Topic 1: Inorganic Chemistry I - Ligands and Complexes

Transition Metal Complexes

Transition metal ions are surrounded (coordinate bond) by molecules called ligands to form coordination compounds called complexes.

Formal Definition of a Complex

A compound is formally a complex when the coordination number is larger than the oxidation state of the center TM ion.

• Example: [Cu(NH₃)₄(H₂O)₂]²⁺.

Complex Notation

1. Contain a complex within square brackets

2. Metal written first.

3. Ligands listed alphabetically.

4. Donor atom written first (e.g: water can be written as OH₂).

5. Overall charge outside brackets.

6. In salts, cations are written before anions.

Counterions

Ions outside the square brackets are counterions. Transition metal complexes can be positively charged or negatively charged (or neutral) and the counterions balance that

Ligand

A ligand is an ion or molecule that binds to a metal ion by donating a lone pair of electrons to form a coordinate bond.

Ligands as Lewis Bases

Ligands act as Lewis bases (electron donors), and the metal center acts as a Lewis acid (electron acceptor).

Donor Atom

The atom providing the lone pair to the metal is the donor atom.

Charges of Ligands

Ligands are usually neutral (e.g., H₂O, NH₃) or negatively charged (e.g., Cl⁻).

Denticity

Denticity is the number of coordinate bonds a ligand forms with a single metal center

Monodentate Ligands

Form one coordinate bond and occupy one coordination site

Multidentate Ligands

Form more than one coordinate bond with the same metal center (bidentate, tridentate, tetradentate, etc.).

Chelate Complexes

Complexes formed by multidentate ligands are called chelate complexes.

Bidentate Ligands

Form two coordinate bonds with the same metal center

EDTA Ligand

Ethylenediaminetetraacetic acid (EDTA₄⁻) is a hexadentate, forming six coordinate bonds (two nitrogens and four oxygens).

EDTA is used in chelation therapy to bind toxic metals for excretion from the body.

Ambidentate Ligands

Ligands that can bind through more than one atom

• Example: Thiocyanate (SCN^-) can bind through N or S, and nitrite (NO₂^-) can bind through N or O.

Kappa Notation

The Greek letter kappa (κ) denotes which donor atom is bound

• Example: thiocyanato-κN means bound through nitrogen.

Bridging Ligands

• Ligands that link two or more metal centers.

• Denoted by Greek letter mu (μ).

Hapticity Ligands

Eta (η) notation indicates how many contiguous atoms in a ligand are bound to the metal

• Example: Benzene rings binds as η6 when all six carbons interact equally.

Coordination Number

Coordination number (CN) is the number of donor atoms directly bonded to the metal ion.

Geometry: CN = 2

• Linear (180 degrees)

• Rare

• Found in d¹⁰ ions like Cu⁺, Ag⁺, Au⁺, Hg²⁺.

Geometry: CN = 3

• Trigonal planar

• Also rare

• Found in Cu⁺ and Hg²⁺ (if not linear)

Geometry: CN = 4

• Either tetrahedral or square planar.

• Tetrahedral forms with halides ligands or d¹⁰ TM ions

• Square planar common for d⁸ TM ions (Rh(I), Ir(I), Pd(II), Pt(II), Au(III), Ni(II)).

Geometry: CN = 5

• Square pyramidal or trigonal bipyramidal

• Rare, and in reality are often distorted shapes from metal-ligand interactions

Geometry: CN = 6

Octahedral (most common) or trigonal prismatic (extremely rare – need ‘special’ arrangement of ligands)

IUPAC Naming Rules

1. Cation is named before the anion in an ionic coordination compound.

2. Within a complex, ligands are named first (alphabetically, ignoring prefixes), followed by the metal.

3. Metal oxidation state is shown in parentheses in Roman numerals.

4. Anionic ligands change their suffix: ide → ido, ite → ito, ate → ato.

5. Neutral ligands usually retain their names except: H₂O = aqua, NH₃ = ammine, CO = carbonyl, NO = nitrosyl.

6. Use prefixes: di-, tri-, tetra-, penta-, hexa- for simple ligands

7. Use prefixes: bis-, tris-, tetrakis-, pentakis-, hexakis- when ligand names already include a prefix.

8. Latin stems are used where available (e.g., iron → ferrate, copper → cuprate).

Charge of a Complex

• In a neutral complex (neutral salt): (Amount of + charges) = (amount of - charges)

• In a charged one: Charge of the metal (TM’s oxidation state) + ∑(charge on ligands)