Coordination Compounds — Revision Notes

⚡ 30-Second Revision

Central Metal — Lewis acid, usually transition metal.
Ligand — Lewis base, electron pair donor (monodentate, bidentate, polydentate).
Coordination Number — Number of metal-ligand bonds (e.g., 4, 6).
IUPAC Naming — Cation first, ligands (alphabetical) then metal, oxidation state (Roman numeral), '-ate' for anionic complex.
Isomerism — Structural (Ionization, Hydrate, Linkage, Coordination), Stereoisomerism (Geometrical, Optical).
VBT — Hybridization ($\text{sp}^3$, $\text{dsp}^2$, $\text{d}^2\text{sp}^3$, $\text{sp}^3\text{d}^2$), Geometry (Tetrahedral, Square Planar, Octahedral), Magnetic properties (unpaired electrons).
CFT — d-orbital splitting ($\Delta_o$, $\Delta_t$), Spectrochemical Series (ligand field strength), High spin/Low spin, CFSE, Color (d-d transitions), Magnetic properties.
Magnetic Moment — $\mu = \sqrt{n(n+2)}\,\text{BM}$ (n = unpaired electrons).
Spectrochemical Series (partial) — $\text{I}^- < \text{Br}^- < \text{Cl}^- < \text{F}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$.

2-Minute Revision

Coordination compounds feature a central metal ion bonded to ligands via coordinate bonds. Key terms include coordination number (number of bonds), coordination sphere (metal + ligands), and counter ions.

IUPAC nomenclature involves naming the cation first, then ligands alphabetically, followed by the metal with its oxidation state. Anionic complexes use an '-ate' suffix for the metal. Isomerism is crucial: structural isomers (ionization, hydrate, linkage, coordination) differ in connectivity, while stereoisomers (geometrical, optical) differ in spatial arrangement.

Valence Bond Theory (VBT) explains geometry and magnetism through hybridization (e.g., $\text{sp}^3$ for tetrahedral, $\text{dsp}^2$ for square planar, $\text{d}^2\text{sp}^3$/$\text{sp}^3\text{d}^2$ for octahedral).

Crystal Field Theory (CFT) provides a more detailed picture, explaining d-orbital splitting in the presence of ligands ($\Delta_o$ for octahedral, $\Delta_t$ for tetrahedral). The spectrochemical series orders ligands by their ability to cause splitting, determining whether a complex is high spin (weak field, more unpaired electrons) or low spin (strong field, paired electrons).

CFT also explains the color of complexes via d-d transitions and quantitatively predicts magnetic moments using the spin-only formula. Remember that strong field ligands lead to larger $\Delta_o$, shorter absorbed wavelengths, and often diamagnetism or low spin, while weak field ligands lead to smaller $\Delta_o$, longer absorbed wavelengths, and high spin.

5-Minute Revision

Coordination compounds are central to inorganic chemistry, characterized by a central metal (Lewis acid) and surrounding ligands (Lewis bases) forming coordinate covalent bonds. The coordination number defines the number of these bonds, dictating geometry (e.

g., 4 for tetrahedral/square planar, 6 for octahedral). Werner's theory established primary (oxidation state) and secondary (coordination number) valencies, distinguishing complexes from double salts.

IUPAC nomenclature is systematic: name cation first, then ligands (alphabetical, with prefixes like 'di-', 'tri-', or 'bis-', 'tris-' for complex ligand names) before the metal. The metal's oxidation state is in Roman numerals; if the complex is anionic, the metal name ends in '-ate'.

Isomerism is diverse. Structural isomers include: Ionization (exchange of ions inside/outside coordination sphere), Hydrate (water as ligand vs. solvent), Linkage (ambidentate ligands bonding via different atoms, e.

g., $\text{NO}_2^-$ via N or O), and Coordination (ligand exchange between cationic and anionic complex ions). Stereoisomers include: Geometrical (cis/trans in square planar and octahedral, fac/mer in octahedral $\text{Ma}_3\text{b}_3$ type) and Optical (non-superimposable mirror images, common in octahedral complexes with bidentate ligands like $[\text{Co(en)}_3]^{3+}$).

Bonding is explained by VBT and CFT. VBT uses hybridization: $\text{sp}^3$ (tetrahedral), $\text{dsp}^2$ (square planar), $\text{d}^2\text{sp}^3$ (inner orbital octahedral), $\text{sp}^3\text{d}^2$ (outer orbital octahedral).

It predicts magnetism based on unpaired electrons. CFT is more robust, treating metal-ligand interaction as electrostatic. It explains d-orbital splitting (e.g., $\text{t}_{2g}$ and $\text{e}_g$ in octahedral).

The Spectrochemical Series (e.g., $\text{I}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{CN}^-$) ranks ligands by their ability to cause splitting ($\Delta_o$). Strong field ligands cause large $\Delta_o$, leading to electron pairing (low spin, often diamagnetic).

Weak field ligands cause small $\Delta_o$, leading to high spin (more unpaired electrons, paramagnetic). CFT explains the vibrant colors of complexes via d-d transitions, where absorbed energy ($E = hc/\lambda$) equals $\Delta_o$.

