AbstractThree different silver melonates, Ag3[C6N7(NCN)3]·H2O (2a), Ag3[C6N7(NCN)3]·0.5NH3 (2b) and Ag3[C6N7(NCN)3]·2C2H8N2 (2c), and a nickel melonate, Ni3[C6N7(NCN)3]2·10NH3·6H2O (3), have been synthesised by the reaction of potassium melonate with silver nitrate either in an aqueous solution (2a), aqueous ammonia solution (2b) or an aqueous solution of ethylenediamine (2c) and with nickel nitrate in an aqueous ammonia solution (3). The crystal structure of 2c was determined by XRD [triclinic, P$\bar {1}$, a = 7.5499(4), b = 11.3438(6), c = 12.5741(7) Å, α = 72.138(4), β = 80.148(4), γ = 75.945(4)°, V = 984.90(9) Å3]. Silver atoms are coordinated to the terminal nitrogen atom of the cyanamide substituent as well as directly to the N atoms of the s‐heptazine core. Complex 2c crystallises in a layered structure. Adjacent Ag–melonate cation–anion units are connected by Ag–Ag interactions. The preferred coordination mode of metal ions at the melonate anion has been considered by quantum chemical calculations. Natural atomic charges calculated for the four nonequivalent, nucleophilic N atoms of the [C6N7(NCN)3]3– anion are (a) –0.596, (b) –0.619, (c) –0.675 and (d) –0.644. The metal coordination found experimentally in the melonates correlates with these relatively small charge differences and with the hard‐soft acid–base concept. However, (mono)protonation of the [C6N7(NCN)3]3– anion exclusively occurs at the terminal N atoms (b) of the s‐heptazine core, which is indicated experimentally and theoretically. Silver melonates 2a, 2b and 2c and nickel melonate 3 were further characterised by FTIR and 13C solid‐state magic‐angle spinning NMR spectroscopy, thermogravimetric/differential thermal analysis and elemental analysis. Solvent‐free complexes 2a and 2b, i.e. Ag3[C6N7(NCN)3], are thermally stable up to 500 °C. In contrast, 2c and 3 are thermally less stable.