George - 0. Kouffman
California State University Fresno, 93710
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Crystals as Moletular Compounds Paul Pfeiffer's application o f coordination theory to crystallography
Crystallography and stereochemistry have always been closely related (I). In fact, Louis Pasteur's resolution of potassium and ammonium tartrates (2) led to Le Bel (3) and van't Hoffs (4) proposal of the tetrahedral carbon atom in 1874, which is considered to mark the foundation of stereochemistry. As early as 1877, August KekulB, in his Rektoratsrede at the Universitat Bonn, expressed the view that the forces of crystal structure are identical with chemical valence forces and that a close relationship exists between molecules of higher order (in modern terminology, coordination compounds) and crystals (5) It must be assumed that the atoms which are united in a molecule and are therefore saturated with respect to their valence exert an attraction not only upon one another but also on the atoms of neighboring molecules and that in this way there occurs a molecular attraction which is caused by the attraction of the individual atoms and therefore is conditioned by their quality. Only in this way can one explain the process of chemical decompositions and the existence of that infinite number of complicated things which are viewed as molecular additions or as molecules of higher order. . . . since the attraction of the atoms is dependent upon their quality, it is clear that the molecular attraction caused by such atomic attraction must produce under suitable conditions an orientation of molecules joined to one another and must in this way lead to compounds of regular molecular structure, i.e., to crystals. Alfred Werner. the founder of coordination chemistry (6),'possessed a tremendous capacity for visualization and thinkine in terms of three dimensions: according to Frau Dr. ~ n n aElisaheth Ernst (nke ~ o r n j ,who re&ived her doctorate under Werner in 1905, he spoke as if he had actually seen atoms (7). Two years before proposing his coordination theory (8). Werner, in his Habilitationsschrift (1891), proposed that "affinity is an attractive force acting equally from the center of the atom toward all parts of its spherical surface" (9). From this definition he concluded that "separate valence units do not exist" and that valence is "dependent not upon one atom alone hut simultaneously upon the nature of all elementary atoms which combine to form the molecule"(9). In view of Werner's propensity for thinking in three-dimensional geometric terms, his ideas of valence just quoted, and his concepts of coordination numbers and their associated geometric configurations, it is surprising that he did not apply his coordination theory directly to the domain of crystallography. Yet he did not. For example, Werner apparently did not realize that the polynuclear complexes which he investigated so extensively (10) constituted a transition between the usual mononuclear coordination compounds and the infinite structure of the crystal lattice (11). Inasmuch as he knew that certain groups, expecially hydroxide, could coordinate with two metal atoms simultaneously to form bridges, it is possible that he might have considered the possibility of infinite structures with metal atoms bonded in this manner. In the case of phydroxo (01) bridges, this infinite olation process would result in the lattice structure of crystalline
metal hydroxide$, e.g., hydrargyllite, as later established by X-ray crystallography. However, Werner did not reach such conclusions, and it remained for Paul Pfeiffer (12), Paul Niggli (13), and others to point out that crystal structures were in beautiful agreement with his coordination theory. Pteitfer's Contributions
Inasmuch as little biographical information on Paul Pfeiffer is available in English (14), this may he an appropriate spot to present a brief, thumbnail sketch of his life and work. Born at Elberfeld in the German Rhineland on April 21, 1875, Pfeiffer first studied for two semesters at the Universitat Bonn under Friedrich August Kekul6 (1829-96) and Richard Anschutz (1852-1937) before entering the Universitat Zurich (1894), where he became Alfred Werner's best known student, prot6g8, and eventually "chief of staff." After receiving his doctorate in 1898 for a study of "Molekulverhindungen der Halogenide des 4-wertigen Zinns und der Zinnalkyle" (Molecular Compounds of Halides of Tetravalent Tin and of Tin Alkyls), he studied for one semester each with Wilhelm Ostwald (18531932) at Leipzig and Arthur Hantzsch (1857-1935) at Wurzbure. On Aueust 14. 1901 he married his cousin Julie ~ u t t e n h i f f . ~ i t gthe' acceptance of his Habilitationsschrift. "Beitrag zur Chemie der Molekulverhindungen" ' ( ~ o n t r i h u t i k to the Chemistry of Molecular Compounds), in 1901 he became Priuat-Dozent at the Universitat Zurich, where in 1908 he was appointed Extraordinarius Professor of theoretical chemistry. In 1916, as a result of personal and political conflicts with Werner, Pfeiffer left Zurich for the Universitat Rostock even though Werner was ill at the time and Pfeiffer was certain to be appointed as his successor. In 1919, Pfeiffer moved to the Technische Hochschule a t Karlsruhe. Three years later he was appointed to the directorship of the Chemical Institute at the Universitat Bonn, the chair formerly occupied by Kekul6. Here he remained until his retirement in 1947. He died on March 5,1951 at Bonn. Pfeiffer's work encompassed both inorganic and organic chemistry as well as the borderland between these areas. As the chemical and intellectual (hut not academic) successor to Alfred Werner, his main interest was in the field of coordination compounds, particularly those of chromium. He investieated their constitution, confieuration. isomerism, acid-Lase and hydrolysis reactions, and thei; relationshi~sto double salts and salt hydrates. Furthermore, he studied both inorganic and organic tin compounds, inner complexes, metal organic compounds, and the chemistry of dyes. He was a pioneer in the field of halochromism-the formation of colored substances from colorless organic bases by the addition of acids or solvents. His contributions to pure organic chemistry include studies of cyclic compounds, quinhydrones, stilhene compounds, unsaturated acids, and the relationship of ethylene compounds to ethane and acetylene compounds. His monograph "Organische Molekulverbindungen" (Organic Volume 50, Number 4. April 7973
