The A mixed crystal compound [Co0.8Fe0.2(C16-terpy)(2)](BF4)(2) (2) (C16-terpy is 4'-hexadecyloxy-2,2':6',2 ''-terpyridine) with long alkyl chains was prepared by mixing [Co(C16-terpy)(2)](BF4)(2) (1), which exhibits unique magnetic behavior, and the diamagnetic iron(II) compound [Fe(C16-terpy)(2)](BF4)(2) (3). The long-lived metastable state in the frozen-in effect was observed for the first time in the spin-crossover cobalt(II) compound 2. Furthermore, relaxation from metastable to stable states was very slow because of a large structural transition resulting from the long alkyl chains.