Question

Cyano complexes of transition metal ions (such as \(\mathrm{Fe}^{2+}\) and \(\mathrm{Cu}^{2+}\) ) are often yellow, whereas aqua complexes are often green or blue. Explain the basis for this difference in color.

Short answer

The color difference between cyano and aqua complexes of transition metal ions such as Fe2+ and Cu2+ is due to the different extent of crystal field splitting induced by the ligands. Cyanide, being a strong field ligand, causes greater splitting and absorbs higher energy light (blue), thus appearing yellow. In contrast, water is a weak field ligand, causes smaller splitting, absorbs lower energy light (red), and therefore the complex usually appears green or blue.

Step-by-step solution

Understanding the Structure of Transition Metal Ions

The 3d transition metal ions such as Fe2+ and Cu2+ have partially filled d-orbitals. The electrons in the d-orbitals can transition between the different energy levels upon absorption of light, causing the complexes to display various colors.

Effects of Different Ligands

The type of ligand attached to the metal ion can greatly influence the energy gap between the d-orbitals. This is because ligands can cause what is known as crystal field splitting, where the d-orbitals are split into two energy levels: lower energy 't2g' orbitals and higher energy 'eg' orbitals. The extent of splitting depends on the nature of the ligand.

Examining Cyanide and Aqua Complexes

Cyanide is a strong field ligand and will cause greater splitting of the d-orbitals compared to water, which is a weak field ligand. Because of this, a larger energy of light (towards the blue end of the spectrum) is needed to promote electrons from the t2g to the eg in cyanide complexes compare to aqua complexes. As a result, cyanide complexes often appear yellow as they absorb light from the blue end of the spectrum and transmit more of the yellow and red light. Conversely, aqua complexes absorb lower energy light (towards the red end of the spectrum) and hence often appear green or blue as they transmit more of the higher energy, green and blue light.

Key concepts

Crystal Field Splitting

Transition metal complexes, such as those involving iron oindent( Fe}^{2+} ) and copper oindent( Cu}^{2+} ), get their colors through the interaction of ligands with the central metal ion's d-orbitals. This interaction creates a phenomenon called crystal field splitting. When ligands approach a metal ion, they create an electrostatic field that influences the energy of the d-orbitals, causing them to split into different energy levels.
The splitting of these orbitals is not uniform, resulting in two distinct sets of orbitals: the lower-energy "t2g" orbitals and the higher-energy "eg" orbitals.
The gap between these two sets of orbitals, known as the crystal field splitting energy ( Δ ), dictates how much energy is required to promote an electron from one set to the other. The magnitude of this splitting directly affects the color we observe in metal complexes because a specific portion of the visible spectrum is absorbed for the electron transition, leaving the complementary color to be observed.

• Larger splitting means higher energy is needed (bluer light absorbed).
• Smaller splitting implies lower energy is needed (redder light absorbed).

Strong Field Ligands

"Strong field ligands" have a pronounced effect on the crystal field splitting. These ligands create a large energy gap between the \left( \text{t}_{2g} \right) and \left( \text{eg} \right) orbitals. This is due to their ability to approach the metal ion more closely and create a strong electrostatic interaction. Strong field ligands include CN\(^-\), CO, and NH\(_3\).
Cyanide \left( \text{CN}^{-}\right) as a strong field ligand is particularly noteworthy.
Because of its strong field nature, it enhances the splitting significantly, leading to absorption in the higher energy blue region of the electromagnetic spectrum. Thus, cyanide complexes often appear yellow, since they absorb blue light and reflect the complementary color, which is yellow.

• High splitting energy \left( \Delta \right) results in absorption of shorter wavelengths (blue-violet).
• Observed color is the complement of the absorbed light frequency.

Weak Field Ligands

"Weak field ligands," on the other hand, result in smaller splitting of the d-orbitals in transition metal complexes. This is because the interactions they form with the central metal ion are weaker, leading to a smaller crystal field splitting energy (Δ). Typical weak field ligands include H\(_2\)O, F\(^-\), Cl\(^-\), and I\(^-\).
When water (H\(_2\)O), acts as a ligand, it causes less splitting compared to strong field ligands like cyanide. Consequently, the electrons in a water-ligated complex absorb lower energy, longer wavelength light (such as from the red or yellow part of the spectrum), causing the compound to appear blue or green, which are the complementary colors of the absorbed wavelengths.

• Low splitting energy\left( \Delta \right)leads to absorption of longer wavelengths (red-yellow).
• Observed color, such as green or blue, results from transmission of higher energy light.