Coordination Compounds Summary Notes

Addition Compounds

• Addition compounds are formed by combining two or more stable compounds and are categorized into double salts and coordination compounds.

• Double Salts: Retain individual ionic identities in aqueous solution by completely ionizing into constituent simple ions.

  • Potash Alum: $K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O$

  • Mohr's salt / FAS (Ferrous ammonium sulphate): $FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O$

  • Carnallite: $KCl \cdot MgCl_2 \cdot 6H_2O$

• Coordination Compounds: Lose individual ionic identities in aqueous solution because complex ions do not dissociate into simple constituent ions.

  • Examples include $K_4[Fe(CN)_6]$ and $[Cu(NH_3)_4]SO_4$.

Coordination Terminology and Chelation

• Coordination Entity / Sphere: Consists of the central metal atom or ion and its bound ligands, enclosed in square brackets (e.g., $[Co(NH_3)_6]^{3+}$).

• Ligand: Donor atom, ion, or molecule that donates an electron pair to the central metal atom/ion.

• Denticity ($D$): The number of coordinate bonds formed by a ligand with the central metal ion.

• Chelation: Formation of ring structures when a bidentate or polydentate ligand coordinates to a central metal ion.

  • Forms stable $5$- or $6$-membered rings; $3$-membered rings are unstable.

  • Chelation increases complex stability via the chelate effect.

  • Hydrazine ($NH_2NH_2$) is not a chelating ligand as it would form an unstable $3$-membered ring.

Classification and Types of Ligands

• Monodentate / Unidentate ($D = 1$): Contains $1$ donor site (e.g., $F^-$, $Cl^-$, $Br^-$, $I^-$, $H_2O$, $NH_3$).

• Bidentate ($D = 2$): Contains $2$ donor sites.

  • Ethane-1,2-diamine / Ethylenediamine ($en$): Neutral, symmetrical, forms a $5$-membered ring.

  • Oxalato ($ox$): Oxalate ion ($C_2O_4^{2-}$), charge $-2$, symmetrical, forms a $5$-membered ring.

  • Glycinato ($gly$): Glycinate ion ($NH_2CH_2COO^-$), charge $-1$, unsymmetrical, forms a $5$-membered ring.

  • Dimethylglyoximate ($dmg$): Charge $-1$, forms a $5$-membered ring.

  • Acetylacetonate ($acac$): Charge $-1$, symmetrical, forms a $6$-membered ring.

  • Biurate (Di-urea): Neutral, forms a $6$-membered ring.

  • Dipyridyl ($dipy$): Neutral, forms a $5$-membered ring.

  • Ortho-phenanthroline ($o\text{-}phen$): Neutral, forms a $5$-membered ring.

  • Propane-1,2-diamine / Propylene diamine ($pn$): Neutral, unsymmetrical, forms a $5$-membered ring.

  • Propane-1,3-diamine / Trimethylene diamine ($tn$): Neutral, symmetrical, forms a $6$-membered ring.

• Polydentate ($D > 2$): Contains more than $2$ donor sites.

  • Ethylene diamine tetraacetate ion ($EDTA$): Hexadentate ligand with a $-4$ charge.

Ligand Naming Conventions

• Anionic Ligands:

  • Acids ending in -ic acid form -ate ions, which become -ato ligands (e.g., Sulphate $SO_4^{2-} \rightarrow$ Sulphato, Nitrate $NO_3^- \rightarrow$ Nitrato).

  • Acids ending in -ous acid form -ite ions, which become -ito ligands (e.g., Sulphite $SO_3^{2-} \rightarrow$ Sulphito, Nitrite $NO_2^- \rightarrow$ Nitrito).

  • Anions ending in -ide convert to -ido or -o (e.g., $Cl^- \rightarrow$ Chlorido, $O^{2-} \rightarrow$ Oxo/oxida, $CN^- \rightarrow$ Cyano/cyanido).

• Neutral Ligands: Retain specific IUPAC names such as $H_2O$ (Aqua), $NH_3$ (Ammine), $CO$ (Carbonyl), and $NO$ (Nitrosyl).