Why Silver Nitrate Pulls Out Only Some of the Chloride in a Cobalt Complex

Haemoglobin in blood, chlorophyll in leaves and the cisplatin used in cancer treatment are all coordination compounds — a central metal atom or ion surrounded by molecules or ions bonded to it. Naming them precisely and understanding how they hold together is the foundation for the rest of this chapter.

This lesson covers IUPAC nomenclature of coordination compounds, Werner's theory with the terms ligand, coordination number and coordination sphere, and the effective atomic number.

IUPAC names for coordination compounds give the cation before the anion, list ligands alphabetically before the metal, show the metal's oxidation state in Roman numerals, and add -ate to the metal when the complex is an anion.

Rules:

- Cation first, then anion, even when the complex is the anion
- Ligands before the metal, in alphabetical order, ignoring numerical prefixes
- Anionic ligands end in -o: chlorido, cyanido, hydroxido, oxalato
- Neutral ligands keep their names, except aqua for water, ammine for ammonia, carbonyl for CO and nitrosyl for NO
- Number prefixes are di, tri and tetra, or bis, tris and tetrakis for ligands whose names already contain numbers, such as ethane-1,2-diamine (en)
- Anionic complexes add -ate to the metal, often with its Latin root: ferrate, cuprate, argentate

Worked examples:

- $[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_5\mathrm{Cl}]\mathrm{C}{\mathrm{l}}_2$ — pentaamminechloridocobalt(III) chloride
- ${\mathrm{K}}_4[\mathrm{Fe}(\mathrm{CN}{)}_6]$ — potassium hexacyanidoferrate(II)
- $[\mathrm{Pt}(\mathrm{N}{\mathrm{H}}_3{)}_2\mathrm{C}{\mathrm{l}}_2]$ — diamminedichloridoplatinum(II)
- $[\mathrm{Co}(\mathrm{en}{)}_3{]}^{3+}$ — tris(ethane-1,2-diamine)cobalt(III) ion

Finding the oxidation state. For ${\mathrm{K}}_4[\mathrm{Fe}(\mathrm{CN}{)}_6]$: $4(+1)+x+6(-1)=0$, so $x=+2$.

An everyday example. Some packets of salt list potassium ferrocyanide as an anti-caking agent — its IUPAC name is potassium hexacyanidoferrate(II).

The substance. The -ate ending belongs only to a complex anion — $[\mathrm{Fe}(\mathrm{CN}{)}_6{]}^{4-}$ becomes ferrate, but the cation in $[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_6]\mathrm{C}{\mathrm{l}}_3$ is simply hexaamminecobalt(III).

Werner's theory states that a metal in a coordination compound has two kinds of valence — a primary, ionisable valence equal to its oxidation state and a secondary, non-ionisable valence equal to its coordination number, directed to fixed positions in space.

Postulates:

- Primary valence — ionisable, satisfied by negative ions, and equal to the oxidation state
- Secondary valence — non-ionisable, satisfied by neutral molecules or negative ions, and equal to the coordination number
- Secondary valences point in definite directions, giving shapes such as octahedral, tetrahedral or square planar

Worked example — cobalt ammines. With excess silver nitrate, one mole of each compound gives a different amount of silver chloride:

- $\mathrm{CoC}{\mathrm{l}}_3\cdot 6N{\mathrm{H}}_3$ gives 3 mol AgCl, so it is $[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_6]\mathrm{C}{\mathrm{l}}_3$
- $\mathrm{CoC}{\mathrm{l}}_3\cdot 5N{\mathrm{H}}_3$ gives 2 mol AgCl, so it is $[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_5\mathrm{Cl}]\mathrm{C}{\mathrm{l}}_2$
- $\mathrm{CoC}{\mathrm{l}}_3\cdot 4N{\mathrm{H}}_3$ gives 1 mol AgCl, so it is $[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_4\mathrm{C}{\mathrm{l}}_2]\mathrm{Cl}$

Only chloride ions outside the square brackets can precipitate; those inside are bonded to cobalt.

Key terms:

- Ligand — an ion or molecule that donates a lone pair to the metal: unidentate such as $\mathrm{N}{\mathrm{H}}_3$, bidentate such as en or oxalate, or polydentate such as hexadentate EDTA
- Chelate — a ring formed when one ligand binds through two or more donor atoms; chelates are especially stable
- Ambidentate ligand — binds through either of two atoms, such as $\mathrm{N}{\mathrm{O}}_2^{-}$ through N or O
- Coordination number — the number of donor atoms bonded to the metal
- Coordination sphere — the metal and its ligands inside square brackets; ions outside are counter ions

An everyday example. EDTA in hospital blood-collection tubes grabs calcium ions as a stable chelate, which stops the blood sample from clotting.

The substance. Coordination number counts donor atoms, not ligands — three ethane-1,2-diamine ligands give $[\mathrm{Co}(\mathrm{en}{)}_3{]}^{3+}$ a coordination number of 6.