Reaction of nickel(ii) thiocyanate and tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) afforded a 2p-3d-2p heterospin triad [Ni(phpyNO)2(NCS)2]. The compound crystallizes in the orthorhombic Pbcn space group. The whole molecule is crystallographically independent. The torsion angles around Ni-O-N-C2py are 26.8(4) and 27.3(4)° at 400 K, indicating appreciable orbital overlaps of the radical π* and nickel(ii) 3d x 2-y 2 /3d z 2 orbitals. In a low-temperature region, the torsion was enhanced, and the space group changed to monoclinic P21/c with a doubled asymmetric unit volume. The χ m T value was practically null below ca. 140 K and, on heating to 400 K, gradually increased and reached 1.30 cm3 K mol-1. A van't Hoff analysis suggests a spin transition at T 1/2 = 530(20) K. Density functional theory calculation reproduced ground S total = 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 T 1/2 located above the experimental temperature window.
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