The transition metals and their compounds are known for their catalytic activity due to their ability to adopt multiple oxidation states and to form complexes.

Interstitial compounds are those that are formed when small atoms like H, C or N are trapped inside the crystal lattices of metals.

The oxidation states of chromium in CrO₄^2- and Cr₂O₇^2- are not the same.

Cr²⁺ (d⁴) is a stronger reducing agent than Fe²⁺ (d⁶) in water.

CrO₄^2-

Cr₂O₇^2–

MnO₄^-

MnO₄^2-

HgI₂, I^$$-$$

HgI$$_4^{2 - }$$, I$$_3^{ - }$$

Hg₂I₂, I^$$-$$

HgI₂, I$$_3^{ - }$$