Chemistry · Ch 5 — Coordination Compounds

Summary

Coordination entity: A central metal ion/atom bonded to a fixed number of ligands (ions/molecules) via coordinate bonds. Example: $[\text{Co}(\text{NH}_3)_6]^{3+}$ .
Central atom/ion: The metal in the coordination sphere; usually a transition metal (e.g., $\text{Fe}^{2+}$ , $\text{Cu}^{2+}$ ).
Ligands: Species donating a lone pair to the metal. Types: unidentate (one donor atom, e.g., $\text{NH}_3$ ), bidentate (two, e.g., $\text{en}$ ), polydentate (many, e.g., $\text{EDTA}^{4-}$ ). Chelating ligands form rings with the metal.
Coordination number: Number of ligand donor atoms directly bonded to the metal. Common values: 4 (tetrahedral/square planar) and 6 (octahedral).
Oxidation number: Charge on the metal after removing ligands as their usual ions. Calculated from overall charge of complex.
IUPAC naming: Cation before anion; ligands in alphabetical order (ignoring prefixes like di-, tri-); metal name ends with -ate if complex is anionic. Oxidation state in Roman numerals in parentheses.
Werner’s theory: Metals have primary (ionizable) and secondary (coordinate) valencies. Secondary valency equals coordination number; primary valency equals oxidation number.
Isomerism:
- Structural: Ionization (e.g., $[\text{Co}(\text{NH}_3)_5\text{Br}]\text{SO}_4$ vs $[\text{Co}(\text{NH}_3)_5\text{SO}_4]\text{Br}$ ), linkage (e.g., $\text{NO}_2^-$ vs $\text{ONO}^-$ ), coordination, hydrate.
- Stereoisomerism: Geometrical (cis/trans for square planar/octahedral; fac/mer for octahedral $[\text{Ma}_3\text{b}_3]$ ) and optical (non-superimposable mirror images, e.g., $[\text{Co}(\text{en})_3]^{3+}$ ).
Valence Bond Theory (VBT): Metal uses hybrid orbitals (e.g., $\text{d}^2\text{sp}^3$ for inner orbital, $\text{sp}^3\text{d}^2$ for outer orbital) to accept lone pairs. Predicts geometry and magnetic behavior (paramagnetic/diamagnetic).
Crystal Field Theory (CFT): Ligands as point charges; d-orbital splitting in octahedral field into $\text{t}_{2g}$ (lower energy) and $\text{e}_g$ (higher energy). Splitting energy $\Delta_o$ depends on ligand field strength (spectrochemical series: $\text{I}^- < \text{Br}^- < \text{Cl}^- < \text{F}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$ ).
High-spin vs low-spin: For $\text{d}^4$ to $\text{d}^7$ in strong field (large $\Delta_o$ ), electrons pair in $\text{t}_{2g}$ (low-spin); weak field (small $\Delta_o$ ) gives high-spin. Affects magnetic moment: $\mu = \sqrt{n(n+2)}$ BM ( $n$ = unpaired electrons). …

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