The I3C chemical shifts and a large collection of coupling constants have been measured for 169 N-substituted azoles with no other substituents on the ring. Simple additive models have been used to discuss both the chemical shifts and the coupling constants. The chemical shift of the substituent depends on the nature of the azole which can, accordingly, be classified into three to six families. Some structural problems (ringchain isomerism of 1-trifluoromethylsulphonyl-1,2,3-triazole, silylotropy and stannotropy) are also discussed.

The effects of substitution, lanthanide-shift reagents, solvent changes, and tautomerism were investigated on the 13C chemical shifts of pyrrole, pyrazole, imidazole, s-and u-triazole, and tetrazole. It was concluded that 1SC chemical shifts are of limited value to ascertain the positions of tautomeric equilibrium for rapidly interconverting azole tautomers.Several molecules in the five-membered N-heterocycle series have been investigated by means of their 13C nuclear magnetic resonance, but not in a systematic way: pyrrole as a neat liquid3 or in acetone;4 pyrazole in water,5 in acetone,4•6 and some of its N-substituted derivatives as neat liquids;6'7 imidazole in water,5•8 acetone,4 and ethanol;9 1,2,3-triazole in acetone;4 1,2,4-triazole in acetone4 or in dimethyl sulfoxide;10 and tetrazole in dimethyl sulfoxide.4 However, in none of these researches have the 13C nmr data been used to study tautomerism. Although a great deal of work has been done on the annular tautomerism of such azoles11 by various methods, often no definite conclusions can be reached. We will give attention here to the usefulness of 13C spectroscopy in the determination of tautomeric equilibrium constants compared with 1H or 14N spectroscopy.

The preparations and properties of ruthenium(II) complexes containing the bidentate ligand l-(2-pyridyl)-3,5-dimethylpyrazole, L, are described. The tris complex RuL32+ is shown to readily undergo photosubstitution in acetonitrile solution to produce RuL2(CH3CN)22+, which in the presence of other bidentate ligands undergoes thermal substitution of the coordinated acetonitrile. , 13C, and "Ru NMR spectra are reported for all the complexes. Proton and carbon-13 NMR reveal the presence of geometrical isomerism, where it exists; proton and ruthenium-99 NMR allow the evaluation of the percentage of these isomers. Ruthenium-99 NMR proves to be an excellent probe of electron density at the metal and is possibly useful to evaluate the amount of x-back-bonding depending on the ligands. Electronic spectral data and oxidation potential measurements are given for comparison with those of the well-known Ru(bpy)32+ complex but do not lead to consistent results.

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