Question

Why the compounds of $T{l}^{+}$ are much more stable than those of $T{l}^{3+}$ ?

Answer 1

$Tl=n{s}^{2}n{p}^1$
$T{l}^{3+}=n{s}^{o}n{p}^o$
But $T{l}^{+}=n{s}^2,n{p}^o$
This metal gas d & f orbital which has poor shielding due to which $n{s}^2{e}^{-}$ pair does not participate in bonding.