Werner's Theory and Naming Complexes: Quick Revision

A brief note on Werner's Theory and Naming Complexes. Read it before a test. Try each question at the end before you open its answer.

Werner's theory

• A double salt gives all the ions of its parts in water. A coordination compound keeps its complex ion.
• Primary valence: the oxidation state. It can be ionised and is satisfied by negative ions.
• Secondary valence: the coordination number. It cannot be ionised, and neutral molecules or negative ions satisfy it.
• The groups of the secondary valence point in fixed directions: linear (2), tetrahedral or square planar (4), octahedral (6).
• The coordination sphere is the metal and its ligands, in square brackets. Only the counter ions outside the brackets are free. Silver nitrate precipitates free chloride only.

Ligands and the coordination number

• A ligand donates a lone pair through a donor atom. It can be neutral or negative.
• Denticity: monodentate ($\text{NH}_{3}$, $\text{Cl}^{-}$), bidentate (en, $\text{C}_{2}\text{O}_{4}^{2-}$), hexadentate (EDTA).
• A chelating ligand uses several donor atoms at once and closes a ring. An ambidentate ligand has two different donor atoms and uses one at a time.
• Coordination number = the number of donor atoms bonded to the metal, not the number of ligands.
• Homoleptic: one kind of ligand. Heteroleptic: more than one kind.

The oxidation state of the metal

• Charge on the complex = $x$ + the ligand charges, so $x$ = charge $-$ the ligand charges.
• Counter ions give the charge on the complex. In $\text{K}_{4}[\text{Fe}(\text{CN})_{6}]$ it is $-4$.

Naming

• Cation first, then anion. Inside the complex: ligands in alphabetical order (prefixes ignored), then the metal, then the state in Roman numerals.
• Anionic ligands end in -o: chlorido, cyanido, hydroxido, oxalato. Neutral: aqua, ammine, carbonyl, nitrosyl.
• Prefixes: di, tri, tetra, penta, hexa. Bis, tris and tetrakis with brackets for long names.
• An anionic complex gets -ate: ferrate, cuprate, argentate, aurate, plumbate, stannate, and others such as cobaltate and zincate.

A worked example

Name $\text{K}_{2}[\text{PtCl}_{4}]$ and find the oxidation state of platinum.

1. Complex charge. Two $\text{K}^{+}$ ions mean the complex ion has a charge of $-2$.
2. Platinum. $x + 4(-1) = -2$, so $x = +2$.
3. Name. The complex is an anion, so platinum becomes platinate: potassium tetrachloridoplatinate(II).

Practice with new numbers

A program to try

The program works out the Roman numeral and the name ending for three complexes.

Mistakes to avoid

• Counting ligands as donor atoms. $[\text{Ni}(\text{en})_{3}]^{2+}$ has a coordination number of 6, not 3.
• Amine for ammine. The ligand $\text{NH}_{3}$ has two m.
• Cation names for anions. An anionic complex of iron is ferrate.
• Free chloride. Chloride inside the brackets gives no silver chloride.
• Ligand charges added, not subtracted, when finding the oxidation state.

Check yourself

1. What is the coordination number of $[\text{Zn}(\text{en})_{2}]^{2+}$?

2. The complex ion $[\text{CoCl}_{4}]^{2-}$ contains cobalt. What oxidation state does cobalt have?

3. Name $\text{Na}[\text{Au}(\text{CN})_{2}]$.

4. How many moles of silver chloride form from $0.50\ \text{mol}$ of $[\text{Co}(\text{NH}_{3})_{4}\text{Cl}_{2}]\text{Cl}$ with excess silver nitrate?