ba-1967-0062.ch009

9 Werner Coordination and Crystal Structure R A L P H W. G. W Y C K O F F

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Department of Physics, University of Arizona, Tucson, A r i z .

The

early interaction between Werner's coordination

theory and crystal structure is discussed in terms of the problems about valence that then prevailed.

\ V 7 e r n e r ' s theory of coordination must be counted one of the great steps forward i n our understanding of chemical combination. C o n cerned with the distribution of atoms i n molecular complexes and coming not long before the discovery of x-ray diffraction, i t was particularly impor tant for those of us who were then beginning crystal analysis. This analysis, i n establishingforthe first time exactly where the atoms are i n a solid, offered the most direct check imaginable of how correct Werner's notions about valence were, and, conversely, the ideas' about coordination arising from this theory could suggest many compounds that i t would be profitable to examine with x-rays. We can best understand this interaction between theory and experi ment i n the light of the actual problems of chemical combination to which coordination theory was then addressed. They were entirely different from those which occupy us today. Inevitably, valence theory has passed through a series of stages i n accord with the prevailing state of chemical knowledge. T h e first objective of chemistry was necessarily determining the composition of compounds. W i t h the adoption of the atomic theory, these compositions were expressed through formulas which stated the amounts of the elements found to be present. A s a second stage, these compositional formulas developed into structural formulas which sought also to express the relations of atoms to one another within a compound. The structural formulas of organic chemistry and the Werner type formulas we can writeforthe more complicated inorganic compounds are such sum mary statements of what has been learned about atomic associations. Nowadays, we know what these associations are or how to establish them experimentally. This has made it possible to move into the third, current stage which, as we all know, defines the forces responsible for the bondings we observe. 114 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


9.

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Crystal Structure

Although early chemical analyses told the proportions of different chemical elements i n a compound, they had little to say about atomic relationships. Structural chemistry and the ideas about valence it incor porates developed rapidly around the preoccupation with organic com pounds that began a century ago. Most of the elements entering into such compounds have fixed powers of combination and this fact, together with the idea of the tetrahedral carbon atom, set the form which valence theory had acquired by the end of the last century. For the inorganic chemist, however, this theory failed to explain two frequently encountered sets of phenomena: (a) the multiple valence of many elements other than carbon, oxygen, and hydrogen; and (b) the innumerable stable "addition compounds" formed from simple molecules. The multiple combining powers of many elements for oxygen and their very different valences referred to hydrogen and to oxygen were early recognized. Thus, it was seen that in H S sulfur was, like oxygen, divalent, but that in S 0 and S 0 it was tetra- and hexavalent. Similarly, chlorine was monovalent i n HC1 and N a O C l , but t r i - , penta-, and heptavalent i n chlorites, chlorates, and perchlorates. The multiplicity of valence thus expressed was an empirical fact of inorganic chemistry long before Werner propounded his theory of coordination (17). This theory stated it suc cinctly, but satisfying explanations emerged only later through electron theories of valence arising out of the facts of x-ray spectroscopy and the theory of Bohr. Coordination may be said to have had its beginnings i n the attempt to understand such simple "molecular compounds" as S 0 - H 0 and N H • HC1. Obviously there was little meaning to representing these com mon substances as molecules held together by some form of "secondary valence." Especially after the development of electrochemistry it was apparent that sulfuric acid should be expressed as 2

2

3

3

2

3

0 H

5

0

O

\

0

even though sulfur was hexavalent, and ammonium chloride was expressed as ^H H

V

_H

/

Cl \ H_

though nitrogen had a valence of five. One of the slightly more compli cated series of compounds used by Werner to illustrate his coordination theory included the numerous ammoniates and hydrates of platinic chloride. H e pointed out that among the ammoniates P t C l • 2 N H is not 4

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

3

WERNER CENTENNIAL

116

dissociated in aqueous solution, but that P t C l • 2 N H C 1 is. The latter does not, however, yield C l ~ anions; instead it dissociates into N H + and P t C l ~ . On the other hand, there are other ammoniates, P t C l - 3 N H and P t C l - 4 N H , which i n solution are dissociated to yield C l ~ anions and to include the platinum as part of complex cations. The proper ties of all these were expressed by him through writing their formulas as [ ( N H ) P t C l ] , (NH ) +[PtCl ] -, [ ( N H ) P t C l ] + C l - and [ ( N H ) P t C l ] C l j f . The characteristics of a large number of compounds of tetravalent platinum are well stated by such coordination formulas i n which a central platinum atom is always directly bound to six neighboring atoms. In some, water molecules replace ammonia molecules i n the foregoing for mulas; in others, six-fold coordination may be provided by atoms of cya nide, nitro, or oxalate radicals, or of relatively complex molecules such as thiourea, ethylenediamine, dimethylglyoxime, or acetylacetone. Werner not only noted that i n all these cases a [Pt • 6X] cluster behaved as an u n dissociated unit, even though the normal valence of platinum remained four, but that a similar six-fold coordination occurs around many other elements, including chromium, cobalt, iron, iridium, vanadium, and molybdenum. H e also pointed out that metallic coordination is not a l ways six. It is sometimes eight while, for such metals as beryllium and nickel and the nonmetallic boron and nitrogen, it is four. M a n y of the compounds for which coordination formulas were written show an isom erism analogous to that found among organic compounds; he explained this by saying that the coordinated radicals and atoms are distributed at the corners of polyhedra: an octahedron for six-fold and a tetrahedron for four-fold associations. 4

