Which of the following complex ions does not exist?

On adding ammonium hydroxide solution to $Al_2(SO_4)_3$ (aq) :

Write balanced chemical equations for the following reactions:
$(i)$ $BF_3 + LiH \rightarrow$
$(ii)$ $B_2H_6 + H_2O \rightarrow$
$(iii)$ $NaH + B_2H_6 \rightarrow$
$(iv)$ $H_3BO_3 \xrightarrow{\Delta}$
$(v)$ $Al + NaOH \rightarrow$
$(vi)$ $B_2H_6 + NH_3 \rightarrow$

Borax is actually made of two tetrahedra and two triangular units joined together and should be written as: $Na_2[B_4O_5(OH)_4] \cdot 8H_2O$. Consider the following statements about borax:
$a.$ Each boron atom has four $B-O$ bonds
$b.$ Each boron atom has three $B-O$ bonds
$c.$ Two boron atoms have four $B-O$ bonds while other two have three $B-O$ bonds
$d.$ Each boron atom has one $-OH$ group
Select correct statement$(s)$:

The material used in the nuclear industry as protective shields and control rods is

Consider the following statements about group $13$ elements:
$A$. $AlCl_3$ achieves stability by forming a dimer.
$B$. $BCl_3$ is an electron-deficient molecule.
$C$. $E^0_{M^{3+}/M^0} \ (V)$ is $+1.26 \ V$ for aluminium.
$D$. In $+1$ oxidation state,thallium is unstable.
The incorrect statements are: