17
$\begingroup$

In his book on molecular orbital theory, Molecular Orbitals and Organic Chemical Reactions, Ian Fleming notes that Pauling formulated an early alternative model to Hückel theory for explaining the bonding of simple conjugated polyenes. Evidently, this came to be called τ-bonding.

Fleming describes it as a modification to, or offshoot from, the hybridization model, in which orbitals similar to sp3-hybridized orbitals are combined. Fleming says that the τ-bond model makes the extent of the conjugation less obvious, but that the model

[…] might have some virtues, not present in the Hückel model, especially in trying to explain some aspects of stereochemistry.

I don't have access to the relevant primary literature, and the treatment of the subject in Fleming's book amounts to one paragraph with an accompanying diagram. A search online was relatively fruitless.

  1. Is τ-bonding just the historical precursor to the bent/banana-bond model, or does it have some unique independent significance that makes it directly relevant to modern chemists?
  2. What are the specific "virtues" that Fleming might have been referring to, and what, if any, advantages does the τ-bond model have over Hückel theory and/or other approaches grounded in MO theory?
$\endgroup$

2 Answers 2

7
$\begingroup$
  1. τ-Bonds provide an alternative description of electron density in alkenes and alkynes valid even in the modern chemistry.
  2. A good example is the conformational preferences in propene with the lower energy of the “eclipsed” conformation [1].

Reference

  1. Deslongchamps, G.; Deslongchamps, P. Bent Bonds, the Antiperiplanar Hypothesis and the Theory of Resonance. A Simple Model to Understand Reactivity in Organic Chemistry. Org. Biomol. Chem. 2011, 9 (15), 5321. DOI: 10.1039/C1OB05393K.
$\endgroup$
6
$\begingroup$

While the question has been already answered, I think it worth to mention some controversial aspects of Pauling's theoretical work [1, p. 751]:

But first on Pauling and the ways he damaged his own credibility: he ignored MO theory to a degree that was clearly perceived by the community as blind, if not unethical. His interests (and great, great creative powers) also shifted to biological problems. And, finally, he lost touch with the explosion in structure and reactivity, to an extent that the third edition of The Nature of the Chemical Bond is … an embarrassment.

Reference

  1. Hoffmann, R.; Shaik, S.; Hiberty, P. C. A Conversation on VB vs MO Theory: A Never-Ending Rivalry? Acc. Chem. Res. 2003, 36 (10), 750–756. DOI: 10.1021/ar030162a.
$\endgroup$
1
  • $\begingroup$ Thanks for the reference, it certainly gives an interesting historical perspective. However posterity may evaluate Pauling, and whatever the faults of VB theory, I'll always have a nostalgic fondness for his book General Chemistry. $\endgroup$
    – Greg E.
    Commented Aug 3, 2014 at 22:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.