Electronic configuration of Thallium is $[ Xe ] \,4 f ^{14} \,5 d ^{10}\, 6 s ^2 \,6 p ^1$.
$d$ and $f$ electrons of inner shells provide poor shielding, thus the attraction of the nucleus on the $s$ electrons of the outermost shell increases. This is called the inert pair effect.
Whereas, $s$ electrons provide strong shielding to $p$ electrons, thus making it easy for the $p$ electrons to be released.
Thus, although Thallium shows $+3$ and $+1$ oxidation states, $+1$ is more common.
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$A + O_2 \to B$
$B + O_2+ H_2O \to C$
$A,\, B$ and $C$ are

$(i)\, [Cr(NO_3)_3 (NH_3)_3]$ $(ii)\, K_3[Co(C_2O_4)_3]$
$(iii)\, K_3[CoCl_2(C_2O_4)_2]$ $(iv)\, [CoBrCl(en)_2]$