ISOELECTRONIC MOLECULES1 The Effect of Number of Outer-Shell Electrons on Structure RICHARD G. GILLIS Defence Standards Laboratories, Department of Supply, Melbourne, Australia
I~OELECTRONIC molecules of a certain
class z/y may be defined as those which have x atoms and y valence or outer-shell electrons; they may be anions, cations, or un~harged.~Whatever the individual atoms may be, such molecules have the same over-all structure with the same arrangement of electron orbitals. There are, however, differences in fine structure, which have provided a fruitful starting point for spectroscopists for many years. The purpose of this discussion is to demonstrate that the concept of isoelectronic molecules can be of considerable value to the instructor in developing the principles of structural chemistry, to the student in helping to bridge the apparent gap between inorganic and organic chemistry, and t o the research worker in suggesting analogies which may yield interesting fields for investigation. The simplest class of isoelectronic molecule is the 2/2 class containing two atoms and two electrons, namely hydrogen, deuterium, tritium, and combinations of these such as HD. Another simple class is the 2/14 class which contains two atoms and fourteen electrons and includes the halogens and interhalogens. These are both structures having only one a bond. A little more complex is the 2/8 class which comprises the hydrogen halides. These have a single a bond which is unsymmetrical because of the difference in electronegativity of the two atoms. More complex still is the 2/10 class. Each molecule may be written X=Y with resonance contributions from structures with appropriate formal charges. They may be also represented
all structure is independent of the atoms involved, although differences in electronegativity, etc., lead to differences in physical properties and reactivity. Topics which may be discussed here include the dipole moment of CO (and of NO+ and CN- if they had independent existence) whereas Nn has no dipole; the relative acidity of CzHzand HCN; and the acidity and conditions for existence of NO+. The anomalous radical nature of the 2/12 molecules 0%) SO, and SSZis brought out by representing thcm as
rather than X=Y, and the comparison uvith strictly singly-bonded 2/14 molecules Cl,, OC1-, etc., is equally . . insiructive. A very important class is the 3/16 molecule shown in Table I t mav be written as X=Y=Z or X-Y=Z or resonance hylqrids of these valence bond structures TABLE 1 T m i c a l Isolectronic Molecule. Class 2/10
3/16
Ionic charge NO NN, CO CN CC
+2
OUO ON0 OCO, OCS SCS NNO, ClCN, BrCN, I ~ N C , ~H~C! NCO, NCS NCSe, CPrO, NNN, ClAgCl, 6 1 ~ ~ ~ 1 NCN
+1
0
-1
-2
using the convention that the pairs of s electrons shown by dots above and below the a bond have mutually perpendicular nodal plane^.^ Here is a simple but effective introduction for students t o the idea that over5/32 Presented s t the 32nd Meeting of the Australian and New Zealand Assooiation for the Advancement of Science, Dunedin, N. Z., January, 1957. "Val2The definition of isoeleet~onicfollows W. H . COULSON, ence," Oxford University Press, 1952, p. 126. However, many chemists understand isoeleetronic to include inner-shell electrons, and refer to the total number of electrons in thomolecule. If this is accepted, then homoelectronic is suggested as a suitable descrip tion of molecules having the same number of outer-shell electrons. a GILLIS,R. G., AND P. F. NELSON,J. CHEM.EDUC.,31, 546 (1954).
Ezamples
+1 0 -1 -2
+1 0 -1
-2 -3
PCL, PBr4 CF., SiBr, TiCL, OsOd MnO* ~ 1 6 , BF* , SO,, &o+ MnOs Cr01 PO4, As04
The tsbles are based on the structural data of WELLS,A. F., "Structural Inorganic Chemistry," 2nd ed., Clarendon Preas, N. V., "The Chemical Elements and Oxford, 1950, and SIDGWICK, Their Compounds," Oxford University Press, London, 1950, where the leading original references are given.
