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Usually Lewis bases as my textbook says:

2.3 Ligands
The ligands are the ions or molecules bound to the central atom/ion in the coordination entity. This is better visualized as the combination of a Lewis acid (the central atom/ion) with a number of Lewis bases (ligands). The atom of Lewis base that forms the bond to the Lewis acid (central atom/ion) is called donor atom ...

According to me its not compulsory for a ligand to be a Lewis base, as $\ce{BF3}$ is a Lewis acid and can act as a ligand by accepting electrons from a metal in vacant orbital of boron. Is it true?

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    $\begingroup$ In that situation, the thing that the boron is binding to is the ligand. $\endgroup$
    – Zhe
    Commented Oct 17, 2018 at 15:04
  • $\begingroup$ I can't understand. Please clarify $\endgroup$
    – Harsh jain
    Commented Oct 17, 2018 at 15:22
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    $\begingroup$ @Zhe Actually OP is right. Ligands can have both acceptor and donor properties like CO, or indeed be only acceptors like BF3. $\endgroup$
    – Mithoron
    Commented Oct 17, 2018 at 15:52
  • $\begingroup$ Can H+ also act as ligand for electron rich metals. If it can its a pure acid . AlCl3 might also be such a ligand due to vacant d orbital of Al $\endgroup$
    – Harsh jain
    Commented Oct 17, 2018 at 16:29
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    $\begingroup$ @Harshjain You're going to far there. While H+ and AlCl3 are Lewis acids (and don't even mention this "d-orbital" nonsense) it's a matter of formalism when something can be called Z-Ligand. $\endgroup$
    – Mithoron
    Commented Oct 17, 2018 at 17:17

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Yes it can happen. This paper offers an example, in which $\ce{BF3}$ or $\ce{B(C6F5)3}$ adds to a low oxidation state iridium complex. There is really not a lot of mystery behind it. The iridium in its low oxidation state has electrons to offer and the boron with its vacant orbital can take them up.

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