Ambidentate ligand: A monodentate ligand that can bond through two different donor atoms. Example: $NO_{2}^{-}$ (bonds via N → nitro, or via O → nitrito); S$CN^{-}$ (bonds via S → thiocyanato, or via N → isothiocyanato).

Chelate effect: The enhanced stability of a complex formed by a polydentate ligand compared to equivalent monodentate ligands. Results from the large positive entropy change ($\Delta S$) when one polydentate ligand replaces multiple monodentate ligands.

Coordination number (CN): The number of donor atoms (ligand atoms directly bonded to the central metal) in a coordination entity. CN = 6 for most octahedral complexes; CN = 4 for tetrahedral and square planar.

Crystal Field Stabilisation Energy (CFSE): The net energy lowering achieved when electrons preferentially occupy the lower-energy d-orbitals after crystal field splitting. Expressed in units of $\Delta o$. Greater CFSE → more thermodynamically stable complex.

Denticity: The number of donor atoms a single ligand can use to bond to a central metal. Monodentate (1), bidentate (2), tridentate (3), tetradentate (4), pentadentate (5), hexadentate (6).

Effective Atomic Number (EAN): Total electrons on the metal atom/ion after counting ligand electron pairs; tends toward the next noble gas configuration (18 electrons).

Inner orbital complex: A complex in which the metal uses (n−1)d orbitals in hybridisation (e.g., d^{2}$$sp^{3} for octahedral). Formed with strong field ligands, typically low spin.

Lanthanoid contraction: The regular decrease in atomic and ionic radii across the lanthanoid series (La to Lu) due to poor shielding by 4f electrons, causing increasing effective nuclear charge on outer shells.

Ligand: An ion or neutral molecule that donates one or more lone pairs of electrons to a central metal atom/ion. Must be a Lewis base.

Outer orbital complex: A complex in which the metal uses nd orbitals (same period) in hybridisation (e.g., sp^{3}$$d^{2} for octahedral). Formed with weak field ligands, typically high spin.

Primary valence (Werner): The oxidation state of the central metal; these valences are ionisable and satisfied by anions.

Secondary valence (Werner): The coordination number of the central metal; these valences are non-ionisable and are satisfied by ligands directed in space.

Spectrochemical series: An empirical ordering of ligands from weakest to strongest crystal field, based on their ability to split d-orbital energy levels.

Trans influence: The ability of a coordinated ligand to weaken the bond trans to it (thermodynamic, ground-state effect). Not to be confused with the trans effect (kinetic, relates to substitution rate).