Revision: Coordination Chemistry Chemistry HSC Science Class 12 Tamil Nadu Board of Secondary Education

Isomers in which there is exchange of solvent (water) ligands between coordination and ionization spheres are called hydrate isomers.

Two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms are called distereoisomers.

Isomers which show interchange of ligands between cationic and anionic spheres of different metal ions are called co-ordination isomers.

In a disubstituted complex molecule/ion, when two same ligands are at right angles (90°), the geometrical isomer is known as a cis-isomer.

When two ligands are in opposite directions, i.e., at 180° to each other, the isomer formed is the trans-isomer.

Optical isomers are mirror images that cannot be superimposed on one another. These are called enantiomers.

Isomerism is the phenomenon in which compounds have the same molecular formula but differ in their physical or chemical properties due to a different arrangement of atoms or groups in space or structure.

Alfred Werner (1893) proposed the first systematic theory to explain the structure and bonding in coordination compounds. His key postulates are:

Main Postulates:

1. In coordination compounds, metals show two types of valencies — Primary (ionisable) and Secondary (non-ionisable).
2. Primary valences are normally ionisable. They are satisfied by negative ions (counter ions/anions). They correspond to the metal's oxidation state.
3. Secondary valences are non-ionisable. They are satisfied by neutral molecules or negative ions (ligands). The secondary valency is equal to the coordination number and is constant for a metal.
4. Ion groups bound by secondary valencies to the metal have a characteristic spatial arrangement (geometry). This geometry is decided by the secondary valences, not the primary valences.

Werner's Formula Examples:

• CrCl₃·6H₂O: In [Cr(H₂O)₆]Cl₃, all three Cl⁻ are outside the coordination sphere and hence ionisable.
• CrCl₃·5H₂O: [Cr(H₂O)₅Cl]Cl₂ — two Cl⁻ are ionisable, one is inside the coordination sphere.
• CrCl₃·4H₂O: [Cr(H₂O)₄Cl₂]Cl — one Cl⁻ ionisable.
• CrCl₃·3H₂O: [Cr(H₂O)₃Cl₃] — no ionisable Cl⁻, no precipitate with AgNO₃.

Rules for Writing Formulae:

• The cation is written first, then the anion.
• In the formula of the complex ion/entity, the central metal atom is written first, then the ligands in alphabetical order.
• The formula of the entire coordination entity is enclosed in square brackets.

Rules for Naming:

Rule 1: Names of neutral coordination complexes are given without spaces. Cation is named first, separated by a space from the anion.

Rule 2 (Naming ligands first):

• Ligands that act as anions end in –o: Cl⁻ = chlorido, Br⁻ = bromido, I⁻ = iodido
• Anions ending in –ite and –ate are replaced with –ito and –ato: SO₄²⁻ = sulphato, CO₃²⁻ = carbonato, NO₂⁻ = nitrito, CH₃COO⁻ = acetato
• Neutral ligands get the same name as the uncoordinated molecule (with spaces omitted): C₅H₅N = pyridine, (CH₃)₂SO = dimethylsulfoxide (DMSO)

Exceptions — neutral ligands with special names:

Rule 3 (Prefixes): Greek prefixes (di, tri, tetra) are used for simple ligand names. For polydentate ligands (i.e., those with a binding site name containing di/tri already): bis-, tris-, tetrakis-, pentakis-, hexakis- are used instead. e.g., bis(ethane-1,2-diamine) not "diethylenediamine".

Rule 4: Oxidation state of the metal is indicated by a Roman numeral in parentheses after the metal name. NO = nitrosyl.

Rule 5 (Complex ion is a cation): Metal is named same as the element. e.g., Co in a cationic complex = cobalt. Name = Ligands + Metal name (with OS)

Rule 6 (Complex ion is an anion): Metal name ends in –ate + oxidation number.

Anionic Complex Metal Names:

IUPAC Name Examples

• Na₂[Fe(CN)₅NO]: Sodium pentacyanonitrosatoferrate(II) (Note: pentacyanonitrosylferrate(II))
• [Fe(CN)₆]³⁻: hexacyanidoferrate(III) ion
• [Pt(NH₃)₂(Br)(NO₂)Cl]Cl: triamminebromochloronitroplatinum(IV) chloride
• K₃[Cr(C₂O₄)₃]: potassium trioxalatochromate(III)

Order of naming ions: Positive ion (cation) first, then negative ion (anion). In naming the complex ion, ligands first (alphabetically), then metal.