Definitions of Some Important Terms Pertaining to Coordination Compounds

Full Notes

There are some key terms for Coordination chemistry that you need to understand and recall.

Coordination Entity

A coordination entity is formed when a central metal atom or ion is bonded to a fixed number of ions or neutral molecules called ligands.

• The metal and its ligands are written inside square brackets [ ]
• Any species outside the brackets are counter ions
• The coordination entity can be neutral, cationic, or anionic

For Example:

Cu(H₂O)₆Cl₂ contains [Cu(H₂O)₆]^2⁺ (cationic) and counter ions Cl⁻.

Central Atom/Ion

The central atom or ion is usually a transition metal that acts as a Lewis acid, accepting electron pairs from ligands.

• It forms coordinate covalent bonds with surrounding ligands
• It can carry a positive, negative, or zero charge

For Example:

• Co³⁺ in [Co(NH₃)₆]³⁺
• Ni²⁺ in [NiCl₄]²⁻
• Fe²⁺ in [Fe(CN)₆]⁴⁻

Ligands

Ligands are ions or neutral molecules that donate at least one lone pair of electrons to the central atom and form a coordinate bond to it.

Example Water is a common example of a ligand.

Ligands are Lewis bases.

They may be:

• Anionic (e.g. Cl⁻, CN⁻)
• Neutral (e.g. NH₃, H₂O)

Classification by denticity:

Monodentate – one donor atom, form one coordinate bond

Examples H₂O, Cl⁻, NH₃

Bidentate – two donor atoms

Examples Ethanedioate and ethylenediamine (en)

Polydentate – more than two donor atoms

ExampleEDTA⁴⁻

The Chelate Effect

There is a tendency for bidentate or multidentate ligands to replace monodentate ligands in ligands.

This is driven by an increase in entropy and is called the Chelate Effect.

Example Reaction of [Cu(H₂O)₆]²⁺ with C₂O₄²⁻ ions

Entropy increases (+ΔS) as the number of free particles increases (4 reactant particles compared to 7 product particles).

ΔG is more negative, making the reaction more feasible.

Coordination Number

The coordination number is the number of ligand donor atoms directly attached to the central atom.

It is not the number of ligands, but the number of donor atoms.

It determines the geometry of the coordination compound.

Matt’s exam tip

Don’t forget the co-ordination number of a complex and the number of ligands don’t have to be the same.

Examples:

• CN = 6 in [Cr(H₂O)₆]³⁺ → octahedral
• CN = 4 in [NiCl₄]²⁻ → tetrahedral
• CN = 4 in [PtCl₄]²⁻ → square planar

Coordination Sphere

The coordination sphere includes the central atom and all ligands attached to it, represented within square brackets [ ].

It is the non-ionisable part of the complex.

Ions outside the coordination sphere do ionise in solution.

Example:

In [Cu(H₂O)₆]Cl₂:
Coordination sphere = [Cu(H₂O)₆]²⁺
Counter ions = 2 Cl⁻

Coordination Polyhedron

The coordination polyhedron refers to the geometric shape formed by the ligand donor atoms around the central atom.

Shapes depend on the coordination number.

Examples:

• [Co(NH₃)₆]³⁺ → octahedral
• [NiCl₄]²⁻ → tetrahedral
• [PtCl₄]²⁻ → square planar

Oxidation Number of Central Atom

The oxidation number is the charge on the central atom after accounting for the charges on all ligands and the overall complex.

Calculation method:

Let x = oxidation number of metal
Sum of charges on ligands + x = overall charge of complex

For Example:

In [Cu(H₂O)₆]²⁺:
Let oxidation number of Cu = x x + (6 × 0) = +2 → x = +2 So Cu is in the +2 oxidation state.

Homoleptic and Heteroleptic Complexes

A homoleptic complex contains only one type of ligand bonded to the central atom.

Examples:

• [Co(NH₃)₆]³⁺
• [Fe(CN)₆]³⁻

A heteroleptic complex contains two or more different types of ligands.

Examples:

• [Co(NH₃)₄Cl₂]⁺
• [Pt(NH₃)₂Cl₂]

Summary

• Coordination entities consist of a central metal and ligands inside square brackets.
• Ligands donate lone pairs and may be mono, bi, or polydentate.
• Coordination number counts donor atoms and sets geometry.
• Oxidation number follows from ligand charges and overall complex charge.
• Homoleptic complexes have one ligand type while heteroleptic have more than one.