Citrate, in Collaboration with a Guanidinium Ion, as a Generator of Cubane-like Complexes with a Range of Metal Cations: Synthesis, Structures, and Magnetic Properties of [C(NH 2 ) 3 ] 8 [(M II ) 4 (cit) 4 ]·8H 2 O (M = Mg, Mn, Fe, Co, Ni, and Zn; cit = Citrate)