Which one of following acts as Lewis acid but not as a Lewis base

1)$\underset{..}{\overset{-}{\mathrm{C}}}\equiv \underset{..}{\overset{+}{\mathrm{O}}}$ is a strong ligand, a Lewis base but not a Lewis acid;

2)$\ddot{\mathrm{S}}{\mathrm{O}}_2, \ddot{\mathrm{S}}{\mathrm{Cl}}_4...$contain lone pair on cental atom thus act like Lewis base ;

They can expand valency thus act like Lewis acid ;

3)${\mathrm{PF}}_5...$can expand valency thus Lewis acid;

No lone pair on central atom thus cannot act like Lewis base ;