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Abstract. The concept of ligand coupling is explained and the actual examples of many important reactions in which not only sulfur and phosporus centered hypervalent species, but iodine, silicon and copper centered hypervalent ones are presented. It was also mentioned that many other reactions in which the central metal atoms in the nickel, triad elements are considered to behave as the catalytic site for ligand coupling reaction, such as the Waker process and the Heck reaction.Ever since we found the following reaction,(l) we have extended our concept of ligand coupling(2) and have so many examples of these or similar reactions. However, there were prior many reactions to enable us to postulate the concept. A typical example of a three-centered, four electron bond, called a hypervalent bond by Musher,(3) can be found in the substance in the structure of quternary florinated sulfur as demonstrated by X-ray analysis by Rundle et a1. (4) and Pimentel, (5) in the following figure. (Fig. 1 -tcThe central atom in a hypervalent species is valence-shell-expanded and tends to to extrude one pair of electrons to the normal valency of an octet. This is the driving force of the reaction and there are three conceivable ways for hypervalent species to collapse to form stable octet species; self-decomposition as exemplified by the wellstudied Wittig reaction,( 6)ligand exchange, (7,8) and ligand coupling.(9-13) It is well known that the equatorial ligands form sp2 and the axial linkages assume 805

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