Coordination Chemistry Lecture Notes

A set of vocabulary-based flashcards covering the fundamental concepts, ligand classifications, and nomenclature rules of coordination chemistry.

Double salt (Lattice compound)

An addition compound that loses its identity in aqueous solution and completely ionises in $H_2O$, such as Potash alum ($K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O$).

Coordination compounds

Complexes that retain their identity in aqueous solution and do not dissociate into separate ions, such as Potassium ferrocyanide ($K_4[Fe(CN)_6]$).

Coordination bond

A bond formed when a donor (ligand) gives its lone pair of electrons to an acceptor (central metal atom/ion).

Coordination number

The total number of coordinate bonds between the ligands and the central metal atom/ion.

Homoleptic complexes

Complexes in which all ligands attached to the central metal are identical, for example $[Co(NH_3)_6]^{3+}$.

Heteroleptic complex

A complex where the ligands attached to the central metal atom are not identical, for example $[Fe(en)_2Cl_2]^+$.

Ambidentate ligand

Ligands that possess two different donor atoms but can donate an electron pair from only one donor atom at a time during coordination ($CN^-$, $NC^-$, $SCN^-$, $NCS^-$).

Non-classical ligand ($\pi$-acceptor ligand)

Also known as $\pi$-acid ligands, these ligands act as $\sigma$-donors and $\pi$-acceptors, using empty antibonding orbitals to receive electrons from the metal ($CO$, $CN^-$, $NO_2^-$).

Chelating ligands

Bidentate or polydentate ligands that attach to the central metal ion in a way that forms a ring structure; $5$ and $6$ membered rings are generally more stable.

Synergic bonding

A bonding interaction where a stronger $\sigma$ bond leads to a stronger $\pi$ bond, which is common in metal carbonyls.

Normal (Classical) ligands

Ligands that act only as $\sigma$ donors to the empty orbital of a metal, such as $H_2O$, $Cl^-$, or $Br^-$.

Oxidation state

The electric charge present on the central metal atom of a complex ion.

Ligands

The anions, cations, or neutral molecules that act as donors and combine with a central metal ion to form a complex ion.

Denticity

The number of lone pairs donated by a ligand to the central metal atom to form coordinate bonds.

Monodentate ligand

A ligand that donates a single lone pair of electrons, such as $NH_3$ (Ammine), $H_2O$ (Aqua), or $NO$ (Nitrosyl).

Bidentate ligand

A ligand that has two donor sites and donates two lone pairs, such as ethylene diamine (en) or Oxalato (ox) ($C_2O_4^{2-}$).

Polydentate ligand

A ligand that donates more than two lone pairs, such as EDTA (Ethylenediamine Tetra Acetate), which is a hexadentate ligand.

Coordination sphere

The central metal atom and the ligands directly attached to it, typically enclosed in square brackets in a chemical formula.

Cationic ligands naming rule

Ligands with a positive charge are typically given an "-ium" suffix, such as Hydrazinium ($NH_2-NH_3^+$) or Nitrosonium ($NO^+$).

Anionic ligands naming rule

Anionic ligands typically have their names modified with an "-ido" or "-o" suffix, such as Chlorido ($Cl^-$) or Cyanido ($CN^-$).