1. To save space, many formulae of the complexes appear in the review only once. If the formula of a complex cannot be found in the text, it is given in Table II at the end of the review.
2. Metal complexes of heterocycles II. N-pyrrolyl complexes of iron
3. This can be shown by comparing the energy levels of the highest occupied π-molecular orbital (HOMO) of pyrrole and of the substituted pyrroles containing electron-withdrawing (formyl, acetyl) or electron-donating substituents (methyl) as calculated by the HMO method. Examples: compound, HOMO (in β units): pyrrole, 0.618; 2-formylpyrrole, 0.709; 3-formylpyrrole, 0.653; 2-acetylpyrrole, 0.699; 3-acetylpyrrole, 0.649; 2-methylpyrrole, 0.450; 3-methylpyrrole, 0.529 (unpublished results; details concerning the calculations and additional quantum-chemical data are available upon request from the authors).
4. Metal complexes of heterocycles III. Aceẗylpyrrolyl and indolyl complexes
5. K. J. Coleman, C. S. Davies and N. J. Gogan, J. Chem. Soc., Chem. Commun., 1414 (1970).