The crystal structures of two phases (I) and (II) of the title compound have been determined by X-ray diffraction and refined by least squares to R 0.12 (I) and 0.064 (II) for 1 136 and 660 observed reflections. Crystals of both forms are triclinic, Z = 2, space group P(I)a= 20.821 (4). b= 9.119 (2), c= 8.700(2)Å, α= 91.41(2), β= 91.48(2), γ= 101.69(2)° : (II)a= 20.71(1), b= 9.036(5), c= 8.746(3)Å, α= 91.11(4), β= 91.19(4), γ= 104.29(5)°. Phase (II) with unit cell volume 30 Å3 less than for (I) appears to be produdced by X-ray-stimulated phase transition, which is accompained within the lattice by an ordering of one of the thiocyanate groups. The compound is a ‘crossover’ derivative and the central Co–N distances are longer (1.98 and 1.95 Å) than in the corresponding pseudo-isostructural bromide (1.89 Å) in which the proportion of the low-spin component is higher.