An element of 3d-transition series shows two oxidation states x and y, differing by two units. Then·

When a 3d- transition element exhibits two oxidation states that differ by two units, the compounds in the lower oxidation state are ionic, while the compounds in the higher oxidation state are covalent.
For the most part, ionic bonds develop at the +2 and +3 oxidation levels while covalent bonds form in the higher oxidation states.