Two heterometallic oxalate-bridged dinuclear anions associated with a non-H-bond donor bisamidinium cation (Cat2+), leading to compounds of formula Cat[MnII(H2O)4MIII(ox)3]2·6H2O (M = Fe (1) and Cr (2)), are presented. Their structural analysis reveals that the anion is the combination of a tris(oxalato)metallate(III) moiety with a tetra(aqua)manganese(II) entity. A 3D H-bonded network is formed between the crystallisation and coordination water molecules and the terminal and bridging oxalate ligands. The exchange interaction between both metal ions mediated by the oxalate bridge is −4.9 cm−1 for 1 and +1.6 cm−1 for 2 (H = −JS1S2).