The charges in the structural formula of BF3 + NH3 on Wikipedia < http://commons.wikimedia.org/wiki/File:NH3-BF3-adduct-bond-lengthening-2D.png > look strange to me. I've asked on the discussion page there < http://commons.wikimedia.org/wiki/File_talk:NH3-BF3-adduct-bond-lengthening-2D.png > if it is correct. I think the charges should be written as ⊕ and ⊖ because they are formal charges, but I am not sure. So far I did not get a reply.
Could you please comment on that? What would be the best way to draw that structural formula? What is the actual charge on boron and nitrogen?
Thank you!
It is merely a question of writing. The 'dative bond', written as an arrow from the donor to the acceptor atom, indicates that one atom shares an electron pair with another atom. Once this is done, the electron (or electron pair, for that matter) doesn't know which atom it stems from - it just occupies a particular molecular orbital. If the orbital is located between/shared by two atoms it constitutes a covalent bond. Therefore a standard dash notation for a covalent bond is ok.
In my experience it's not used anymore (or rarely) nowadays, at least not in organic chemistry.
The charges are, as you say, formal charges - again it's a question of notation.
Hope this helps.
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