Coordination Compounds: Structure, Nomenclature, and Crystal Field Theory

Coordination Compounds

Introduction to Coordination Compounds

Coordination compounds are substances in which a central metal atom or ion is bonded to a group of surrounding molecules or ions, known as ligands. These compounds exhibit unique chemical and physical properties, including distinctive colors and magnetic behaviors, due to the interactions between the metal center and its ligands.

• Complex ion: A charged species consisting of a central metal ion bonded to one or more ligands.

• Ligand: An ion or molecule that binds to a central metal atom to form a coordination complex. Ligands can be neutral molecules (e.g., H2O, NH3) or anions (e.g., Cl-, CN-).

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

Notation and Nomenclature of Coordination Compounds

Writing Formulas and Identifying Components

Coordination compounds are written with the central metal ion surrounded by its ligands in square brackets. The overall charge is shown outside the brackets if the complex is charged.

• Example: [Cu(NH3)4]Cl2

• Ligands are listed in alphabetical order within the formula, regardless of charge.

Naming Coordination Compounds

The systematic naming of coordination compounds follows specific rules:

1. Name the cation before the anion, as in ionic compounds.

2. Within the complex ion, name the ligands in alphabetical order, using prefixes (di-, tri-, tetra-, etc.) to indicate the number of each type of ligand.

3. If the complex ion is an anion, the metal name ends with "-ate" (e.g., ferrate for Fe, cuprate for Cu).

4. Specify the oxidation state of the central metal in Roman numerals in parentheses.

Example: K4[Fe(CN)6] is named potassium hexacyanoferrate(II).

Table: Names and Formulas of Common Ligands

Ligand	Name in Complex Ion
H2O	Aqua
NH3	Ammine
Cl-	Chloro
CN-	Cyano
NO2-	Nitro
OH-	Hydroxo
Oxalate (C2O42-)	Oxalato

Table: Names of Common Metals in Anionic Complexes

Metal	Name in Anionic Complexes
Iron	Ferrate
Copper	Cuprate
Lead	Plumbate
Silver	Argentate
Gold	Aurate
Zinc	Zincate

Examples of Naming

• [PtCl4]2-: Tetrachloroplatinate(II)

• [Cu(NH3)4]2+: Tetraamminecopper(II)

• [Cr(NH3)2(C2O4)2]-: Diamminedioxalatochromate(III)

Practice Problems

• Select the correct name for [Cu(NH3)4]Cl2: Tetraamminecopper(II) chloride

• Select the correct formula for tetraaquadichlorochromium(III) chloride: [Cr(H2O)4Cl2]Cl

Geometry and Coordination Number

Common Geometries

The geometry of a coordination compound depends on its coordination number (the number of ligand atoms directly bonded to the metal center).

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

• d8 or d9 metals often form square planar complexes; otherwise, tetrahedral is more common for coordination number 4.

Bonding in Coordination Compounds: Crystal Field Theory

Crystal Field Theory (CFT)

Crystal Field Theory explains the optical and magnetic properties of coordination compounds by considering the electrostatic interactions between the metal ion and the surrounding ligands.

• Bonding is primarily electrostatic between the metal ion and the ligands.

• The arrangement of ligands causes the d orbitals of the metal ion to split into groups of different energies.

• The pattern of splitting depends on the geometry (octahedral, tetrahedral, square planar).

Crystal Field Splitting in Octahedral Complexes

• In an octahedral field, the five d orbitals split into two sets: t2g (lower energy: dxy, dxz, dyz) and eg (higher energy: dz2, dx2-y2).

• The energy difference between these sets is called the crystal field splitting energy ($\Delta$).

Strong Field and Weak Field Ligands

• Strong field ligands (e.g., CN-, CO) cause a large splitting ($\Delta$), often leading to low-spin complexes.

• Weak field ligands (e.g., H2O, F-) cause a small splitting, often resulting in high-spin complexes.

Color and Magnetism in Coordination Compounds

The color of a coordination compound arises from the absorption of visible light, which promotes an electron from a lower-energy d orbital to a higher-energy d orbital (d-d transition). The color observed is complementary to the color absorbed.

• Example: [Ti(H2O)6]3+ absorbs red light and appears green.

• Magnetic properties depend on the number of unpaired electrons in the d orbitals.

Converting Wavelength to Energy

The energy associated with the absorption of light can be calculated using the equation:

• $\Delta E = \frac{hc}{\lambda}$

• Where $h$ is Planck's constant, $c$ is the speed of light, and $\lambda$ is the wavelength of light absorbed.

• To convert to kJ/mol, multiply by Avogadro's number ($N_A$): $\Delta E = \frac{hcN_A}{\lambda}$

Summary Table: Color and Absorption

Complex	Color Observed	Color Absorbed (Wavelength)
[Ni(H2O)6]2+	Green	~680 nm (red)
[Ni(NH3)6]2+	Blue	~610 nm (orange)
[Ni(en)3]2+	Purple	~575 nm (yellow)
[Ni(gly)3]2+	Violet	~565 nm (green)

Key Takeaways

• Coordination compounds consist of a central metal ion and surrounding ligands, with specific rules for notation and nomenclature.

• The geometry and coordination number determine the shape and properties of the complex.

• Crystal Field Theory explains the color and magnetism of these compounds based on d orbital splitting.

• Strong and weak field ligands influence the electronic structure and observable properties of the complex.