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Abstract Until recently, phosphorus, arsenic, antimony and bismuth ligands were nearly always understood to be molecules such as R 3 E, R 2 E(CH 2 ) n ER 2 , and RC(CH 2 ) n ER 2 3 (E = P, As, Sb, Bi) in which the lone pair of the atom E functions as a 2e donor. Current research interests, however, increasingly involve substituent‐free E n ligands (n = 1–6), the various types of which are mainly accessible via EX 3 (X = F,Cl,Br,Ph), E (SiMe 3 ) 3 , (RAs) n , P 4 S 3 , As 4 S 4 , gray arsenic, and especially P 4 and As 4 ; these ligands can be stabilized in the coordination spheres of certain transition metal fragments. P 1 , As 1 and Sb 1 units with multiple M‐E bonds are found in bi‐ and trinuclear complexes; E 1 units can also be encapsulated (partially or completely) in cavities of a variety of metal cluster frameworks. Metallatetrahedranes with up to three E atoms are especially common. Besides E 1 units, the greatest variety of coordination modes is exhibited by E 4 ligands, which are found as intact tetrahedra and also as parts of chains, polycycles, cubes and a trigonal prism. The phosphorus and arsenic species cyclo ‐E n , isoelectronic to the carbocyclic (CH) n species, are suitable ligands for forming sandwich complexes (n =3, 4, 5) and triple‐decker sandwich complexes (n = 3, 5, 6). In addition to the wide variety of chemical reactions, remarkable parallels are found in organic chemistry as well as in solid‐state chemistry for many of these substance classes. Some of the molecules have received lively interest from theoretical chemists.
Otto J. Scherer (Mon,) studied this question.