The structures of two derivatives of Meldrum's acid, 2,2-dimethyl-5-[N-(1-pyrrolyl)aminomethylene]-l,3-dioxane-4,6-dione, ClIH12N204, (1), and 2,2-dimethyl-5-[N-(1-pyrazolyl)aminomethylene]-l,3-dioxane-4,6-dione, C10HIlN304, (2), which differ only at one position in the five-membered ring, are described.

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The structure determination of 1,1-dimethyl-5-phenyl-1,2,5-triazapentadienium picrate, C10H]aN~.C6H2N307-, illustrates the effects of two factors on the internal geometry of the 1,2,5-triazapentadienium system. The first is the presence at the 5 position of an aryl group, the second the existence of hydrogen bonding in the form of an N...O interionic contact of 2.784 (6)/~ between the cation and the phenolic O atom of the picrate anion. The triazapentadienium system is planar and the phenyl ring is twisted relative to it by only 1.4 °. Comparison is made with published structures containing the analogous cations with 1-aryl and 1,1,5,5-tetramethyl substituents.