In his book on molecular orbital theory, Molecular Orbitals and Organic Chemical Reactions, Ian Fleming notes that Pauling formulated an early alternative model to Huckel theory for explaining the bonding of simple conjugated polyenes. Evidently, this came to be called $\tau$-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 $\tau$-bond model makes the extent of the conjugation less obvious, but that the model

[...] might have some virtues, not present in the Huckel 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.

My questions are:

  1. Is $\tau$-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 $\tau$-bond model have over Huckel theory and/or other approaches grounded in MO theory?

2 Answers 2

  1. Tau-bonds provide an alternative valid description of electron density in alkenes and alkynes. Even in the modern chemistry
  2. A good example is the conformational preferences in propene (lower energy of the "eclipsed" conformation). You can find more information here: http://pubs.rsc.org/en/Content/ArticleLanding/2011/OB/c1ob05393k#!divAbstract

While the question has been already answered, I think it worth to mention some contraversial asspects of Paulings theoreticla work:

"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."

This is from an Acc. Chem. Ress. (2003) paper about VB-MO history http://www.roaldhoffmann.com/sites/all/files/493s_0.pdf

  • $\begingroup$ 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.
    Aug 3, 2014 at 22:46

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