Why metal in its low oxidation state forms ionic compound whereas in high oxidation state it forms covalent compounds?

To form an ionic bond, the neutral atoms need to lose or gain electrons - this change in the electronic state is not notional but real; this is where ionization enthalpy or electron gain enthalpy come into the picture; losing more than two electrons is very difficult (needs lots of energy) and similarly gaining more than two electrons is also very difficult; which is why ionic bonds are formed at lower oxidation states (between -2 to +2);

Whereas higher oxidation states are based on relative charge on the central atom; for example if we were to take the dichromate ion, Cr has an oxidation number of +6 but that does not mean Cr has lost six electrons to attain this configuration; it is relative; The Cr-oxygen bonds in the dichromate ion are all covalent;

Hence the observation, that higher oxidation states are not found in ionic bonds;