Group 13 Elements: The Boron Family — General Trends

Group 13, the boron family, comprises boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl), all sharing the general outer-shell configuration $ns^2np^1$. All five elements can, in principle, show the group oxidation state $+3$, using all three valence electrons in bonding. However, the inert pair effect becomes progressively more important down the group: the $6s^2$ pair of thallium is held so tightly (partly due to relativistic effects on heavy, high-$Z$ atoms) that thallium's $+1$ oxidation state — using only the single $6p$ electron — is in fact more stable than $+3$. Consequently $\text{Tl}^{3+}$ compounds are strong oxidising agents that readily revert to $\text{Tl}^{+}$, whereas boron and aluminium show essentially no tendency towards a stable $+1$ state under ordinary conditions.

Boron itself stands apart from the four metals below it, and this anomalous behaviour of the first member is one of the most instructive features of the group. Three structural/electronic reasons underlie it. First, boron's atomic radius is exceptionally small (about 85 pm) compared with even aluminium (about 143 pm) — the usual "first-member anomaly" seen at the top of several p-block groups, arising because boron has no intervening $d$ or $f$ electrons to poorly shield its comparatively high effective nuclear charge. Second, this small size combined with boron's high ionisation enthalpy means boron never loses its three valence electrons to form a simple $\text{B}^{3+}$ cation the way aluminium can approach ionic character in some of its compounds; boron's bonding is essentially always covalent. Third, boron has no low-lying $d$ orbitals available (its valence shell is $n=2$), so its maximum coordination number is capped at 4 (as in $\text{BF}_4^{-}$), whereas aluminium and its heavier congeners, with accessible $d$ orbitals or simply larger size, can expand their coordination number in certain complexes. …