Lewis Structures for Compounds with Expanded Octets Charles J. Malerich Baruch College. New York, NY 10010 Compounds and complex ions such as PF5, SF4, SFa, BrC1-. ... or I?" are examnles of chemical snecies in which the central, period 111-VI nonmetal atom ex'hibits an expanded octet. Comnounds of this tvDe also show novel and interesting molecuiar shapes, and'observation of these shapes is a criterion for octet expansion (1,2). Here a simple method is presented for recognizing expanded octets given only the molecular formula of the compound. The method also facilitates writing the dot formula for these compoundsand, when combined with valence shell electron pair repulsion theory, the molecular structure (3). Recognition of octet expansion given the molecular formula requires a systematic procedure for counting the number of two-electron bonds predicted by the octet rule. The procedure used in this article was develo~edhv Miller (4) although alternatiw procedures such as those presented hy \lorrimer r i , or Davis, Gail\., and Whitten ( 6 )work equally well. Miller's procedure consists of the four steps:
its octet to 10 electrons, and then the predicted bonding is given as follows: Step 1. Divide the tots1 number of valence e- in the molecule into bonding and nonbonding electrons with an expanded octet of 10 electrons on S. S has
6 valence e- shares 4 bonding e-
Step 2. Select an arrangementof the atoms
Step 3. Distribute the bonding e- in pairs.
I. Divide the total number of valence electrons in the molecule or
complex ion into bonding and nonbonding electrons by counting the number of valence electrons on e k h atom and the number of electrons each atom must share to complete its octet. 2. Select an arrangement of the atoms. 3. Distribute the bonding electrons into the selected atomic arrangement by forming two-electron single bonds and, if necessary, multiple bonds. 4. Where neeessary,eompleOeoctetson eachatom with nonbonding electrons.
Step 4. Complete octets (and expanded octets)withnonbondinge-.
When this orocedure is aonlied to a molecule with an expanded o c t k , the number bf bonds predicted is not sufficient to bind the molecule toeether. Consider SFa. for example.
-
Step 1. Divide the total number of valence e- in the molecule into bonding and nonbonding e-. S has 6 valence e- shares 2 bonding e4 F have 28 valence e- share 4 bondine eTotals: 34 valence e6 bonding eStep 2. Select an arrangement of the atoms. (To observe octet expansion, the period 111-IV nonmetal element must be present in the molecule or complex ion as a central atom. An arrangement with the four fluorine atoms around the sulfur satisfies this criterion.)
F S F F
Step 3. Distribute the honding e- in pairs.
The SF4 molecule is completely bound, and the number of electrons in the dot formula equals the total number of valence e- determined in step 1. The dot formula corresponds to a molecule of the type AB4E (E is a nonhondiug pair of e-) and for this formula VSEPR predicts a distorted tetrahedron shape. This shape corresponds to experiment. The SF4 example also illustrates the point that the central atom expands its octet by the number of underestimated bondine electrons. This orocedure for countine the number of elec&ons in the expanded octet has heen tested with examnles from Cotton and Wilkinson (2) and found to hold l in moiecules where the central, period f i b ~ n o n m e t aatom has an expanded octet of 10,12, or 14 electrons. Hence the procedure outlined here represents a useful general tool for recognizing and writing Lewis structures for molecules in which the Eentral atomhas an expanded octet. Literature Cited I. ~ i e k e y ,D.J. ~ . cham E ~ U C .1ssn.57.211. 2. cutton. F. A,; Wilkinaon, G. Adoancad 1norgonie Chemistry. A Comprehensiue T e d , 4th ed.; Wiley: NevYork. 1980:Chapter 5. I. Gil1espie.R. J. J. Chpm Educ. 1970.47, 18. 4. Miller, G. T., Jr. ChrmVtry: Principles and Applications; Wadsaorth: Belmont, CA, 1976: Chapter 4, Supplement 1. 5. Mortirner.C.E.C h ~ n i b t ~ Concegtuol ~,A Approach,4lhed.:VanNodrand: New York, 1979 Chapter 3, Section 9. 6. Davis. R. E.: Galley. K. D.: Whitten, K. W. Principles of Chemistry Saunden: New Ysrk. 1984:ChapfsrG. p L9&
.
One fluorine is not bonded because the octet rule is not valid for the central S atom. To hind this F, the central S expands
Volume 64
Number 5 May 1987
403
