Colors and Magnetism

Transition metal complexes exhibit a variety of different colors, attributed to the absorption of specific wavelengths of visible light by these compounds.

Light is absorbed when it has the needed energy to excite an electron from a lower energy level to a higher one. Consequently, transition metal complexes generally absorb light matching the crystal field splitting energy, or delta, of the complex, which is typically in the visible light range.

For example, hexafluorocobaltate(III) strongly absorbs red light but minimally absorbs green light, leading it to appear green in color.

Hexaamminecobalt(III), which has a higher delta, strongly absorbs high-energy blue light but minimally absorbs yellow light. Accordingly, hexaamminecobalt(III) appears yellow in color.

The effects of the smaller delta of hexafluorocobaltate(III) are not limited to its color. When delta is low enough, like in hexafluorocobaltate(III), electrons singly occupy the higher-energy orbitals before pairing in the lower-energy orbitals.

Here, delta is smaller compared to the spin-pairing energy — the energy of the electrostatic repulsion between electrons in the same orbital. As such, it is more energetically feasible for electrons to overcome delta and occupy high-energy orbitals than to overcome the spin-pairing energy to pair in the low-energy orbitals.

In contrast, in hexaamminecobalt(III), delta is greater than the spin-pairing energy. Accordingly, electrons pair in the lower-energy orbitals, leaving the higher-energy orbitals vacant, as expected from Hund’s rule.

As a consequence of this difference in electronic distribution, while the Co(III) ion has four unpaired electrons in hexafluorocobaltate(III), it has zero unpaired electrons in hexaamminecobalt(III). Accordingly, the former is classified as a high-spin complex and the latter is labeled as a low-spin complex.

In general, weak-field ligands, which are associated with small values of delta, lead to high-spin complexes, while strong-field ligands, which promote high values of delta, form low-spin complexes.

High-spin and low-spin complexes can exhibit very different magnetic properties. For example, the high-spin hexafluorocobaltate(III) is attracted by a magnet owing to its unpaired electrons and is called paramagnetic.

Meanwhile, the low-spin hexaamminecobalt(III) is repelled by a magnet and labeled as diamagnetic.