Reaction of the lanthanide metallocene allyl complexes, (C5Me5)2Ln(eta(3)-CH2CHCH2)(THF) (Ln = Ce, Sm, Y) with 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, Hhpp, forms a series of metallocene complexes, (C5Me5)(2)Ln(hpp) (Ln = Ce, Sm, Y) in which the (hpp)(1-) anion coordinates as a terminal bidentate ligand. Isomorphous structures were observed by X-ray crystallography regardless of the size of the metal. The acetonitrile adduct, (C5Me5)(2)SM(hpp)(MeCN), was also crystallographically characterized to provide an unusual pair of eight- and nine-coordinate complexes. The coordination mode of the (hpp)(1-) anion in these complexes is compared with that in other heteroallylic metallocenes like the caprolactamate (C5Me5)(2)Y(ONC6H10) and the dithiocarbamate (C5Me5)(2)SM(S2CNEt2), which was also structurally characterized.