Question

The maximum covlaency for boron and aluminium respectively are

• A. 4 for both
• B. 6 and 4
• C. 4 and 6
• D. 6 for both

Answer 1

The correct option is C 4 and 6

$A{l}^{+3}$ stable oxidation state
$\therefore$ it can accomodate 6 ligand
$\therefore$ covalency 6
(b) $1s^{2}2s^{2}2p^1$
No vacant d-orbital.
$B^{+3}$ can maximum accommodate 4 ligand
$\left[B\right(OH{)}_3{]}^{-}$
Hence, covlaency for boron and aluminium respectively are
4, 6.

THEORY
Important Trends and Anomalous Properties of B:

1. Tri-chlorides, tri-bromides and tri-iodides of group13 elements are covalent in nature and hydrolysed in water.
2. Tetrahedral $\left[M\right(OH{)}_4{]}^{–}$ and octahedral $\left[M\right(H_{2}O{)}_6{]}^{3+}$ exist in an aqueous medium except for boron. Because in boron, d-orbital is absent, so maximum covalency of B is 4. But for other elements (Al, Ga, In, Tl) d-orbitals are available, so maximum covalency can be expected beyond 4.
3. 
4. The monomeric trihalides being electron-deficient, are strong Lewis acids.
5. $F_{3}B+\stackrel{\cdot \cdot }{N{H}_3}\to F_{3}B\leftarrow N{H}_3$
6. Most of the other metal halides are dimerized through halogen bridging e.g. $AlC{l}_3$