Aqueous reaction mixtures containing citric acid, guanidinium carbonate, and a range of metal cations (Mg
2+, Mn
2+,Fe
2+, Co
2+, Ni
2+, and Zn
2+) at room temperature give crystalline products of composition [C(NH
2)
3]
8[(M
II)
4(cit)
4]·8H
2O (cit = citrate). In all cases, the crystals are suitable for single-crystal X-ray diffraction studies, which revealthat the compounds are isostructural (space group
P4
2/
n;
a ~ 16.2 Å, and
c ~ 11.5 Å). As was intended, cubane-like [M
4(cit)
4]
8- complex anions are present. The individual citrate units are chiral, but each cubane unit containstwo of one hand and two of the other, related around an
S4 axis. The cubane units are involved in no less than40 H-bonding interactions with guanidinium cations and lattice water molecules. Detailed susceptibility andmagnetization studies show that the intracluster magnetic coupling within the Mn
II, Fe
II, Co
II, and Ni
II cubanes isvery weak in all cases with
J values of -0.82, -0.43, and -0.09 cm
-1 for the Mn, Fe, and Co species, respectively.A two-
J model gave the best agreement with the susceptibility and high-field magnetization data for the Ni
II case,over the whole temperature range studied, and the sign of the parameters,
J12 = -0.3 cm
-1 and
J13 = +2.97cm
-1, correlated with the two Ni-(
3-O)-Ni angles observed in the cluster structure. All members of the 3d-block[M
4(cit)
4]
8- family have spin ground states,
ST, of zero, with the higher
ST levels just a few reciprocal centimetersaway in energy.