4

4

6

2

4

4


3

2

3

3

4

4

2

6

2

3

3

3

3

4

2

2+

Werner's book contains these essentially spatial formulas for hundreds of obscure as well as common inorganic compounds. A t the time, it was argued that the composition of many of these substances needed verifica tion and further study; nevertheless, the coordination formulas in his com pilation continue to this day to suggest useful experiments, and many of the properties they predict have since been verified. In evaluating the work of Werner we must never lose sight of the fact that the writing of meaningful formulas, and not an "explanation" of valence forces, was the goal within reach of the structural chemistry of his time. Though coor dination formulas did make understandable many of the new facts being brought to light by physical chemistry, they were not immediately accepted as representing the actual atomic associations. W i t h the discovery of x-ray diffraction and the opportunity this gave to determine exactly where the atoms are i n a crystal, there arose an unex pectedly direct way to ascertain the measure of reality behind the Werner theory and its implied equivalence of some primary and secondary bonds. As a graduate student majoring in inorganic chemistry, I chose to study the structure of sodium nitrate (18) as part of my doctoral thesis


9.

WYCKOFF

117

Crystal Structure

because it seemed important to find out how a pentavalent nitrogen atom was related to the oxygen atoms which, together with it, formed the nitrate ion. Due mainly to the insights of electrochemistry, we no longer wrote the formula of this salt as N a 0 - N 0 , but we still needed to discover if the three nitrogen-to-oxygen bonds were equivalent. I then chose a m monium chloroplatinate (19) as a crystal that should provide a clear-cut test of Werner coordination. Soon after this structure was worked out and published, Dickinson (9) published the atomic arrangement in the isomorphous chlorostannate, and similar confirmations of this type of structure were offered by Scherrer, Stoll (12, 18), and Bozorth (5). A l l these studies agreed i n showing that the six halogen atoms of the molecule formed part of a complex anion centered around the metallic atom. A l l six chlorine atoms i n ( N H ) P t C l , for instance, were crystallographically identical. They were equally dis tant from the metal atom, and hence there was no difference in the bonds they formed with it. Furthermore, chlorines were found to be at the corners of a regular octahedron having the platinum atom at its center. A more complete agreement with the predictions of the Werner theory could scarcely have been imagined. Subsequent studies of crystal structures provided, and are still supply ing, other confirmations. We showed early that the molecules of water and of ammonia i n most crystalline hydrates and ammoniates have the same association with a metallic atom as do the coordinated atoms and radicals of a complex ion. For example, [ N i - 6 N H ] C 1 (20) has the same crystal structure as ammonium chloroplatinate with [ N i - 6 N H ] i n place of [PtCl ]; and there are numerous hydrates, such as N i S 0 - 6 H 0 (1, 7), i n which the water molecules are octahedrally distributed about the metal atom. As an additional point it was later found, as Werner had predicted, that in N i S 0 • 7 H 0 (2) this octahedral coordination still exists, the seventh water lying elsewhere in the structure unattached to metal. It is impractical to list even representative examples of the different types of compound that crystal structure investigations have shown to be appropriately represented by coordination formulas; nevertheless the following, randomly chosen from among recent determinations, are typical. In contrast to the coordinated complex ions discussed above, crystals of compounds such as C o C l ( N 0 ) - 3 N H (15) and the two, cis and trans, forms of P t C l - 2 N H (8, 4, 8) are built of definite molecules that do not dissociate in solution. Among compounds of somewhat greater com plexity we have i n N i C l - 4 ( N H ) C S (6, 10) an octahedral coordination of nickel provided by the chloride ions and four sulfur atoms of the thiourea molecules; the same coordination is supplied i n ferric acetylacetonate (11) by two oxygen atoms from each of three acetylacetone groups and in nickel nitrate tris-ethylenediamine (14, 16) by the two nitrogens of each H N ( C H ) N H molecule. Results such as these have put the basic


2

2

5

4

3

2

3

6

4

4

2

2

4

2

2

2

3

3

2

2

2

2

2

2


2

6


118

WERNER CENTENNIAL

correctness of coordination theory beyond dispute; its formulation will remain permanently useful even though crystal structure methods have advanced until we no longer need rely on its predictions. The terminology of coordination has quite naturally found a place in our everyday descriptions of crystal structures. Thus, we refer to the coordination of an atom as n-fold, or more specifically as triangular, square, tetrahedral, octahedral, cubic, etc. Often it is convenient to go a step further and state the relation of these coordination figures to one another throughout the crystal, which can then be imagined as built up by their sharing of corners, edges, or faces. We have learned from the study of crystal structures that the coordination of an atom is often related to its characteristic combining power, i.e., its valence. This is illustrated by the square coordination of the platinum atom in its P t condition and by the fact that in the Pt + state the planar [PtCl ] ~ association is converted into a [PtCl ] ~ octahedron by adding two more chlorine neighbors on a line normal to the plane of the P t complex. It is significant that a square coordination for the P t and an octahedral association for the Pt + atoms are preserved whatever may be the four and six atoms that are coordinated. 2 +

