Which of the following pairs contains structurally similar substances?

Which of the following complex ions does not exist?

Aluminium $(III)$ chloride forms a dimer because aluminium

Which of the following show diagonal relationship?

$A$ Lewis acid '$X$' reacts with $LiAlH_4$ in ether medium to give a highly toxic gas,'$Y$'. '$Y$' when heated with $NH_3$ gives a compound known as inorganic benzene. '$Y$' burns in oxygen and gives $H_2O$ and '$Z$'. '$Z$' is

Inorganic benzene is