15A.4-15A.10 - Ligands, Complex Ions And Colour

Colour In Transition-Metal Complexes

Aqueous transition-metal chemistry is often visible. Some solutions are blue, green, yellow or purple; others that contain metal ions are colourless. A colour change may therefore be useful evidence, but it is not enough to say "transition metals are colourful" and stop there.

The model has three layers:

The observed colour is the light transmitted through the solution after selected wavelengths have been absorbed.

Ligands And Coordinate Bonds

A surrounding molecule or ion binds by donating an electron pair from a particular donor atom.

Once formed, the coordinate bond behaves like an ordinary covalent bond, but its origin matters because the metal ion is accepting electron pairs from surrounding species.

The central metal ion is usually positive, so it can attract electron-pair donors. Ligands may be neutral or negatively charged.

Worked example: identify the ligand and bonding

In $\ce{[Cu(H2O)6]2+}$, the central metal ion is $\ce{Cu2+}$ and the ligands are six $\ce{H2O}$ molecules. Each water molecule donates a lone pair from oxygen to the copper ion, forming a coordinate bond. The square brackets show that the metal and ligands are being treated as one complex ion with an overall $2+$ charge.

Complex-Ion Charge

The charge on a complex ion comes from the charge on the central metal ion plus the charges on all the ligands. Neutral ligands such as $\ce{H2O}$ and $\ce{NH3}$ do not change the overall charge. Charged ligands such as $\ce{Cl-}$ and $\ce{OH-}$ do.

Worked example: charge of a complex ion

For $\ce{[CoCl4]2-}$, assume the metal ion is cobalt(II), $\ce{Co2+}$.

So the complex ion is written as $\ce{[CoCl4]2-}$.

This charge accounting is useful because questions may ask you to deduce an oxidation number, write a complex ion formula, or explain a colour change from a formula. The brackets and charge are not decorative: they show that the central metal ion and its ligands behave as a single charged species.

Common-error contrast
Do not give the charge on the ligand as the charge on the whole complex. In $\ce{[CoCl4]2-}$, each chloride ligand is $-1$, but the whole ion is $2-$ because the cobalt ion contributes $+2$.

d-Orbital Splitting And Colour

A free transition-metal ion has five d orbitals at the same energy in a simple model. When ligands surround the metal ion, their electron pairs interact with the metal's d orbitals. The ligands do not affect all five d orbitals equally, so the d orbitals split into different energy levels.

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If the metal ion has suitable d electrons, one d electron can absorb energy from visible light and move from a lower d level to a higher d level. The energy absorbed corresponds to a particular wavelength or range of wavelengths. The colour observed is the light not absorbed by the complex.