Electron defficiency is a common feature in some class of compounds of Group 13 elements (B,Al,Ga). In some covalent compounds of Be too the same nature prevails. To compansate this electron defficiency they undergo dimerization forming bridges('3c 2e' or coordination) or in some cases they get stability through backbonding (halides of group 13 elements). Hydrides of them are known to stay in dimeric form through bridging.

Halides of B are found to remain in monomeric form due to backbonding where as 'Be, Al'-halides highly favour bridging structures through coordination of halide. Why is such difference? In Be,Al, vacant p orbital is present too to favour backbonding.

Then why Be,Al favour dimerization (or polymerization) over backbonding in their halides?

  • $\begingroup$ Why my questions are getting downvoted everytime? What is the reason?, I can't get it. What's wrong with this question? $\endgroup$ Jul 1, 2020 at 9:09
  • $\begingroup$ Generally because there are some people around with nothing better to do than downvote questions and answers anonymously. The people who actually have a good reason for their votes and who want to help others rather than put them down will post a comment with their reasons for downvoting and suggestions on how to make your question or answer better. The rest should be ignored as the bottom feeders they are. $\endgroup$
    – Gwyn
    Jul 1, 2020 at 18:39
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    $\begingroup$ @Gwyn That is a very wild accusation you throw out there. This question has probably gotten a down-vote (and close votes), because it does not really fit the scope of this site. There are plenty of excellent textbooks, which explain this very well, see Resources for learning Chemistry for a list or recommendations. $\endgroup$ Jul 9, 2020 at 14:39
  • $\begingroup$ @Martin : I stand by my comment. Doing someone the courtesy of explaining a down vote is simple good manners (what us fossils would call proper 'netiquette). How can they learn if the just get slapped down with no explanation. And really? Doesn't fit the scope of the site? I would have thought voting to close as a duplicate would be more likely, since you are one of those who once spent ages debating a very similar question at: chemistry.stackexchange.com/questions/34842/… $\endgroup$
    – Gwyn
    Jul 11, 2020 at 0:48
  • $\begingroup$ Does this answer your question? Why is BCl3 a monomer whereas AlCl3 exists as a dimer? $\endgroup$
    – Gwyn
    Jul 11, 2020 at 2:17

1 Answer 1


I believe this may be a duplicate of this question: Why is BCl3 a monomer whereas AlCl3 exists as a dimer?

From what I remember, the stronger the backbond, the smaller the tendency to undergo dimerisation, however, way back in the dark ages when I was a student and asked my second-year chem prof, his answer was that the simple reason why none of the tri-halides of boron undergo dimerisation is steric hinderance - i.e. the boron atom is just too small to accommodate the fourth larger Cl/Br/I atom. He then pointed me to several sets of further reading.

However, you will probably find a better answer at one of the following links:

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    $\begingroup$ That is a very crude oversimplification. $\endgroup$ Jul 9, 2020 at 14:41
  • $\begingroup$ @Martin:Yup. Still better than nothing. $\endgroup$
    – Gwyn
    Jul 11, 2020 at 0:53

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