a b s t r a c t Four isostructural heterometallic octanuclear complexes have been synthesized of general formulae [Co II 4 Ln III 4 (OH) 4 (chp) 10 (acac) 6 ]Ásolv (Ln = Y (1), Gd (2), Tb (3) and Dy (4); solv = 0.5MeCN for 1, 2 and 4 and Et 2 O for 3), where Hchp = 6-chloro-2-hydroxypyridine and acac À = acetylacetonate. A fifth complex of formula [Co II 11 Dy III 6 (OH) 14 (chp) 14 (piv) 8 (NO 3) 4 (MeCN) 4 ]Á2H 2 O (5) was also isolated using the Hchp ligand with pivalic acid as a co-ligand. X-ray crystallographic studies reveal that complexes 1–4 all dis-play a central planar butterfly motif consisting of the four Ln III ions, with the Co II ions ''capping'' around the periphery of the butterfly. Compound 5 displays a complex metallic core with an unusual disc-like motif capped by two non-planar butterfly units. DC magnetic susceptibility measurements for the {Co II 4-Ln III 4 } family of compounds (1–4) reveals that the diamagnetic Y analogue {Co II 4 Y III 4 } (1) displays antifer-romagnetic exchange interactions between the Co–Co ions. An overall decrease in the susceptibility (v M T) is then observed upon lowering the temperature for {Co II 4 Gd III 4 } (2) and {Co II 4 Tb III 4 } (3), while the {Co II 4-Dy III 4 } (4) complex displays a decrease before a small increase at the lowest temperatures measured, indi-cating a ferrimagnetic ground state. AC susceptibility measurements reveal an absence of any SMM behaviour for 1–3 but show the onset of slow magnetic relaxation below 4 K in the case of 4. The DC and AC behaviour of complex 5 {Co II 11 Dy III 6 } follows a similar profile to compound 4 revealing a small increase in v M T at low temperatures, with the observation of possible SMM behaviour as observed via low temperature 'tails' of peaks in the out-of-phase susceptibilities.