Trend 4.3 of the Carbon Group (Group IVA) Trends says that the element-element mean bond enthalpies decrease in the order:
C-C > Si-Si > Ge-Ge > Sn-Sn > Pb-Pb and therefore the range of ring and polyhedral molecules diminish down the group.
Carbon not only forms an extensive range of chain and ring compounds, but also polyhedral molecules such as prismanes, C6H6, and cubane (C8H8). Analogous compounds are known for Si, Ge, and Sn if the hydrogens are replaced by bulky organic substituents. However, few examples exist for Pb, which form compounds containing the anionic Zintl polyhedral anoin Pb94- analogous to Sn52-.
Catenation is the tendency for covalent bonds to form between atoms of a given element (e.g. C—C). In group four, the element-element mean bond enthalpies decrease in the order: C—C > Si—Si > Ge—Ge > Sn—Sn > Pb—Pb Carbon, being the smallest and most electronegative of the column is able to form Pπ-Pπ bonding between Carbons. This is what makes the bond enthalpies so strong. Moving down the column, elements become larger and less electronegative. As a result, orbitals overlap less efficiently, and Pπ-Pπ bonding is weaker (or nonexistent), making the mean bond enthalpies decrease.
Carbon’s ability to make stable, strong single, double, and triple bonds with itself allows it to form
- rings (such as Bezene)
- polyhedral molecules (such as Cubane, C8H8, and prismane, C6H6)
As catenation decreases down the group, the ability to make such structures, as well as the range of structures, decreases. The larger elements (Si, Ge, and Sn) form these structures more often with bulky organic substituents.
A substituent is an atom or group of atoms that is substituted in place of Hydrogen.These form because larger atoms are more stable with the larger compounds bonded to them
Few examples or rings or polyhedral molecules exist for Pb, which form compounds containing the anionic Zintl polyhedral anion Pb94-. Zintl ions are clusters of group 4 elements. So, Pb prefers to form anionic rings of nine leads.