Coordination Compounds: Structure, Nomenclature, and Geometry

Coordination Compounds

Introduction to Coordination Compounds

Coordination compounds are chemical species consisting of a central metal ion bonded to surrounding molecules or ions called ligands. These compounds are important in both inorganic and biological chemistry due to their diverse structures and reactivity.

• Complex ion: A charged species consisting of a central metal ion and its attached ligands.

• Ligand: An ion or molecule that binds to a central metal atom to form a coordination complex.

• Coordination number: The number of ligand donor atoms directly bonded to the central metal ion.

Notation for Coordination Compounds

Coordination compounds are represented by formulas that show the central metal ion, its ligands, and any counterions needed to balance charge.

• Example: $\text{PtCl}_4^{2-}$, $\text{Cu(NH}_3)_4^{2+}$, $\text{Cr(NH}_3)_2(\text{C}_2\text{O}_4)_2^{-}$

• Oxidation state: The charge of the central metal ion, determined by the charges of the ligands and overall complex.

• Ligands: Shown inside brackets, with their number and type indicated.

Naming Coordination Compounds

The nomenclature of coordination compounds follows specific rules to ensure clarity and consistency.

1. Cation before anion: Name the cation first, then the anion, as in ionic compounds.

2. Ligand names: List ligands in alphabetical order, using prefixes (di-, tri-, tetra-) to indicate quantity.

3. Metal name: If the complex ion is an anion, the metal name ends in "-ate". Specify the oxidation state of the metal using Roman numerals in parentheses.

4. Example: $\text{K}_3[\text{Fe(CN)}_6]$ is named potassium hexacyanoferrate(III).

Common Ligands and Their Names

Ligands can be anions, neutral molecules, or chelating agents. Their names in complex ions often differ from their standard chemical names.

Ligand	Name in Complex Ion
Bromide, Br-	bromo
Chloride, Cl-	chloro
Hydroxide, OH-	hydroxo
Cyanide, CN-	cyano
Nitrite, NO2-	nitrito
Oxalate, C2O42-	oxalato
Ethylenediaminetetraacetato (EDTA4-)	ethylenediaminetetraacetato
Water, H2O	aqua
Ammonia, NH3	ammine
Carbon monoxide, CO	carbonyl
Ethylenediamine (en)	ethylenediamine

Names of Common Metals in Anionic Complexes

Metal	Name in Anionic Complex
Chromium	chromate
Cobalt	cobaltate
Copper	cuprate
Gold	aurate
Iron	ferrate
Lead	plumbate
Manganese	manganate
Nickel	nickelate
Palladium	palladate
Platinum	platinate
Silver	argentate
Zinc	zincate

Examples of Naming Complex Ions

• $\text{PtCl}_4^{2-}$: tetrachloroplatinate(II)

• $\text{Cu(NH}_3)_4^{2+}$: tetraamminecopper(II)

• $\text{Cr(NH}_3)_2(\text{C}_2\text{O}_4)_2^{-}$: diamminebis(oxalato)chromate(III)

Practice: Naming and Writing Formulas

• Given formula: [Cu(NH3)4]Cl2

• Correct name: tetraamminecopper(II) chloride

• Given name: tetraaquadichlorochromium(III) chloride

• Correct formula: [Cr(H2O)4Cl2]Cl

Geometry and Coordination Number

The geometry of a coordination compound depends on its coordination number and the arrangement of ligands around the central metal ion.

Coordination Number	Shape	Example
2	Linear	[Ag(NH3)2]+
4	Square planar	[Ni(CN)4]2-
4	Tetrahedral	[Zn(NH3)4]2+
6	Octahedral	[Fe(CN)6]3-

• Coordination number 2: Linear geometry (e.g., [Ag(NH3)2]+)

• Coordination number 4: Can be square planar (d8 metals) or tetrahedral

• Coordination number 6: Octahedral geometry

Summary Table: Coordination Compounds Geometries

Geometry	Coordination Number
Octahedral	6
Tetrahedral	4
Square planar	4

Additional info:

• For square planar geometry, d8 metals (such as Ni2+, Pd2+, Pt2+) are common.

• When naming, always specify the oxidation state of the metal in Roman numerals.

• Ligands with multiple donor atoms (chelating ligands) can increase the stability of the complex.