why transition metals in low oxidation states act as acids...???plzzz explain...

Generally in Transition Metal chemistry, the central metal ion with vacant bonding orbitals can act as a Lewis acid by accepting an electron pair to form a dative covalent bond. Ligands act as Lewis bases by electron pair donation to form the metal-ligand co-ordinate bond forming.

HSAB concept (hard and soft (Lewis) acids and bases) -- 'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable.

Transition metals in higher oxidation states with few ‘d’ valence electrons act as hard Lewis acids and form complexes with hard lewis bases such as F- or O2-, while transition metals in lower oxidation states with many ‘d’ valence electrons act as soft Lewis acids and form complexes with soft lewis bases such as CO, CN-, PR3, alkenes, alkynes. Hence transition metals act as Lewis acid, by accepting pair of electrons from lewis bases i.e. ligands to form metal complexes.