Volume II/26 supplements the previous compilations II/l, II/9 and II/17 of the magnetic properties of free radicals which were published in 1965, 1977–1980 and 1986–90. Due to the still rapid growth of the field and the necessary inclusion of new subjects the volume is divided into subvolumes which will appear in fast succession. Together with the earlier publications volume II/26 offers an up-to-date and comprehensive survey and collection of structures and data on the important chemical intermediates, namely radicals, polyradicals and related species such as carbenes, nitrenes, etc. As before the species have been grouped according to chemical aspects. The contents of the individual subvolumes are indicated on the inside of the front covers. For each group of substances the literature has been compiled and extracted by experts in the fields. A small overlap between the chapters is intentional and allows a maximum of coherence and comprehensiveness of the display. We wish to thank allthe authors for their careful and experienced work and the most agreeable cooperation, the Landolt-Börnstein office, especially Mrs. A. Endemann, for the layouts and the untiring and careful checking of manuscripts and galley proofs, and Springer-Verlag for the customary care in the preparation of the volume which is published without external financial support. Zürich, December 2003 The Editor Table of contents Magnetic Properties of Free Radicals Subvolume E: Phosphorus-Centered Radicals, Radicals Centered on Other Heteroatoms, Organic Radical Ions Part 2 I Introduction I General introduction [H. Fischer]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 A Definition and substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .