Coordination Compounds — Revision Notes

⚡ 30-Second Revision

Coordination Compound: — Metal + Ligands via coordinate bonds.
Werner's Theory: — Primary valency (oxidation state, ionizable), Secondary valency (coordination number, non-ionizable, directional).
Ligands: — Monodentate ($NH_3$), Bidentate (en), Polydentate (EDTA). Ambidentate ($NO_2^-$, $SCN^-$).
Coordination Number (CN): — Number of donor atoms bonded to metal.
Oxidation State: — Charge on metal.
Nomenclature: — Cation first, then anion. Ligands (alphabetical) $o$ Metal (with '-ate' if anionic) $o$ Oxidation State (Roman numeral).
Isomerism:

- Structural: Ionization, Hydrate, Linkage, Coordination. - Stereo: Geometrical (cis/trans, fac/mer), Optical (chiral).

VBT: — Hybridization ($sp^3$, $dsp^2$, $d^2sp^3$, $sp^3d^2$), Geometry, Magnetic properties (unpaired electrons).
CFT: — d-orbital splitting ($Delta_o$, $Delta_t$), Spectrochemical Series ($Cl^- < H_2O < NH_3 < CN^-$), High spin/Low spin (compare $Delta_o$ vs. P), Color (d-d transitions).
Magnetic Moment: — $mu = sqrt{n(n+2)}$ BM (n = unpaired electrons).
Chelate Effect: — Increased stability with chelating ligands (entropy driven).

2-Minute Revision

Coordination compounds feature a central metal ion bonded to ligands via coordinate bonds. Werner's theory explains primary (oxidation state) and secondary (coordination number) valencies. Ligands can be monodentate, polydentate, or ambidentate, with polydentate ligands forming stable chelates.

IUPAC nomenclature is crucial for systematic naming, requiring careful determination of the metal's oxidation state and proper ligand naming. Isomerism is diverse, including structural types like ionization, hydrate, linkage, and coordination isomerism, and stereoisomers such as geometrical (cis/trans, fac/mer) and optical isomers.

Valence Bond Theory (VBT) predicts hybridization, geometry, and magnetic properties by considering electron pairing and orbital overlap. Crystal Field Theory (CFT) explains d-orbital splitting, color, and magnetic behavior based on electrostatic interactions.

The spectrochemical series ranks ligands by their splitting ability, determining whether a complex is high spin or low spin. Stability is influenced by the metal ion, ligand basicity, and the chelate effect.

These compounds are vital in biological systems, catalysis, and medicine.

5-Minute Revision

Coordination compounds are central to inorganic chemistry, characterized by a metal ion (Lewis acid) accepting electron pairs from ligands (Lewis bases). Werner's theory laid the foundation, distinguishing between primary (ionizable, oxidation state) and secondary (non-ionizable, coordination number) valencies.

Ligands are classified by their denticity (monodentate, bidentate, polydentate) and donor atoms (ambidentate ligands like $NO_2^-$). The coordination number dictates geometry (e.g., 4 for tetrahedral/square planar, 6 for octahedral).

Nomenclature: Follow IUPAC rules: Cation first, then anion. Within the complex, ligands are named alphabetically, followed by the metal. Anionic ligands end in '-o' (e.g., chloro), neutral ligands have special names (e.g., ammine for $NH_3$). Prefixes like di-, tri- or bis-, tris- are used for multiple ligands. The metal name takes an '-ate' suffix if the complex is anionic (e.g., ferrate), and its oxidation state is given in Roman numerals.

Isomerism:

Structural: — Ionization (ligand vs. counter ion exchange, e.g., $[Co(NH_3)_5Br]SO_4$ vs. $[Co(NH_3)_5SO_4]Br$), Hydrate (water as ligand vs. solvent), Linkage (ambidentate ligand binding through different atoms, e.g., nitro vs. nitrito), Coordination (ligand exchange between cationic and anionic complexes).
Stereoisomerism: — Geometrical (cis/trans for $MA_2B_2$ square planar, $MA_4B_2$ octahedral; fac/mer for $MA_3B_3$ octahedral). Optical (non-superimposable mirror images, chirality, common in octahedral complexes with chelating ligands like $[Co(en)_3]^{3+}$).

Bonding Theories:

Valence Bond Theory (VBT): — Explains bonding via hybridization of metal orbitals ($sp^3$ for tetrahedral, $dsp^2$ for square planar, $d^2sp^3$ or $sp^3d^2$ for octahedral). Ligands are categorized as strong field (cause electron pairing, leading to inner orbital/low spin complexes) or weak field (do not cause pairing, leading to outer orbital/high spin complexes). Magnetic properties are predicted by the number of unpaired electrons.
Crystal Field Theory (CFT): — Treats metal-ligand interaction as electrostatic. Explains d-orbital splitting (e.g., $t_{2g}$ and $e_g$ in octahedral). The spectrochemical series ($I^- < dots < H_2O < dots < NH_3 < dots < CN^-$) ranks ligands by their splitting ability ($Delta_o$). For $d^4-d^7$ configurations, compare $Delta_o$ with pairing energy (P) to determine high spin ($Delta_o < P$) or low spin ($Delta_o > P$) complexes. CFT successfully explains color (d-d transitions) and magnetic properties. Magnetic moment $mu = sqrt{n(n+2)}$ BM.

Stability: Measured by stability constant ($K_f$). Factors include metal ion charge/size, ligand basicity, and the chelate effect (enhanced stability with chelating ligands due to entropy gain).

Applications: Crucial in biology (hemoglobin, chlorophyll), medicine (cisplatin), catalysis, and analytical chemistry.

Prelims Revision Notes

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Definitions:

* Coordination Compound: Central metal atom/ion + ligands. * Ligand: Electron pair donor (Lewis base). Monodentate (1 donor atom), Bidentate (2 donor atoms, e.g., 'en', $C_2O_4^{2-}$), Polydentate (many donor atoms, e.

g., EDTA). Ambidentate (binds via different atoms, e.g., $NO_2^-$, $SCN^-$). * Coordination Number (CN): Number of donor atoms directly bonded to metal. * Oxidation State: Charge on central metal.

* Coordination Sphere: Metal + ligands (non-ionizable). * Counter Ions: Outside coordination sphere (ionizable).

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Werner's Theory:

* Primary Valency: Oxidation state, ionizable, non-directional. * Secondary Valency: Coordination number, non-ionizable, directional (determines geometry).

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Nomenclature (IUPAC):

* Cation named first, then anion. * Within complex: Ligands (alphabetical) $o$ Metal $o$ Oxidation State. * Anionic ligands: '-o' suffix (e.g., chloro, hydroxo, cyano). * Neutral ligands: ammine ($NH_3$), aqua ($H_2O$), carbonyl ($CO$), nitrosyl ($NO$), ethylenediamine (en).

* Prefixes: di-, tri- (simple ligands); bis-, tris- (complex ligands). * Metal name: Unchanged if cationic/neutral complex; '-ate' suffix if anionic complex (e.g., ferrate, cuprate). * Oxidation state: Roman numeral in parentheses.

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Isomerism:

* Structural: * Ionization: Exchange of ligand with counter ion (e.g., $[Co(NH_3)_5Br]SO_4$ vs. $[Co(NH_3)_5SO_4]Br$). * Hydrate: Water as ligand vs. water of crystallization (e.g., $[Cr(H_2O)_6]Cl_3$ vs.

$[Cr(H_2O)_5Cl]Cl_2 cdot H_2O$). * Linkage: Ambidentate ligand binds via different atoms (e.g., nitro ($NO_2$) vs. nitrito ($ONO$)). * Coordination: Ligand exchange between cationic and anionic complex parts (e.

g., $[Co(NH_3)_6][Cr(CN)_6]$ vs. $[Cr(NH_3)_6][Co(CN)_6]$). * Stereoisomerism: * Geometrical (cis-trans): Different spatial arrangement. * Square planar ($MA_2B_2$): cis (adjacent), trans (opposite).

* Octahedral ($MA_4B_2$): cis (adjacent B's), trans (opposite B's). * Octahedral ($MA_3B_3$): fac (ligands on a face), mer (ligands on a meridian). * Optical (Enantiomers): Non-superimposable mirror images (chiral).

Common in octahedral complexes with chelating ligands (e.g., $[Co(en)_3]^{3+}$).

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Bonding Theories:

* VBT: * Assumes coordinate bond formation by overlap of vacant metal hybrid orbitals with ligand orbitals. * Hybridization: $sp^3$ (tetrahedral), $dsp^2$ (square planar), $d^2sp^3$ (inner orbital, octahedral), $sp^3d^2$ (outer orbital, octahedral).

* Strong field ligands (e.g., $CN^-, CO, NH_3, en$) cause electron pairing $o$ inner orbital/low spin/diamagnetic (if all paired). * Weak field ligands (e.g., $X^-, H_2O, OH^-, F^-$) do not cause pairing $o$ outer orbital/high spin/paramagnetic.

* CFT: * Electrostatic interaction between metal ion and ligands (point charges). * d-orbital splitting: Degeneracy of d-orbitals lifted. * Octahedral: $d_{x^2-y^2}, d_{z^2}$ ($e_g$) higher energy; $d_{xy}, d_{yz}, d_{xz}$ ($t_{2g}$) lower energy.

Splitting energy = $Delta_o$. * Tetrahedral: $d_{xy}, d_{yz}, d_{xz}$ ($t_2$) higher energy; $d_{x^2-y^2}, d_{z^2}$ ($e$) lower energy. Splitting energy = $Delta_t approx 4/9 Delta_o$. * Spectrochemical Series: $I^- < Br^- < Cl^- < F^- < OH^- < C_2O_4^{2-} < H_2O < NH_3 < en < CN^- < CO$ (increasing $Delta_o$).

* High/Low Spin: For $d^4-d^7$ (octahedral): If $Delta_o > P$ (pairing energy) $o$ low spin (pairing); If $Delta_o < P \to$ high spin (no pairing). * Color: Due to d-d transitions (absorption of specific wavelength, transmission of complementary color).

* Magnetic Moment (spin-only): $mu = sqrt{n(n+2)}$ BM, where n = number of unpaired electrons.

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Stability:

* Thermodynamic stability: Measured by formation constant ($K_f$). Higher $K_f$, more stable. * Chelate Effect: Chelating ligands form more stable complexes due to favorable entropy change.

Vyyuha Quick Recall

To remember the spectrochemical series (common ligands, increasing field strength):

I Brought Some Cold Fresh Oysters, With Nice Eggs, Crabs, Corn.

I$^{-}$ < Br$^{-}$ < SCN$^{-}$ < Cl$^{-}$ < F$^{-}$ < OH$^{-}$ < Oxalate ($C_2O_4^{2-}$) < Water ($H_2O$) < NH$_3$ < Ethylenediamine (en) < CN$^{-}$ < CO