A novel bis(oxalato)chromium(III) salt of a ferromagnetically coupled, oxalato-bridged dinuclear chromium(III)-cobalt(II) complex of formula [CrL(ox)(2)-CoL'(H2O)(2)] [CrL(ox)(2)]center dot 4H(2)O (1) has been self-assembled in solution using different aromatic alpha,alpha'-diimines as blocking ligands, such as 2,2'-bipyridine (L bpy) and 2,9-dimethyl-1,10-phenanthroline (L' = Me(2)phen). Thermal dehydration of 1 leads to an intriguing solid-state reaction between the S = 3/2 Cr-III anions and the S = 3 (CrCoII)-Co-III cations to give a ferromagnetically coupled, oxalato-bridged trinuclear chromium(III)-cobalt(II) complex of formula {[CrL(ox)(2)](2)CoL'} (2). Complex 2 possesses a moderately anisotropic S = 9/2 (Cr2CoII)-Co-III ground state, and it exhibits slow magnetic relaxation behavior at very low temperatures (T-B < 2.0 K).