A RULE of COORDINATION RICHARD F. ROBEY? Koselle, New Jersey
I
N THE elementary qualitative chemical analysis course, quite a number of so&lled complex inorganic compounds such as Cu(NHa),SOa, &Co(NO&, and &Fe(CN)a are usually discussed, for this type of compound plays a rather important part in many analytical schemes. The modern texts in qualitative analysis usually present a portion of Werner's theory of coordination valency in order to explain the existence of these compounds and complex ions in general. Some go further and use the theory in an explanation of hydrolysis and amphoterism of hydroxides. In most cases, however, no connection of coordination valency with the theoretical concepts of atomic structure and primary valency is developed. As for the student the recalling of the coordination number of a given cation after it has been discussed is much more a test of memory than of ability to apply general principles already acquired. Teachers of chemistry hear complaints that elementary chemistry courses require too much memorization, and, although i t might be argued that these complaints are not justified, it seems desirable to reduce the amount of material to be committed to memory. STABLE ELECTRONIC CONFIGURATIONS
Ordinarily it is taught that a polar compound is formed as the result of a gain or loss of electrons by reacting atoms, the outer electrons of the resulting ions tending to assume the stable configurations possessed by the inert gases or some other stable electrodynamic system. In sodium chloride, to take a simple example, the sodium atom loses an electron, the chlorine gains one; the resulting exterior electronic constitution of these ions is similar to that of the inert gases neon and argon, respectively. It is then stated that the "effective atomic numbers" (E.A.N.)of these respective ions and inert gases are numerically equal-Na+ and Ne, 10; C1- and A, 18-where the effective atomic number is defined as the total number of electrons outside the nucleus but directly associated with an ionic or combined atom. Coordination, on the other hand, is a form of cop
1 This paper is one of a series presented in participation in a recent general endeavor to modernize the general course in elementary chemistry. For other articles in this series, see literature references (I), ( Z ) , (3), and (4). * F m e r l y assistant (instructor) in general chemistry, The Ohio State University, now research chemist in the Chemical Laboratories of the Standard Oil Development Co., Elizabeth. New Jersey.
valency in which one of the two atoms united together contributes both electrons of the non-polar covalent bond. This results in an increase of the E.A.N. of one atom (the acceptor), while it leaves that of the other (the donor) unchanged. Thus the reaction between ammonia and a hydrogen ion may be written.
The nitrogen of the ammonia has an E.A.N. which remains unchanged in the reaction while it contributes its unused or "lone pair" of electrons to form the bond with hydrogen. The E.A.N. of the hydrogen ion is thereby increased from 0 to 2 and is equal to that of the inert gas, helium, in the resulting ammonium ion. As another example, the zinc atom which has an E.A.N. of 30 loses two electrons on oxidation to give the zinc cation, Zn++ of E.A.N. 28. The twenty-eight electrons form a stable system which is often termed a pseudo-inert gas configuration. Now, it is known that the zinc ion combines with the four molecules of ammonia to form a complex ion, Zn(NH&++. Since each NHa molecule is the donor of two electrons to the covalent linkages, the E.A.N. of the zinc ion increases from 28 to 36. The inert gas krypton has an atomic number of 36.
A sufficientnumber of the examples can be cited to indicate an apparent generalization, namely, that atoms and their simple ions tend to gain electrons by coordinating other atoms, ions, molecules, or groups and thereby accomplish or approach the completion of further rare gas or other stable electronic configurations. However, two points must be kept in mind in the application of this rule. (1) Consideration must be given to molecular dimensions since the number of atoms or groups which can surround another atom acting as a nucleus is, of course, limited. The maximum coordination number is usually six although a few as high as eight are known. The most common coordination numbers are two three, four, and six. The small diameter of units such as the ions of the alkali metals, e. g., Li+, Na+, and so forth, definitely limits the number of groups which can occupy the space of their coordination spheres. The
Melot
Aluminum Antimony Arsenic Barium Bismuth Cadmivm Calcium Chromium Cobalt
Copper Hydrogen Iron Lend Magnesium Manganese
Merrury
Nickel Potassium Silver Sodium
strontium Tin zinc a
b
' d
I
Co6rdinntion number = approximntr diflerrocc dlvldrd hv f r o This may be r t i t t c a AIO, ' ?I110 Co6rdioafion number limited try space. One electron from each Hg atom is used in forming the homoatomic link This may be written HSnO%-.HnO. This mag be written Zn01-.2H20.
