The treatment of the diethyl ester of o-phenylenebis(oxamic acid) (opbaH₂Et₂) with 2/3 of an equivalent of MeNH₂ in MeOH does not result in the formation of the methyl ester of o-phenylene(N′-methyloxamide)(oxamic acid) (opooH₃Me, 1) in pure state, as reported previously. The colourless crude material formed by this reaction was confirmed to be composed of 1 (89% content), the dimethyl ester of o-phenylenebis(oxamic acid) (opbaH₂Me₂, 2, 6%), 1,4-dihydro-2,3-quinoxalinedione (3, 3%) and o-phenylenebis(N′-methyloxamide) (opboH₄Me₂, 4, 1%), respectively. The identities of 1–4 have been verified by IR, ¹H and ¹³C NMR spectroscopy as well as elemental analysis. In addition, the solid state structures of 1 and 2·2DMSO, respectively, were determined by single-crystal X-ray diffraction studies. Successive recrystallization of the crude material from MeOH and MeOH:THF (1:1), respectively, does not give pure 1, but a mixture of 1 and 2. It is shown further that out of this mixture pure bis(oxamato) complexes cannot be obtained, as previously reported. Instead, treatment of the mixture with Ni^II or Cu^II salts, followed by the addition of [ⁿBu₄N]OH, results in the formation of two mixtures of [ⁿBu₄N]₂[Ni(opba)] (5) and [ⁿBu₄N]₂[Ni(opooMe)] (6) as well as [ⁿBu₄N]₂[Cu(opba)] (7) and [ⁿBu₄N]₂[Cu(opooMe)] (8), respectively. The simultaneous formation of 5/6 and 7/8, respectively, has been verified by crystallization of the obtained mixtures and X-ray diffraction studies of the obtained single crystals. Co-crystallization of mixtures of 5/6 (99 mass%) and 7/8 (1 mass%), respectively, results in the formation of single-crystals of diamagnetically diluted 7 in the host lattice of 5 (7@5) accompanied by single-crystal formation of diamagnetically diluted 8 in the host lattice of 6 (8@6), as verified by EPR spectroscopy. It is finally shown that the ethyl ester of o-phenylene(N′-methyloxamide)(oxamic acid) (opooH₃Et, 9), a homologue of 1, can be obtained in pure state by the treatment of opbaH₂Et₂ with 5/6 of an equivalent of MeNH₂ in EtOH.