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with appropriate formal charges. It may a180 be represented
These molecules are all linear, and have two extended r orbitals with mutually perpendicular nodal planes, each containing four electrons. The over-all picture is therefore similar to an elongated nitrogen molecule. The number of electrons in triatomic molecules is critical for linearity. The resonance system cannot accommodate more than four electrons in each r orbital, and as a result molecules with 17 and more electrons are angular. As the number of electrons increases, the angle becomes more acute up to the 3/22 class where there is a sharp reversion to linearity. Within the limit of experimental error of the few measurements recorded, the angle depends only on the number of electrons and is independent of the particular atoms5 as shown in Table 2.4
TABLE 2 Angularity of Triatomie Molecules Class 3/16 3/17 3/18 3/19 3/20 3/21 3/22
Mean apex anole
T?lpical examvles
180" 143" 117' 116' 103'
NO2+etc. (Table 1) NO1 NO>-, Oa, Sot C102 CLO
180"
BrICI-, I$-
Triatomic molecules containing less than 16 valence electrons may still be linear, e.g., C02+
in which a lone pair electron has been removed from CO, without destroying the resonance of the r bond system. One would expect silicon dioxide OSiO to appear among the 3/16 molecules because it is closely related to carbon dioxide. In fact, monomeric silica does not exist since the simple two-electron bond gives the lowest energy state. To avoid misleading students, even unwittingly, silica should be represented (SiO& rather than SiOz. This makes it clear why all of Kippings' attempts t o prepare monomeric compounds of the type R&=O led to polymeric products containing only single bonds. From these our present day "silicones" were evolved. Molecules of the type AB3 in the 4/24 class all have a plane triangular structure; these include C03-- and NO3-. On the other hand sulfite ion SO3--, a 4/26 molecule, is not planar but pyramidal. This again points to the critical influence on structure of the number of electrons. If we allow a class of isoelectronic molecules to include those with additional hydrogen atoms, we may iuclude in the 4/24 class the guanidinium ion C(NH2js+in which the additional hydrogen atoms provide the necessary electrons to make 24, and do not alter the planar arrangement and axial symmetry of the central CN, unit. Similarly we may include in the 3/16 class
' WALSH,A. D., J. Chem. Soc., 1953,2266. VOLUME 35, NO. 2, FEBRUARY, 1958
ketene HzCCO and allene H1CCCH2, in both of which the skeleton is linear. The tetrahedral nature of saturated carbon is always stressed in organic chemistry; less often that of fourcovalent nitrogen. However, all 5 / 8 molecules of the type AH4 are tetrahedral including those shown in Table 1 and the corresponding deuterium compounds. The same tetrahedral structure is found in organic molecules of the 5/32 class of which carbon tetrachloride is a familiar example. However, many metallic and nonmetallic elements other than carbon may occupy the central position in AB4 molecules of this class and they, too, are all tetrahedral. Some of these ions are colored and some are colorless. The reason is that t o obtain all 32 electrons it is necessary in some cases to use electrons from inner orbitals of the central atom, and the color is directly due to the vacant orbitals remaining. For example, in perchlorate ion CIOn-, seven electrons are provided by the chlorine, which has the valence arrangement 3sZ3p5; there are no vacant inner orbitals and the ion is colorless. On the other hand, in permanganate ion Mn04-, manganese with a valence arrangement 3d"s2 must leave five vacant 3d orbitals in providing seven electrons, and the ion is colored. Manganate ion MnOl-- has a different color from permanganate since only six electrons are provided by manganese and there are only four vacant inner orbitals. Colorless sulfate and colored chromate provide another example. A few 5/32 molecules are planar, notably PtClp--, PdC1,-; AuCI4-, and AuBr4-. These may be used as an entry to a detailed discussion of square planar complex cyanides and of octahedral molecules. An extreme case so far as molecular size is concerned is boron nitride (RS),. This has infinite planar sheets constructed of fused 6-rings which have alternate boron and nitrogen atoms and six electrons per ring (two from each nitrogen). The result is a molecule with all the essential characteristics of graphite. The manner of stacking the sheets is the only structural differences between the two.6 In the same way borazole B3NaHsis closely related to benzene.? In the teaching of heterocyclic organic chemistry, it is common to emphasize that replacement of CH by N in aromatic structures does not alter the inherent stability due to resonance. Pyridine and benzene are usually compared first. However, they are not isoelectronic molecules by the definition used here. Rather we should compare pyridinium ion, (I), with benzene, (11), since both are A,H6 molecules of the 12/30 class. The value of this approach is that it suggests an inquiry into the possible existence, stability, and properties of the boron analogue, (111).
The main point of these examples is that the structure of the molecule is determined by the number of atoms and valence electrons, and that the particular 'PEASE,R. S., Nature, 165, 722 (1950). "STONE,F. G. A,, Quavt. Reu. (London), 9, 174 (1955).
atoms atfeet only the stability or reactivity of the molecule. This idea may be presented t,o beginning students as a model, or t o advanced undergraduates and graduate students as a starting point for a critical appraisal of fine structure and reactivity. There are many opportunities for the instructor t o point out that the st,ruct,urr of organic molecules is not unique and
that carbon is extraordinary only in its ability to form stable long chains, whose inorganic analogue is the silicon-oxygen chain. In this way, a rational connection can be made between structural organic and inorganic chemistry. This paper is published with the permission of the Chief Scientist, Depart,ment of Supply, Australia.
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