The complex ion [Cu(H2 O)6]^2+ has an absorption maximum at around 800 nm. When four ammonias r

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The complex ion [Cu(H2 O)6]^2+ has an absorption maximum at...

The complex ion $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ has an absorption maximum at around $800 \mathrm{~nm}$. When four ammonias replace water, $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$, the absorption maximum shifts to around $600 \mathrm{~nm} .$ What do these results signify in terms of the relative field splittings of $\mathrm{NH}_{3}$ and $\mathrm{H}_{2} \mathrm{O}$ ? Explain.

The complex ion $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ has an absorption maximum at around $800 \mathrm{~nm}$. When four ammonias replace water, $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}$, the absorption maximum shifts to around
$600 \mathrm{~nm} .$ What do these results signify in terms of the relative field splittings of $\mathrm{NH}_{3}$ and $\mathrm{H}_{2} \mathrm{O}$ ? Explain.

Key Concepts

Crystal Field Splitting

Crystal field splitting refers to the separation of the d-orbital energy levels in a transition metal when surrounded by ligands. This splitting, ?, is directly related to the energy required for d-d electronic transitions. A larger splitting energy implies that higher-energy light (shorter wavelength) is absorbed, which explains the shift in absorption maximum when the ligand environment changes.

Spectrochemical Series

The spectrochemical series is an empirically determined list that ranks ligands based on the field strength they impose on a transition metal ion. Ligands that produce a larger splitting of the metal d-orbitals are termed 'strong field' ligands. In this context, ammonia (NH3) is a stronger field ligand than water (H2O), meaning it induces a larger splitting, which results in a shift of the absorption band to a shorter wavelength.

Electronic Transitions in Transition Metal Complexes

Electronic transitions, particularly d-d transitions, are responsible for the absorption of light in transition metal complexes. The energy difference between the split d-orbitals determines the wavelength of light absorbed. The observed shift in the absorption maximum from 800 nm to 600 nm indicates that replacing water with ammonia increases the crystal field splitting, thus requiring absorption of light with higher energy.

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