Magnetic moments are calculated using $\mu = \sqrt{n(n+2)}\,\text{BM}$. Applications range from biological (hemoglobin, chlorophyll) to medical (cis-platin) and industrial (catalysis, metallurgy).

Prelims Revision Notes

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Definitions — Central metal (Lewis acid), Ligand (Lewis base), Coordination number (no. of M-L bonds), Coordination sphere (metal + ligands in brackets), Counter ions (outside brackets).
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Werner's Theory — Primary valency (oxidation state, ionizable), Secondary valency (coordination number, non-ionizable, determines geometry).
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Double Salts vs. Coordination Compounds — Double salts dissociate completely in solution; coordination compounds retain complex ion identity.
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Nomenclature (IUPAC)

* Cation first, then anion. * Ligands named before metal, alphabetically (ignoring prefixes). * Anionic ligands end in '-o' (e.g., chloro, cyano, hydroxo). Neutral ligands: aqua ($\text{H}_2\text{O}$), ammine ($\text{NH}_3$), carbonyl (CO), nitrosyl (NO). * Prefixes: di-, tri-, tetra- for simple ligands; bis-, tris-, tetrakis- for complex ligand names. * Oxidation state of metal in Roman numerals. * Metal name ends in '-ate' if complex ion is anionic (e.g., ferrate, cuprate).

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Isomerism

* Structural: Ionization (e.g., $[\text{Co(NH}_3)_5\text{Br}]\text{SO}_4$ vs. $[\text{Co(NH}_3)_5\text{SO}_4]\text{Br}$), Hydrate (e.g., $[\text{Cr(H}_2\text{O})_6]\text{Cl}_3$ vs. $[\text{Cr(H}_2\text{O})_5\text{Cl}]\text{Cl}_2 \cdot \text{H}_2\text{O}$), Linkage (ambidentate ligands, e.

g., $\text{NO}_2^-$ via N or O), Coordination (ligand exchange between complex ions). * Stereoisomerism: Geometrical (cis/trans for square planar $[\text{Ma}_2\text{b}_2]$ and octahedral $[\text{Ma}_4\text{b}_2]$, fac/mer for octahedral $[\text{Ma}_3\text{b}_3]$), Optical (chiral, non-superimposable mirror images, e.

g., $[\text{Co(en)}_3]^{3+}$).

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Valence Bond Theory (VBT)

* Hybridization & Geometry: CN=4: $\text{sp}^3$ (tetrahedral), $\text{dsp}^2$ (square planar). CN=6: $\text{d}^2\text{sp}^3$ (inner orbital, low spin), $\text{sp}^3\text{d}^2$ (outer orbital, high spin). * Magnetic Properties: Paramagnetic (unpaired electrons), Diamagnetic (all paired electrons). Strong field ligands cause pairing.

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Crystal Field Theory (CFT)

* d-orbital splitting: Octahedral ($\text{t}_{2g}$ lower, $\text{e}_g$ higher, $\Delta_o$), Tetrahedral ($\text{e}$ lower, $\text{t}_2$ higher, $\Delta_t \approx 4/9 \Delta_o$). * Spectrochemical Series: $\text{I}^- < \text{Br}^- < \text{Cl}^- < \text{F}^- < \text{OH}^- < \text{C}_2\text{O}_4^{2-} < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$ (increasing $\Delta$).

* High Spin vs. Low Spin: If $\Delta_o < \text{P}$ (pairing energy) $\implies$ high spin (weak field). If $\Delta_o > \text{P} \implies$ low spin (strong field). * Color: Due to d-d transitions.

$E = hc/\lambda = \Delta_o$. Shortest $\lambda$ for largest $\Delta_o$. * Magnetic Moment: $\mu = \sqrt{n(n+2)}\,\text{BM}$.

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Applications — Biological (hemoglobin-Fe, chlorophyll-Mg, Vitamin $\text{B}_{12}$-Co), Medical (cis-platin, EDTA), Industrial (catalysis, metallurgy).

Vyyuha Quick Recall

To remember the spectrochemical series for common ligands (weak to strong field):

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Iodide ($\text{I}^-$)
Bromide ($\text{Br}^-$)
Sulfide ($\text{S}^{2-}$)
Chloride ($\text{Cl}^-$)
Sulfate ($\text{SO}_4^{2-}$)
Fluoride ($\text{F}^-$)
Oxalate ($\text{C}_2\text{O}_4^{2-}$)
Oxide ($\text{O}^{2-}$)
Hydroxide ($\text{OH}^-$)
Nitrate ($\text{NO}_3^-$)
Ethylenediamine ('en')
Nitrite ($\text{NO}_2^-$)
Cyanide ($\text{CN}^-$)
Carbonyl (CO)

(Note: This is a slightly expanded version, focus on the most common ones for NEET like $\text{I}^-, \text{Br}^-, \text{Cl}^-, \text{F}^-, \text{H}_2\text{O}, \text{NH}_3, \text{en}, \text{CN}^-, \text{CO}$)