Unusual {⋯HNC2O⋯HCnO}, n = 1 or 2, synthons predominate in the molecular packing of one-dimensional coordination polymers, {Cd[S2P(OR)2]2(3LH2)}n, for R = Me and Et, but are precluded when R = i-Pr; 3LH2 = N, N′-bis(3-pyridylmethyl)oxalamide. Issue 16 (28th March 2022)