Both 'Li' and 'Mg' display several similar properties due to the diagonal relationship however they are   incorrect in with respect to 

1. Magnesium forms basic carbonates (MgCO₃​.Mg(OH)₂​.xH₂​O) while lithium forms normal carbonates, it does not form basic carbonates. 

$5{\mathrm{Mg}}^{+2}+6{\mathrm{CO}}^{2-}3+7{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{MgCO}}_3\cdot \mathrm{Mg}(\mathrm{OH}{)}_2\cdot 5{\mathrm{H}}_2\mathrm{O}+2{{\mathrm{HCO}}_3}^{-}$

While Li can form only carbonate $\left({\mathrm{Li}}_2{\mathrm{CO}}_3\right)$  not basic carbonate

2.Both Li and Mg combine directly with N_2​ to give nitrides Li₃​N_2​ and Mg₃​N_2​. 

3.Bicarbonates of Li and Mg are more soluble in water than carbonates whereas carbonates of alkali metals are more soluble.

4.Hydroxides and nitrates of both Li and Mg decompose on heating to give oxide.

LiNO₃​$\stackrel{∆}{\to }$​Li₂​O+NO₂​+O₂​

Mg(NO₃​)2​ $\stackrel{∆}{\to }$​MgO+NO_2​+O₂​