Lewis acids as ligands 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?
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.