Give reason for the following features of transition metal chemistry:
(i). The lowerst oxide of a transition metal (say, chromium, atomic number 24) is basic whereas the highest oxide is usually acidic.
(ii) . Transitio metals sometimes oxhibit very low oxidation states such as `+1` and 0.

Explanation

(i). Lowest oxide of Cr is CrO which is basic. The highest oxide is `CrO_3` which is acidic. (In between `Cr_2O_3` is amphoteric). Higher the oxidation state of the metal, more easily it can accept alectrons and hence greater is the acidic character.
(ii). The `+1` oxidation state is shown by Cu because after loss of one electron, it acquires stable configuration of `3d^(10)`. Zero oxidation state is shown in forming metal carbonyls, e.g., `Ni(CO)_4` because `pi`-electrons donated by the ligands are accepted into the empty d orbitals.