Transition-metal ions, colour and complex shapes

What you’ll learn

• Write d-block electronic configurations and identify transition elements.
• Explain colour using ligand-induced d-orbital splitting.
• Determine coordination number and complex geometry.

Understand the idea

A transition element forms at least one stable ion with a partially filled d subshell. Remove 4s electrons before 3d electrons when forming first-row d-block cations. Scandium and zinc lie in the d block but their usual ions have empty and full d subshells respectively, so they do not meet this definition. Copper qualifies because copper(II) has a partially filled d subshell. Similar energies of 3d and 4s electrons help explain variable oxidation states.

A ligand donates a lone pair to a central metal ion to make a coordinate bond. Coordination number counts donor atoms bonded to the metal, not the number of separate ligand molecules. Six-coordinate complexes are commonly octahedral; four-coordinate complexes may be tetrahedral or square planar. Cisplatin is a square-planar complex whose cis and trans arrangements are distinct geometric isomers.

Ligands split the d-orbital energies. Absorption of visible light can promote an electron between these levels, and the observed colour comes from the remaining transmitted or reflected light. Changing ligand, oxidation state or geometry can change the energy gap and therefore the colour. Empty or full d subshells cannot give the usual d–d transition, although colour can arise by other mechanisms in some compounds.

Key formulae

$$\mathrm{Fe}:[\mathrm{Ar}]\,3d^64s^2,\quad\mathrm{Fe^{2+}}:[\mathrm{Ar}]\,3d^6$$

$$\mathrm{Fe^{3+}}:[\mathrm{Ar}]\,3d^5$$

$$\Delta E=h\nu=\frac{hc}{\lambda}$$

$$\text{coordination number}=\text{number of donor atoms bonded to the metal}$$

Worked example

Explain why $[\mathrm{Cu(H_2O)_6}]^{2+}$ is a transition-metal complex and state its coordination number.

1. Neutral copper is $[\mathrm{Ar}]3d^{10}4s^1$; removing two electrons gives copper(II), $[\mathrm{Ar}]3d^9$.
2. The incomplete d subshell makes copper a transition element under the stated definition.
3. Each of the six water ligands donates one oxygen lone pair, so six donor atoms are attached.

Answer: Copper(II) has a 3d⁹ configuration; the coordination number is 6, with an approximately octahedral arrangement.