edited by MARYVIRGINIA ORNA. 0.S.U Coilege of New Rochelle New Rochelle, NY 10801
Ligand George B. Kauffman California State University. Fresno, CA 93740
William H. Brock Victorian Studies Cenlre, The University. Leicester, LEI 7RH. England
1929 a UIC commission on the nomenclature of inorganic chemistry adopted a proposal (8) containing detailed rules for naming coordination compounds using Stock numbers, but the term ligand was not used (9). In 1936 this commission (H. Remy, W. P. Jorissen, M. DelBpine, and F. Fichter, with Clarence Smith. the Enelish member, absent) drafted a oro-
K. A. Jensen Department of General and Organic Chemistry. University of Copenhagen. The H. C. mrsted Institute, DK-2100 Copenhagen, Denmark
Christlan Klixbiill Jwgensen
The German version, containinithe word &and, was pub. lished in 1940 (10).In translating this German r e ~ o r thow-
DCpartement de Chimie MinCr.de Analytique et AppliquCe de I'UniversitL., Section Chimie-Sciences 11. 121 1 Genhve 4. Switzerland
The term lieand is now so commonlv in the field . emuloved - of coordination chemistry that it is usually given without definition. While editing an encyclopedia article on coordination chemistry hy ~ a i f f m a n( j ) , rock became interested in the origin and dissemination of the term and turned to a number of authorities active in the field. The results of our search should furnish chemical educators with a valuable case study of the adoption and acceptance of new terms into the language and literature of science. In the course of his now classic research on the boron hydrides at the Kaiser-Wilhelm-Institnt fiir Chemie in Berlin during World War I, Alfred Stock (1876-1946), the originator of the Stock system of nomenclature, began to examine the annloeons silicon hvdrides in the hooe that their volatile and highly r e x t i w proptrtit.s nutrht he UIIIIII i n 5 htw~calaarlnn. Thv h ~ i l o i Itis ircturc. 11, s v r i l ~ ~Ilk n ~rt>st.:ir< h - - ~ -~ ~ u l ~ l i sw.r:im on silicon hydrides was presented before t h e Kaiser-Wilhelm-Institut on November 27,1916. The puhlication contained a footnote in which the term ligand is used for the first time, hut not in its usual present context of coordination chemistry (2): Affinity is the expression far the firmness with whichone element hinds other elements or radicals (generally: "Ligands" (ligare [Latin], to hind); the introduction of a word hitherto lacking simplifies the manner of expression for this immediately clear concept). Valence (Velenz)means the unit of force which can hind a univalent ligand; positive valencies hind negative ligands, negative valencies hind positive ligands. I t is strange that Stock did not use the new term a t other places in this paper or in articles that he wrote later. It is equally strange that authors of standard monographs on coordination chemistry (3) did not use the new term. A Deutsche Chemische Gesellschaft commission on inorganic nomenclature (R. Lorenz, R. J . Meyer, S. Meyer, P. Pfeiffer, A. Rosenheim, and A. Stock) recommended in its published rules (4) the use of roman numerals (Stock numbers) to designate the oxidation states of elements, hut the term lzgand did not anoear in this oreliminarv reoort. Nine G a r s later (in ~ n r n b e ri ~ 9 2, i ) Stock submitted this proposal (4) at the Hanptversammlung des Vereins deutscher Chemiker. From 1927 on, the term ligand began to appear in the German literature. In 1929 it was used in the fourth edition of Ephraim's "Anorganische Chemie" (1929) in introducing the elements of Periodic Group IV. Ephraim cited Stock as its originator (5).By 1930 the word ligand was widely used in German publications (6).In 1938 it appeared in a Japanese journal in an English article on the spe&ochemical series by the Japanese coordination chemist RyWtarii Tsuchida (7). The Union Internationale de Chimie Pure et AppliqnAe was established after World War I, and after meetings in 1926 and ~~
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d e c k o n may have been influenced by the outbreak of World War I1 and bv the absence of the word in Enelish texthooks " (12), Patterson's "German-English Dictionary for Chemists" (131. and Chemical Abstracts. where it did not aooear until the sixth Collective Index (1957-61). Inasmuch a s ~ a s s e t t ' s translation was used in the American version of the report (14) with only minor amendments of spelling and footnotes, it is not surprising that the term was omitted in Scott's 1943review of inorganic nomenclature (15). There has been less reluctance, however, to introduce ligand into other languages directly from German usage or from the German version of the 1938Rules. In the mid-1930s, after first using ligand in a German paper on platinum complexes (161, Jensen introduced the term into Danish (17). On Jensen's suggestion, Jannik Bjerrum adopted the word in his dissertation of 1941 in the context of discussions of stepwise equilibria, e.g., a "step system consisting of a central group M and n ligands A" and "ligand effect" (18). Thus, together with Tsuchida, Bjerrum became the first to use ligand in an influential English puhlication. Although his use of the term was immediately accepted by some Swedish chemists, Englishspeaking chemists did not use it extensively until the 1950s. The long-neglected term ligand suddenly became so widesoread in Enelish for several reasons. One decisive factor W:I- (1 e t ~ l ~ ~ lthe v > .first post-\v.,r mweti~tg,131:i in I.~mdmut the rtvi\.td Iutt.r~u~tii,~~.tl I . l l l i u n ~retitlcdrhr lnrcrnatimal Union of Pure and Applied Chemistry, IUPAC) whose Commission on the Nomenclature of Inoreanic Chemistrv used ligand in a 1949 review paper (19). Foll~wingfurther meetings of the commission, an extended set of rules for inorganic nomenclature was prepared and presented as "~entativeRules" at a 1953 conference in Stockholm. Thus the new rules and the official definition of ligand were known four years before their final adoption hy the IUPAC Council a t the 1957 conference in Paris and oublication as "Nomenclature of Inoreanic Compounds li57" (20): In inoreanic eom~oundsit is eenerallv ~ossiblein a .oalvatomic .
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