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JEE Main levelSingle correct optionDifficulty 2 of 3cc190

Which order of C-O stretching frequency is correct for the isoelectronic ions $[V (CO) X_{6}] X^{-}$ , $Cr (CO) X_{6}$ and $[Mn (CO) X_{6}] X^{+}$ ?

A $[Mn (CO) X_{6}] X^{+}$ $>$ $Cr (CO) X_{6}$ $>$ $[V (CO) X_{6}] X^{-}$
B $[Mn (CO) X_{6}] X^{+}$ $>$ $[V (CO) X_{6}] X^{-}$ $>$ $Cr (CO) X_{6}$
C $Cr (CO) X_{6}$ $>$ $[Mn (CO) X_{6}] X^{+}$ $>$ $[V (CO) X_{6}] X^{-}$
D $[V (CO) X_{6}] X^{-}$ $>$ $Cr (CO) X_{6}$ $>$ $[Mn (CO) X_{6}] X^{+}$

Answer: A

Back-donation is largest when the metal is most electron-rich, i.e. in the anion, which populates

π^{*}

of

CO

and lowers

ν (CO)

. The cation has the least back-donation. Hence

ν

:

[Mn (CO) X_{6}] X^{+} > Cr (CO) X_{6} > [V (CO) X_{6}] X^{-}

.

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