4-Amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one (metamitron) and 4-amino-6-methyl-3-phenyl-1,2,4-triazin-5(4H)-one (isometamitron)

The title structures, both C₁₀H₁₀N₄O, are substitutional isomers. The N—N bond lengths are longer and the C=N bond lengths are shorter by ca 0.025 Å than the respective average values in the C=N—N=C group of asymmetric triazines; the assessed respective bond orders are 1.3 and 1.7. There are N—H...O and N—H...N hydrogen bonds in both structures, with 4-­amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one containing a rare bifurcated N—H...N,N hydrogen bond. The structures differ in their mol­ecular stacking and the hydrogen-bonding patterns.