The oxidation number of a metal increases than ionic character decreases. how?

Generally compounds in higher oxidation state are covalent while the compounds in lower oxidation state are ionic in nature. This is basically in accordance with the Fajan's Rules. Fajan's rules are the rules which determine which are the different factors that favour polarisation and covalent character in a molecule. More is the charge on the cation, more is the extent of polarisation and hence more is the  covalency. Thus, as the oxidation number of the central metal ion increases, its tendency to form covalent compounds increases and that to form ionic compound decreases.