Coordination Compounds - Introduction and terminology

Coordination compounds are those in which a central metal atom or ion is bonded to a fixed number of ions or molecules (ligands) via coordinate covalent bonds. Example: $[Fe(CN)_6]^{4-}$.

Werner's Theory: It postualtes two types of valencies. Primary Valency is ionizable, corresponds to the oxidation state, and is satisfied by negative ions. Secondary Valency is non-ionizable, corresponds to the coordination number, and is satisfied by neutral molecules or negative ions.

Ligands: Ions or molecules capable of donating an electron pair to the central metal atom. They are classified as unidentate ($Cl^-$, $H_2O$), didentate ($en$, $C_2O_4^{2-}$), or polydentate ($EDTA^{4-}$).

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

Chelate Effect: When a di- or polydentate ligand uses its two or more donor atoms to bind a single metal ion, it forms a ring-like structure called a chelate. Chelated complexes are more stable than similar complexes with unidentate ligands.

Coordination Number (C.N.): The total number of coordinate bonds formed between the central metal ion and the ligands. For example, in $[PtCl_6]^{2-}$, the C.N. of $Pt$ is $6$.

Homoleptic and Heteroleptic Complexes: Complexes with only one type of ligand are homoleptic (e.g., $[Co(NH_3)_6]^{3+}$), while those with more than one type are heteroleptic (e.g., $[Co(NH_3)_4Cl_2]^+$).