Question

The absorption spectra of the tetraoxido anions of Group 6 metals, CrO2, MoO42, and WO42- is shown below. Explain, in terms of LMCT bands, why the energies of transitions correlate with the order of the spectrochemical series. Moo Absorbance I wo: cro 250 350450 A/nm

Answer 1

Chromium (Cr), Molybdenum (Mo) and Tungsten (W) are the transition metal belongs to the 3d, 4d and 5d series respectively.

As per the given plot of absorbance, the highest value of $\lambda _{_{max}}$ for
Cro24
is t 360 nm. However,
WO2-
has the lowest value of $\lambda _{_{max}}$ .

It is known that, energy for LMCT varies directly with $\frac{1}{\lambda _{_{max}}}$ . It means it varies inversely with $\lambda _{_{max}}$ . Lower the higher will be the energy for LMCT. Thus,
WO2-
has the highest and
Cro24
has the lowest value of energy for LMCT.

The general trend of energy for LMCT in the group is that it gets increase down the group. As a result of this the absorption wavelength decreases.

The increasing order of energy for LMCT is shown below.

Cro24
<
M00,-
<
WO2-