ii) thiocyanate and -butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) afforded a 2p-3d-2p heterospin triad [Ni(phpyNO)(NCS)]. The compound crystallizes in the orthorhombic space group. The whole molecule is crystallographically independent. The torsion angles around Ni-O-N-C are 26.8(4) and 27.3(4)° at 400 K, indicating appreciable orbital overlaps of the radical π* and nickel(ii) 3d /3d orbitals. In a low-temperature region, the torsion was enhanced, and the space group changed to monoclinic 2/ with a doubled asymmetric unit volume. The value was practically null below 140 K and, on heating to 400 K, gradually increased and reached 1.30 cm K mol. A van't Hoff analysis suggests a spin transition at = 530(20) K. Density functional theory calculation reproduced ground = 0 with singlet-triplet gaps of 910 and 1263 K for the 140 K structure, and the gap was reduced to 297 K at 400 K. Consequently, the present compound can be considered as an incomplete spin-crossover material, as a result of located above the experimental temperature window.