Q. Which pair of molecules has same reason of non-existance ?
(I) $IF_{3}^{2 -}$
(II) $H_{4}Se$
(III) $XeF_{4}_{}^{2 -}$
(IV) $NCl_{5}$

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$IF_{3}^{2 -},XeF_{4}^{2 -}$

$H_{4}Se,NCl_{5}$

$XeF_{4}^{2 -},H_{4}Se$

$IF_{3}\_{}^{2 -},NCl_{5}$

Solution:

$IF_{3}^{2 -},XeF_{4}^{2 -} \rightarrow l.p.$ are present at $90^\circ ,$ so it is unstable and does not exist. $NCl_{5} \rightarrow $ due to the absence of d-orbital contraction
$H_{4}Se \rightarrow $ Se can not form bond with hydrogen in its excited state.

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Q. Which pair of molecules has same reason of non-existance ? (I) $IF_{3}^{2 -}$ (II) $H_{4}Se$ (III) $XeF_{4}_{}^{2 -}$ (IV) $NCl_{5}$