1
$\begingroup$

This is an image of the structure of diborane.

enter image description here

It has a 3-center-2-electron bridge bond.

As we can see in the bridge bond, the electron in the hydrogen atom and one electron in either boron atom is shared between the hydrogen and boron atoms two times, making two $\ce{B-H-B}$ bridges. Does this mean that both boron atoms as well as the hydrogen atom share those 2 electrons? If this is true, then both borons' octet should be satisfied since the 2 electrons are shared equally between all 3 atoms. So why are both boron atoms still electron deficit?

These are the sources confirming that boron has an electron deficit.

$\endgroup$
11
  • 2
    $\begingroup$ In this arrangement, Boron atom is not electron deficient. But the bonds are not typical covalent bonds as they are in $\ce{H2}$ molecules, for example. $\endgroup$
    – Maurice
    Sep 10, 2023 at 15:47
  • 2
    $\begingroup$ Provide references and reasoning of such sources. If there are so many of them, they surely also say why or why not are boron atoms still electron deficient even in B2H6. As depending on a point of view, they are and at the same time they are not electron deficient. ( They are not as they do deal with the full electron octet. They are, otherwise they would not need to achieve that by 3c-2e bond.) $\endgroup$
    – Poutnik
    Sep 10, 2023 at 16:35
  • 1
    $\begingroup$ Quoting the wikipedia article to which you link: "Traditionally, diborane has often been described as electron-deficient, because the 12 valence electrons can only form 6 conventional 2-centre 2-electron bonds, which are insufficient to join all 8 atoms.[28][29] However, the more correct description using 3-centre bonds shows that diborane is really electron-precise, since there are just enough valence electrons to fill the 6 bonding molecular orbitals.[30] Nevertheless, some leading textbooks still use the term "electron-deficient". One of the references is to Bill Lipscomb's Nobel lecture. $\endgroup$
    – Ian Bush
    Sep 11, 2023 at 6:42
  • 1
    $\begingroup$ If you don't know who Professor Lipscomb was see nobelprize.org/prizes/chemistry/1976/lipscomb/facts - if anybody knew about boranes it was him. From his Nobel Prize Lecture: "One of the simple consequences of these studies was that electron deficient molecules, defined as having more valence orbitals than electrons, are not really electron deficient. I mean by this non-sequitur that the three-center two-electron bonds make possible a simple description of these molecules and ions as filled orbital species." nobelprize.org/uploads/2018/06/lipscomb-lecture.pdf $\endgroup$
    – Ian Bush
    Sep 11, 2023 at 6:49
  • 1
    $\begingroup$ Maybe later. I'm not sure I really know what "electron deficient" means. I'm not sure anybody does. $\endgroup$
    – Ian Bush
    Sep 12, 2023 at 5:46

1 Answer 1

3
$\begingroup$

Well at least according to Professor William Lipscomb, who won a Nobel Prize "for his studies on the structure of boranes illuminating problems of chemical bonding", they are not electron deficient. Quoting from his Nobel Prize lecture

"One of the simple consequences of these studies was that electron deficient molecules, defined as having more valence orbitals than electrons, are not really electron deficient. I mean by this non-sequitur that the three-center two-electron bonds make possible a simple description of these molecules and ions as filled orbital species."

However as Wikipedia notes some leading text books still use this description. Personally I'm not sure I really know what is meant by "electron deficient", it being a term that seems to mean less and less with the more chemistry you know (c.f. hybridization). So I will bow to Professor Lipscomb's authority.

$\endgroup$
5
  • $\begingroup$ I think if it is or is not electron deficient depends on what is meant by that by the context. It may be both at the same time. In sense of using less valence electrons than the double of the bond count, it is electron deficient In the sense you have described, it is not. $\endgroup$
    – Poutnik
    Sep 13, 2023 at 6:42
  • $\begingroup$ To be honest nowadays I would just solve for the orbitals and aufbau them with electrons. Let the maths do the talking, there is (usually) no ambiguity then $\endgroup$
    – Ian Bush
    Sep 13, 2023 at 7:15
  • $\begingroup$ What I have meant that sometimes a term is not ambiguous, it just have more meanings and is good to explicitly say which one is assumed. $\endgroup$
    – Poutnik
    Sep 13, 2023 at 7:28
  • $\begingroup$ I just don't think it an especially useful term. Just marvel in the extraordinary variety of ways that atoms can combine. That, for me, is the wonder of Chemistry, Then get down to the maths. $\endgroup$
    – Ian Bush
    Sep 13, 2023 at 8:38
  • $\begingroup$ I agree. Scientists often think nature does things this or that way, what is often true for a long time until they are shown the cases it does not. $\endgroup$
    – Poutnik
    Sep 13, 2023 at 8:41

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.