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Linus Pauling (1901-1994) with George B. Kauffinan (1930(Reproduced from Chemical & Engineering News. Copyright 1993 American Chemical Society.)
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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Chapter 5
Early Structural Coordination
Chemistry
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Linus Pauling† Linus Pauling Institute of Science and Medicine, 440 Page Mill Road, Palo Alto, CA 94306
The history of coordination chemistry may in a sense be said to have begun with the work of Werner. The early crystal-structure determinations by W. L. and W. H. Bragg showed that in crystals such as sphalerite, ZnS, there is tetrahedral coordination around both zinc and sulfur, and in crystals such as sodium chloride there is octahedral coordination about both the anion and the cation. The modern period may be said to have begun in 1921, with the determination of a crystal containing an octahedral complex by Wyckoff and of crystals containing tetrahedral and square planar complexes (1922) by Dickinson. Later developments include application of quantum mechanics, discussion of hybrid orbitals especially suited to bonding, and detailed interpretation of interatomic distances found by careful X-ray diffraction studies. Some of the substances that we now call coordination compounds have been known for hundreds of years. For example, potassium ferrocyanide, K4 [FeCN)6], was made by strongly heating a mixture of a nitrogenous material, iron filings, and wood ashes or potassium carbonate, eluting with water, and crystallizing the yellow substance (7,2). It was used as a source of hydrocyanic acid, Prussian blue, and other compounds containing cyanide. Until about 100 years ago coordination compounds were usually called double salts. The formula of potassium ferrocyanide was written as 4KCN»Fe(CN) . By the 1870s efforts were being made to assign structural formulas to coordination compounds. In his Nobel address, delivered in 1913 (3), Alfred Werner mentions some of the efforts that were being made, in particular those by the Swedish chemist C. W. Blomstrand and the Danish chemist S. M . J0rgensen. For example, the compound cobalt hexammine trichloride was assigned by Blomstrand a structure in which there were three chains attached to the cobalt atom, each chain consisting of two ammonia molecules and a chlorine atom. J0rgensen, on the basis of differences in chemical reactivity of the different ammine groups, assigned to this substance a structure with two short chains and one long chain, two chains with a single ammonia and a chlorine atom and a long chain with four ammonia molecules and a chlorine atom. These chains of ammonia molecules were suggested by hydrocarbon chains and were assumed to involve quinquevalent nitrogen atoms. 2
†Linus Pauling died on August 19, 1994.
0097-6156/94/0565-0069$08.00/0 © 1994 American Chemical Society In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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70
COORDINATION CHEMISTRY
The great step forward in coordination chemistry was taken over a period of years by Alfred Werner himself, beginning in 1893 in his classic work, "Beitrag zur Konstitution anorganischer Verbindungen" on the subject of the spatial arrangement of atoms in the ammonia complexes (4). He rejected the idea of directed primary valence bonds; instead, he suggested that the central metal atom exerts a spherically symmetrical force of attraction on surrounding atoms, a strong force for atoms in the first coordination shell and weaker forces for those in the outer coordination shells. He introduced the concept of coordination and coordination number (the number of atoms in the first coordination shell). He made extensive studies of complexes with coordination number 6. At first he distinguished between primary and secondary valences but later suggested that there was little or no difference between them. The observation that an octahedral complex with two ligands of one kind and four of another kind occurs in two forms characterized by different colors (essentially the same for corresponding complexes of tripositive chromium and cobalt) permitted a decision to be made between the octahedral and the trigonal prismatic coordination polyhedron for coordination number (ligancy) 6. Two isomers are expected for the octahedron, and three for the trigonal prism. Werner and his students culminated this effort by resolving pairs of chiral complexes, with the central atom chromium, cobalt, or rhodium and with two ethylenediamine groups and two chlorine atoms in the first coordination shell. In addition to octahedral complexes, Werner also discussed complexes in which the central atom has ligancy 4. Some of these complexes were found not to form isomers with composition M A 2 B2, while others formed pairs of isomers, cis and trans. The tetraligated complexes of bipositive palladium and platinum are in the second group, to which Werner assigned the square planar configuration, with the other ligancy 4 complexes being tetrahedral. The Werner-J0rgensen controversy and the supersession of the Blomstand-J0rgensen chain theory by Werner's coordination theory is discussed in more detail by George B. Kauffman in his paper, "Theories of Coordination Compounds: Alfred Werner's Triumph," in this symposium volume. In other complexes the central atom has ligancy 2 (as in the dicyanoargentate(I) anion), 8 (as in the octacyanomolybdate (IV) or (V) anion, or 9 (as in the enneahydridorhenate(VI) anion) (5). The last of these complexes was identified through the X-ray diffraction determination of the structure of the salt K2ReH9; the nine hydrogen atoms are at the single-bond distance from the rhenium (
