Coordination Compounds - Definitions of Some Important Terms

Coordination Entity: A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, in $[CoCl_3(NH_3)_3]$, the $Co^{3+}$ ion is the central ion surrounded by three $NH_3$ molecules and three $Cl^-$ ions.

Central Atom/Ion: The atom or ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement. It acts as a Lewis acid by accepting electron pairs from ligands. Examples include $Ni^{2+}$ in $[NiCl_2(H_2O)_4]$.

Ligands: These are the ions or molecules bound to the central atom/ion in the coordination entity. They act as Lewis bases. Common ligands include $Cl^-$, $H_2O$, $NH_3$, and $CN^-$.

Denticity: Refers to the number of donor atoms a single ligand uses to bind to the central metal. Ligands can be Unidentate (e.g., $Cl^-$), Didentate (e.g., $H_2NCH_2CH_2NH_2$ or 'en'), or Polydentate (e.g., $EDTA^{4-}$ which is hexadentate).

Chelating Ligands: When a di- or polydentate ligand uses two or more donor atoms to bind a single metal ion, it forms a ring-like structure known as a chelate. Such ligands are called chelating ligands and they increase the stability of the complex.

Ambidentate Ligands: Ligands which can ligate through two different atoms but use only one at a time. Examples: $NO_2^-$ (can bind via $N$ or $O$) and $SCN^-$ (can bind via $S$ or $N$).

Coordination Number (CN): The total number of ligand donor atoms to which the metal is directly bonded. For example, in $[PtCl_6]^{2-}$, the $CN$ of $Pt$ is $6$. For complexes with polydentate ligands, $CN$ is the sum of (number of ligands $\times$ denticity).

Coordination Sphere: The central atom/ion and the ligands attached to it are enclosed in square brackets $[ ]$, which is collectively termed the coordination sphere. The ionizable groups written outside the bracket are called counter ions.

Oxidation Number of Central Atom: It is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. It is denoted by Roman numerals in parentheses.

Homoleptic and Heteroleptic Complexes: Complexes in which a metal is bound to only one kind of donor group are Homoleptic (e.g., $[Co(NH_3)_6]^{3+}$). Complexes in which a metal is bound to more than one kind of donor group are Heteroleptic (e.g., $[Co(NH_3)_4Cl_2]^+$).