Assertion : The degree of complex formation in actinides decreases in the order $M^{4+} > \mathrm{MO}_2{…
Assertion : The degree of complex formation in actinides decreases in the order $M^{4+} > \mathrm{MO}_2{ }^{2+} > M^{3+} > \mathrm{MO}_2{ }^{+}$.
Reason : Actinides form complexes with p-bonding ligands such as alkyl phosphines and thioethers.
If both assertion and reason are true and reason is the correct explanation of assertion
If both assertion and reason are true but reason is not the correct explanation of assertion
If assertion is true but reason is false
If both assertion and reason are false.
Solution
The higher the charge on the metal ion, smaller is the ionic size and more is the complex forming ability. Thus, the degree of complex formation decreases in the order
$M^{4+} > M_2^{2+} > M^{3+} > M \mathrm{O}_2^{+}$
The higher tendency of complex formation of $\mathrm{MO}_2^{2+}$ as compared to $M^{3+}$ is due to high concentration of charge on metal atom $M$ in $\mathrm{MO}_2^{2+}$.