The cobalt(II) complex incorporating π-conjugated substituent, [Co(Naph-C2-terpy)2](BF4)2 (1;
Naph-C2-terpy = 4’-(2-naphthoxy(ethoxy))-2,2’:6’,2’’-terpyridine), exhibits an abrupt spin transition (ST)
behavior (cooperative factor C = 0.91) while its solvated product, 1·2MeOH, shows gradual spin crossover
(SCO) behavior (C = 0.49). Single crystal X-ray structural analyses demonstrated that the octahedral
coordination core [CoN6] in 1 shows larger distortion in both high-spin and low-spin states than solvated
1·2MeOH or another two derivatives, [Co(R-terpy)2](BF4)2 (R = 2-naphthyl (2), 9-anthracenyl (3)). The
respective distortion parameters (Σ) are compared with those for previously reported SCO cobalt(II) compounds.
The highly-distorted [CoN6] core in 1 (Σ = 126 in the HS state and 101.6 in the LS state) was stabilized
by strong intermolecular interactions and observed an abrupt ST behaviour.