Coordination Compounds (Class 12): Naming, VBT and Crystal Field Basics

Nomenclature

1. Cation first, then anion, as in any ionic compound.
2. Within the complex, ligands before the metal, in alphabetical order.
3. Prefixes for numbers — di, tri, tetra normally; bis, tris, tetrakis where the ligand name already contains a number prefix or is complicated.
4. Anionic ligands end in -o; neutral ligands keep their names, with the standard exceptions for water, ammonia, carbon monoxide and nitric oxide.
5. Oxidation state of the metal in Roman numerals in parentheses.
6. If the complex is an anion, the metal name ends in -ate, often using the Latin stem.

Werner's theory

Werner proposed two kinds of valency: primary, satisfied by anions and equal to the oxidation state, and secondary, satisfied by ligands and equal to the coordination number. Secondary valencies are directed in space, which is what gives complexes definite geometry.

The experimental basis was conductivity and precipitation. Adding silver nitrate precipitates only the chloride outside the coordination sphere, and conductivity measurements reveal how many ions the compound gives in solution. Deducing the structure from such data is a standard question.

Valence bond theory

Ligands donate lone pairs into hybridised metal orbitals, and the hybridisation determines the geometry.

Inner orbital complexes use inner d orbitals and are usually low spin; outer orbital complexes use outer d orbitals and are high spin. Magnetic measurements distinguish them, since the number of unpaired electrons differs.

What valence bond theory cannot do

• It does not explain colour at all.
• It does not predict which complexes will be high or low spin — it accommodates the observation after the fact.
• It gives no account of the spectrochemical series.

Listing these limitations is a frequently asked question, and it motivates crystal field theory.

Crystal field theory basics

Ligands are treated as point charges whose field splits the five degenerate d orbitals. In an octahedral field the two orbitals pointing at the ligands rise in energy and the three pointing between them fall, with the gap called the crystal field splitting.

Whether electrons pair in the lower set or occupy the upper set depends on whether that gap exceeds the pairing energy. Strong field ligands give a large gap and low-spin complexes; weak field ligands give high-spin complexes.

Colour

An electron absorbs light of energy equal to the splitting and jumps to the upper set. The complex appears the complementary colour to what is absorbed, so the size of the splitting determines the colour observed.

This immediately explains two facts: complexes with no d electrons or a full d set are colourless, since no such transition is possible; and changing the ligand changes the colour, because it changes the splitting.

Frequently asked questions

Why is a complex with a full or empty d set colourless?

Because there is no possible d–d transition — either no electron to promote, or no vacancy to promote it into.

How is inner orbital distinguished from outer orbital experimentally?

By magnetic measurement. The number of unpaired electrons differs, so the magnetic moment does too.

Why is alphabetical order by ligand name rather than prefix?

Because the prefix indicates quantity, not identity. Alphabetising by prefix would reorder the same complex depending on how many ligands were present.

What does the spectrochemical series tell you?

The order of ligands by the size of the splitting they produce, which determines whether a complex is high or low spin and what colour it appears.