New insights into the synthesis of high-nuclearity polycarbonatolanthanoid complexes have been obtained from a detailed investigation of the preparative methods that initially yielded the so-called lanthaballs [Ln(13)(ccnm)(6)(CO3)(14)(H2O)(6)(phen)(18)]center dot Cl-3(CO3)center dot 25H(2)O [alpha-1Ln; Ln = La, Ce, Pr; phen = 1,10-phenanthroline; ccnm = carbamoylcyanonitrosomethanide]. From this investigation, we have isolated a new pseudopolymorph of the cerium analogue of the lanthaball, [Ce-13(ccnm)(6)(CO3)(14)(H2O)(6)(phen)(18)]center dot Cl-3 center dot CO3 (beta-1Ce). This new pseudopolymorph arose from a preparation in which fixation of atmospheric carbon dioxide generated the carbonate, and the ccnm ligand was formed in situ by the nucleophilic addition of water to dicyanonitrosomethanide. From a reaction of cerium(III) nitrate, instead of the previously used chloride salt, with (Et4N)(ccnm), phen, and NaHCO3 in aqueous methanol, the new complex Na[Ce-13(ccnm)(6)(CO3)(14)(H2O)(6)(phen)(18)](NO3)(6)center dot 20H(2)O (2Ce) crystallized. A variant of this reaction in which sodium carbonate was initially added to Ce(NO3)(3), followed by phen and (Et4N)(ccnm), also gave 2Ce. However, an analogous preparation with (Me4N)(ccnm) gave a mixture of crystals of 2Ce and the coordination polymer [CeNa(ccnm)(4)(phen)(3)]center dot MeOH (3), which were manually separated. The use of cerium(III) acetate in place of cerium nitrate in the initial preparation did not give a high-nuclearity complex but a new coordination polymer, [Ce(ccnm)(OAc)(2)(phen)] (4). The first lanthaball to incorporate neodymium, namely, [Nd-13(ccnm)(4)(CO3)(14)(NO3)(4)(H2O)(7)(phen)(15)](NO3)(3)center dot 10H(2)O (5Nd), was isolated from a preparation similar to that of the second method used for 2Ce, and its magnetic properties showed an antiferromagnetic interaction. The identity of all products was established by X-ray crystallography.