Five acetate-diphenoxo triply-bridged CoII-LnIII complexes (LnIII = Gd, Tb, Dy, Ho, Er) of formula [Co(μ-L)(μ-Ac)Ln(NO3)2] and two diphenoxo doubly-bridged CoII-LnIII complexes (LnIII = Gd, Tb) of formula [Co(H2O)(μ-L)Ln(NO3)3]·S (S = H2O or MeOH), were prepared in one pot reaction from the compartmental ligand N,N′,N′′-trimethyl-N,N′′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylene triamine (H2L). The diphenoxo doubly-bridged CoII-LnIII complexes were used as platforms to obtain 1,5-dicyanamide-bridged tetranuclear CoII-LnIII complexes (LnIII = Gd, Tb, Dy, Ho, Er). All exhibit ferromagnetic interactions between the CoII and LnIII ions and in the case of the GdIII complexes, the JCoGd were estimated to be ∼+0.7 cm−1. Compound 3 exhibits slow relaxation of the magnetization.