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Molecular Compounds) (Verlag Ferdinand Enke, Stutt-earti - ~ -~.first anneared in 1922. and a second edition was published in i627. Pfeiffer was the first to point out the close relationship between Werner's coordination theory and the structure of crystals as revealed by the then new experimental technique of X-ray diffraction. In a short classic paper of 1915 (12~).translated in its entirety in the following paper, he suggested that crystals he regarded as extremely highmolecular-weight coordination compounds, in which atbms act as cwrdination centers, about which further atoms group themselves in definite symmetrical relationshim. According to Pfeiffer, crystals are constructed according to the same structural-chemical and steric laws as coordination compounds. He regarded the forces holding together the atoms or groups of atoms in crystals as identical with the chemical forces operative in coordination compounds. He thus extended the coordination theory into areas in which it had previously been inapplicable. In his first paper, Pfeiffer dealt with sodium chloride, which he regarded as a high-molecular-weight coordination compound ( N ~ C I ) ,made up of equal amounts of [NaCls] and [ClNa6] units. He showed that in crystals of symmetrical compounds, the difference between Hauptualenzen (primary valencies) and Nebenualenzen (secondary valencies) disappears (Werner himself was never satisfied with his valence dichotomy and always regarded it as provisional (uorliiufig) and purely formal (15). In his second paper (12b), Pfeiffer extended his treatment to other crystals such as the diamond, zinc hlende (ZnS), fluorite (CaFz), copper, silver, intermetallic compounds, anhydrite (CaSOd, and calcite (CaC03). He showed that coordination centers could he groups of atoms as well as single atoms. Niggli (13) extended the coordination theory to more complex crystalline compounds. Within a few short years, the crystal structures of a numher of coordination compounds were determined by X-ray diffraction, e.g., (NH&[PtC16] (16); (NH&[SnCla] (17); KzIPtClel, [Co(NH~)eIClz, Ni(NHddCIz, Rhz[PdBra] (18) (coordination numher 6, octahedral); and Kz[PtCln], KzpdC14], and (NH4)z[PdC14] (19) (coordination numher 4, square planar). These investigations provided a complete and direct confirmation of Werner's views to support his indirect configurational proofs ohtained during the previous decade by resolution of optically active compounds (20). For example, in a compound such as K2[PtC16], three of Werner's structural units (octahedron, cube, and tetrahedron), were found; the Pt4+ ion is surrounded octahedrally by six chloride ions, forming the ptCIe]Z- ion, which acts as a coordination center and is surrounded cubically by eight K+ ions, while the K+ ions group the [PtC161z- ions tetrahedrally around themselves. Today the terminology and concepts of coordination theory are routinely used in crystallography (21, 22). The symbiotic relationship between Werner's coordination theory and crystallography has been well expressed by Ralph W. G. Wyckoff, one of the pioneers in crystallography (22) Concerned with the distribution of atoms in moleeulsr complexes and coming not long before the discovery of X-ray diffraction, it [Werner's coordination theory] was particularly important for those of us who were then beginning crystal analysis. This analysis, in establishing for the first time exactly where the atoms are in a solid, offered the mast direct check imaginable of how
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correct Werner's notions about valence were, and, conversely, the ideas about coordination arising from this theory could suggest many compounds that it would he profitable to examine with Xrays (22). As I have pointed out elsewhere (6), Werner's personality was such that he was unable to allow his former student Pfeiffer the degree of independence that Pfeiffer required in order to develop his own scientific career, and during Werner's lifetime, the reputation of the teacher naturally outshone that of the student. In the application of Werner's coordination theory to crystallography, however, Pfeiffer made a definitive-contribution that provided the impetus for much important experimental and theoretical .work. I t would he unfortunate if this contribution went unrecognized, and it is with a view to rescuing it from the neglect which it has suffered that the following translation of this minor classic of coordination chemistry is offered. Acknowledgment The author wishes to acknowledee the financial assistance of the John Simon Guggenh2m Memorial Foundation for a Guggenheim Fellowship, the California State University, Fresno for a sabbatical leave, and the California State University, Fresno Research Committee. Literature Cited 11) Berry, A. J . . "Madem Chemistry: Some Sketches of Its Historical Development." Cambridge Univprsily Press, Cambridge. 1948. pp. 89-91. 121 Pssteur. L., Campr. rend., 26.535(1848l. (31 Le Bel. J. A , Bull. Soc. Chim. Fmnce. 121 22. 337 (18741. For an English translsfion 3- Benfoy. 0. T.. "Clmsics in the Theory of Chemical Combination." Dovc~Puhlicatians,Ine.. New York, 1363, pp. 161-71. (4) van'f Holf. J. H.. "Vmrstel tot uifbreidung der trpnwmrdig in de schoikunde gebruikfe sfructuurformules in de mimte." J. Greven, Utrecht. 1871. "La Chimie dans iW~paee."P. M . Bazandijk. Rotterdam. 1875. For an English translation see Benfey, 0. T.. t classic^ in the Theory of Chemicsl Combination." Dover Publicstions. Inc., New York. 1963. p p 151-60. (5) Werner, A,. "Newre Anrhsuungen m i d e m Gcbiete der enorganischen Chemie," re,. by P. Pfciifer, Friedrieh Vieweg und Sohn. Braumchweig, 1923. p. 319. (61 Keuffman, G. B., "Alfred Werner. Founder of Caordination Chemistry," SpringerVerlag. Berlin-Heidelberg~NewYork,1966. (7) Ernst, A,, personal communication with G. B. KauEman. 1963. (8) Werner. A , 2. o w r e Chem., 3. 267-330 (1893). For an Eng1i.h translation with commantary se Kauffman, G. 5.. "Classia in Cmrdination ChemlsLry, Pall I. The Selected Papers of Alfred Werner." Dover Publieation% h e . . New Ymk,
