The correct increasing order for the wavelengths of absorption in...

The correct increasing order for the wavelengths of absorption in the visible region for the complexes of

{Co}^{3 +}

is:

Options

A ${\left[Co{\left(en\right)}_{3}\right]}^{3 +}, {\left[Co{\left({NH}_{3}\right)}_{6}\right]}^{3 +}, {\left[Co{\left({H}_{2}O\right)}_{6}\right]}^{3 +}$
B ${\left[Co{\left({H}_{2}O\right)}_{6}\right]}^{3 +}, {\left[Co{\left(en\right)}_{3}\right]}^{3 +}, {\left[Co{\left({NH}_{3}\right)}_{6}\right]}^{3 +}$
C ${\left[Co{\left({H}_{2}O\right)}_{6}\right]}^{3 +}, {\left[Co{\left({NH}_{3}\right)}_{6}\right]}^{3 +}, {\left[Co{\left(en\right)}_{3}\right]}^{3 +}$
D ${\left[Co{\left({NH}_{3}\right)}_{6}\right]}^{3 +}, {\left[Co{\left(en\right)}_{3}\right]}^{3 +}, {\left[Co{\left({H}_{2}O\right)}_{6}\right]}^{3 +}$

Correct answer

A. ${\left[Co{\left(en\right)}_{3}\right]}^{3 +}, {\left[Co{\left({NH}_{3}\right)}_{6}\right]}^{3 +}, {\left[Co{\left({H}_{2}O\right)}_{6}\right]}^{3 +}$

Step-by-step solution

The order of the ligand in the spectrochemical series

{H}_{2}O < {NH}_{3} < en

Absorbed wavelength

\text{∝}\dfrac{1}{Absorbedenergy}

Thus, the order of observed energy is:

{\left[Co{\left({H}_{2}O\right)}_{6}\right]}^{3 +} < {\left[Co{\left({NH}_{3}\right)}_{6}\right]}^{3 +} < {\left[Co{\left(en\right)}_{3}\right]}^{3 +}

Hence, the wavelength absorbed will be in the opposite order i.e.,

{\left[Co{\left(en\right)}_{3}\right]}^{3 +} < {\left[Co{\left({NH}_{3}\right)}_{6}\right]}^{3 +} < {\left[Co{\left({H}_{2}O\right)}_{6}\right]}^{3 +}

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