How are the chemical properties of cobalt different from those of rhodium and iridium?

While cobalt, rhodium, and iridium are all members of Group 9 of the periodic table, they exhibit distinct chemical differences as a result of their positions in different transition series.123

Key Chemical Differences

• Stability of Oxidation States: In the chemistry of cobalt (the first transition series), the +2 oxidation state is characteristic and stable in aqueous solutions. In contrast, the +2 oxidation state for rhodium (second series) and iridium (third series) is rare and relatively unimportant.43
• Higher Oxidation States: For the heavier transition metals like rhodium and iridium, higher oxidation states are generally more stable than they are for cobalt. While cobalt chemistry is dominated by +2 and +3 states, rhodium and iridium exhibit a broader range of oxidation states, including those up to +6.4123
• General Trends: Although the chemistry of rhodium and iridium complexes in the +3 state shows similarities to cobalt in the +3 state, there are more consistent differences than similarities between the first-row element (cobalt) and the heavier elements of the group. Generally, the chemistry of aquo ions in lower oxidation states (+2 and +3) is a dominant feature for cobalt but is relatively unimportant for the heavier rhodium and iridium.4

These differences arise because, for heavier transition metals, the increasing nuclear charge and the effects of the lanthanoid contraction influence the stability and reactivity of their valence-shell orbitals compared to the lighter cobalt.4