size of the coord'mated atoms or groups, of course, has a definite influence also. (2) Certain ions, particularly those of the transition el&ents, cannot attain a rare gas electronic system through coordination due to their odd effective atomic numbers. The most stable arrangement in these cases is an E.A.N. closely approaching that of the inert gas. The iron atom, for instance, bas an atomic number of 26. It loses three electrons on oxidation, yielding the ferric ion, Fe+++, of E.A.N. 23.3 This ion approaches the E.A.N. of luypton, 36, by coordinating six groups (as in Fe(CN)F) each furnishing two electrons which results in a system of thirty-five electrons, one less than the atomic number of krypton. These systems are fairly stable chemically but possess a resultant magnetic moment which gives rise to the property known as paramagnetism (4). Coincidentally, the ions which possess these "odd" electronic arrangements according lo the rule, exhibit characteristic col&. On reducing this empirical rule to a simple formula for simple common ions, one obtains the approximate coordination number,
This formula leads to very high coordination numbers (10 to 20) for certain of the alkali and alkaline earth
ions. The coordination number of these ions is naturally limited to six or less by stereochemical considerations. Table 1 lists a number of the common metals and gives the results of the application of the rule to their cations in the formation of complex ions. CO6RDINATION NUMBERS OF ANIONS In contrast to the simple cations, the ions derived from the less electropositive elements tend to be the donors rather than acceptors of electronic couples. This fact is quite obvious from an inspection of Table 1 in which i t is seen that the donor groups in general consist of electronegative elements or groups. For example, the sulfide ion, S-, may be oxidized to the sulfate ion and in the process associates with four atoms of oxygen, thus
Here the sulfur atom is the donor, the oxygen atom the acceptor, and the charge on the ion the same as the original sulfide ion. In case the central atom is amphoteric in nature, this type of structure is probably in equilibrium with a structure in which the r81e of donor and acceptor is modified or reversed. These types of structures have been discussed from the standpoint of magnetic phenomena in a previous paper (4).
The prominent existence of many simple ions, including the ferric ion. Fc-", is open to question. The properties of the compound anhydrous ferric chloride indicate it to he apparrntlr non-oolar rather than an ionized comoound-it is volsrilr. dissol& in hydrocarbon solvents, and is nan-conductor. ~ k k i n g this into consideration the E.A.N. of the iron atom in ferric chloAPPLICATION ride becomes 29, an approach to the pseudo-rare gas electronic configuration (2. 8, 18 = 28). The formation of the associated The rule presented in this paper has been applied in Fe.Cls, in which hoth iron atoms are assumed to be associated teaching a selected group of 6rst-year college students with all six chlorine atoms, gives each of the iron atoms an possessing special qualifications (5, 6). In this group E.A.N. of 35 in accordance with the proposed rule.
a
the rule was apparently accepted with enthusiasm when given in answer to questions concerning coordination numbers and valency. The simplicity of the rule makes it seem likely that it could be readily comprehended by the average first-year college chemistry student. In teaching this rule, however, it probably should be
emphasized that there are many known exceptions to it and, therefore, i t cannot be considered the complete answer to the coordination number question. From the teaching standpoint it conforms to the general practice of presenting only the most easily comprehensible portions of current theories to students of elementary science.
LITERATURE CITED
(1) FERNELIUS. W.
C. AND R. F. ROBBY, ''The nature of the metallic state." I. CHEM.Enuc.. 12,53 (1935). (2) Roe~y,R. F., “Molecular models in inorganic chemistry," i b d , 12, 378 (1935). L. L. AND R. F. ROBEY, "The magnetic method of (3) QUILL, producing ultra-low temperatures," Sch. Sn'. Math., 36, 871 (1936).
(4) ROEEY, R. F. AND W. M. DIX, "Magnetism and chemical constitution," J. CHEM.EDUC., 14 414 (1937). (5) DAY,J. E.: ibid., 12, 166 (1935). (6) FERNELIUS, W. C:, L. L. QUILL, AND W. L. EVANS, "Experiences teachmg proficiency students in chemistry," ihid., 14, 427 (1937).