4

6

4

2

2

2 +

2 +

4

On the other hand, we have found that there are many types of crystal in which the coordination is set, not by chemical valence, but by the relative sizes of an atom and its immediate neighbors—that is to say, by considera tions of close packing. This is the case both with simple ionic compounds and with compounds like hydrates where a metal atom and its enveloping water molecules are held together by dipolar attractions of some sort rather than by directed valence bonds. Thus, among their hydrates, the coordi nation about the small Be + is tetrahedral, that around the larger C a usually octahedral, while around the still larger B a it may be nine or more. Often we can decide from the experimental results if the observed coordination is caused by packing or directed valence. Valence must, for instance, be responsible for the different coordinations of platinum in its divalent and tetravalent compounds, as well as for the tetrahedral coor dination about Cu+ atoms as contrasted with the square, or distorted, octahedral coordination that occurs about C u . In contrast, the tetra hedral coordination of oxygen about silicon, which prevails throughout silicate chemistry, is to be expected whether or not a strongly directional character is attributed to the S i - 0 bond; evidently a knowledge of crystal structure does not always give the answer we seek. 2

2 +

2 +

2 +

W i t h the development of quantum mechanics, however, this knowledge is providing much of the experimental basis for explanations of the nature of chemical bonds. The first steps, taken by applying quantum mechanical ideas to the earlier electron theories of valence reinforced by the quantita tive measurements of spectroscopy, identified many bonds in terms of the electrons that could form them. Thus, hybridization of one s and three p electrons was shown to account for the tetrahedrally distributed four equal


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119

Crystal Structure

bonds (sp* bonds) of the carbon atom. Similarly, we have become accus tomed to attribute a square coordination to the presence of four sp d bonds obtained when one s, one d, and two p electrons are hybridized. Six equal bonds of the type sp d? can be constructed from the available valence electrons of many of the atoms and ions that show octahedral coordination, such as Pt + or Co ". We have, however, come to realize that it is often necessary to consider all the chemically active electrons of a molecule when seeking to account quantitatively for any one bond. The development of molecular orbital and crystal field theories is now making this possible. I t obviously does not lie within the province of this brief historical review to discuss this newest phase of valence theory, but in conclusion I do want to emphasize the essential role of Werner coordination in preparing the ground for it. It showed how to write realistic formulas for innumerable complex inorganic substances that were not previously comprehensible and thus has gained for itself a permanent place through the qualitative pictures i t supplied of molecular structure and the atomic associations that prevail i n crystalline solids. 2

z


4

Literature

24

Cited

(1) Beevers, C. A., Lipson, H., Z. Krist. 83, 123 (1932). (2) Beevers, C. A., Schwartz, C. M., Z. Krist. 91A, 157 (1935). (3) Bokii, G. B., Kukina, G. A., Porai-Koshits, M. A., Izv. Sektora Platiny Metal., Inst. Obshch. Neorgan. Khim. Akad. Nauk SSSR No. 29, 5 (1955). (4) Bokii, G. B., Porai-Koshits,M.A., Tishchenko, G. N., Izv. Akad. Nauk SSSR Otdel. Khim. Nauk 5, 481 (1951). (5) Bozorth,R.M.,J. Am. Chem. Soc. 44, 1066 (1922). (6) Cavalca, L., Nardelli, M., Braibanti, A., Gazz. Chim. Ital. 86, 942 (1956). (7) Corey, R. B., Wyckoff, R. W. G., Z. Krist. 84, 477 (1933). (8) Cox, E. G., Preston, G. H., J. Chem. Soc. (London) 1933, 1089. (9) Dickinson, R. G., J. Am. Chem. Soc. 44, 276 (1922). (10) Lopez-Castro, A., Truter,M.R.,J. Chem. Soc. (London) 1963, 1309. (11) Roof, R. B., Jr., Acta Cryst. 9, 781 (1956). (12) Scherrer, P., Stoll, P., Arch. Sci. Phys. Nat. 4, 232 (1922). (13) Scherrer, P., Stoll, P., Z. Anorg. Allgem. Chem. 121, 319 (1922). (14) Swink, L. N., Atoji, M., Acta Cryst. 13, 639 (1960). (15) Tanito, Y., Saito, Y., Kuroya, H., Bull. Chem. Soc. Japan 26, 420 (1953). (16) Watanabe, T., Atoji, M., Science (Japan) 21, 301 (1951). (17) Werner, A., "New Ideas on Inorganic Chemistry," Longmans Green, London, 1911. (18) Wyckoff, R. W. G., Phys. Rev. 16, 149 (1920). (19) Wyckoff, R. W. G., J. Am. Chem. Soc. 43, 2292 (1921). (20) Ibid. 44, 1239 (1922). RECEIVED June 7, 1966.


ba-1967-0062.ch009

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