research summaries for teachers
MILTON TAMRES The University of Michigan Ann Arbor, Michigan
physical-inorganic chemistry
Donor-Acceptor Complexes The hest kno\vn and most widely taught examples of dooor-acceptor interactions are those involving format,ion of strong complexes, particularly those which illustrate the Br@nsted-Lowryor the Levis theory of acids and bases. Donors and acceptors can he selected so that the energetics of interaction vary over a wide range, \vit,l~no clear division beheen "strong" and "\\-ealc." The weaker interactions generally have not been correlated with acid-base concepts; but usually have been introduced as special topics, e.g., hydrogen-bonding and va,n der Waals forces. I~cscarchof the past twenty years has shown that measurement of \veal< interactions is a convenient. way to determine relative st,rengths of a series of donors1 (or acceptors). Donor-acceptor terminology now is commonly used in the lkerature for bot,h strong and wealveal< complexes (Dewar, Hanna, Mulliken, and Person). The work on CT complexes has been done almost exclusively in solution, using for the most part solvents considered to be relatively "inert" such as n-heptane and CCI+ Polar solvents like chloroform and methylene chloride also have been used, especially when solubility of reagents was a factor. The presence of solvent complicates matters considerably because of the difficulty in evaluating specific solvation effects. Since the theory is based on the properties of isolated molecules, better comparison is afforded by studying the complexes in the vapor phase. Several laboratories, including ours, are engaged in this work, and since 1965 about a dozen such studies have been reported. The results point out how markedly different the spectral and thermodynamic properties of the complexes can he in the two phases. As an extreme example, results for the durene-tetracyanoethylene complex in the vapor phase (Kroll) relative to those in methylene chloride solution (Merrifield and Phillips) show that r is smaller by a factor of 5, K , (298°K) is larger by about 200, and the enthalpy of reaction is larger by a factor of 2 (-10.8 versus -5.1 kcal/mole). Evidently, specific solvent effectscan he large, especially ~vhena hydrogen-bonding solvent such as methylene chloride is involved. In one case, that of the moderately strong trimethylamine-sulfur dioxide (Christian and Grundnes), all steps in the internal energy cycle for complex formation in the vapor phase and in n-heptane solution have been determined. Even in n-heptane, the internal energies of solvation of gaseous species is appreciable; for the above complex it is -10.5 kcal/mole. As for CClr, there is evidence that it behaves as an electron acceptor toward many donors. I t is apparent that many of the solution studies will have to be reinvestigated in the vapor phase. Spectral band positions are dependent on the medium 222
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Journal of Chemical Education
(Bayliss and AhRae). In one case, the band position of a CT complex in the vapor phase and dissolved in a series of similar "inert" type solvents has been correlated with refractive index (Voigtj. Of the complexes studied so far, CT hand shifts in going from vapor to solution have been about those expected, i.e., the red shift becomes smaller with increasing strength of the complex. By contrast, studies on the visible band of iodine are surprising. Iodine in "inert" solvents has a well-known band at -520 mr which is shifted to lower wavelength on complexation. This blue-shifted band often has been used to determine free energies, enthalpies, and entropies of complex formation and, in fact, the band position has been correlated with the strength of donor-acceptor interaction. In the vapor phase, hoxever, studies to date show no evidence for the presence of the shifted hand. Data are much more available for the less volatile CT complexes, no doubt because they are simpler to study in solution. Since the volatile complexes are well suited for vapor phase work, this area of research undoubtedly vill expand. Other Work
CT complexes are of interest and are being studied in all branches of chemistry. Some general areas of recent research are the follo\ving: (1) Solvent effects on CT complexes (W. LIPTAY, J. N. MURAND A. TRAMER, P. J. TROTTER) RELL,J. PROCHOROW H. W. (2) Presswe effects an CT complexes (H. G. DRICKIMER, OFFEN) (3) Excited state properties of CT complexes (J. F E R G U S ~ N , S. NnG.lrtun~,A. WELLER) (4) Structures of solid CT complexes ( 0 . HASSEL,J. D. MCCULLOUGH) (5) Electrical -properties of CT complexes (F. GUTMANN, M. M. LAIIES) (6) CT complexes involvingpolyme!s (N. C. YANG) (7) CT complexes oi biological interest, inchiding carcinogenic A. SZENT-GYORGI) compauuds ( R . FOSTER,B. PULLMAN, (8) Optically active CT complexes (G. BRIEGLEII) (9) CT complexes in analysis (G. H. SCHENK) (10) CT complexes in chromatographic separation (A. R. COOPER, D. B. PARIKAR) (11) CT complexes in catalysis (M. I c m ~ n w n H. , INOI
