Coordination Compounds

Inorganic Chemistry Weightage: 3–4 Questions (12–16 Marks) NMC Unit 12

“Coordination chemistry is the nexus where transition metals link to molecular architectures. From Alfred Werner's pioneering primary and secondary valencies to Crystal Field stabilization energy (CFSE), stereoisomerism, and synergic bonding in metal carbonyls, this high-yield chapter consistently delivers 12 to 16 marks on the NEET paper.”

— SCORECHEM ACADEMIC TEAM

1. Werner's Theory of Coordination Compounds

Historical Milestone (Alfred Werner, 1898): Postulated that transition metals display two distinct types of valency:
- Primary Valency: Ionisable, corresponds to the formal oxidation state, satisfied strictly by negative ions.
- Secondary Valency: Non-ionisable, corresponds to the coordination number, directional in three dimensions, satisfied by neutral molecules or negative ions.
Precipitation with Silver Nitrate ( $\text{AgNO}_3$ ):

Complex Formulation	Modern Structural Formula	Moles of $\text{AgCl}$ Precipitated	Electrolyte Type	Solution Colour
$\text{CoCl}_3 \cdot 6\text{NH}_3$	$[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$	3	$1:3$	Yellow
$\text{CoCl}_3 \cdot 5\text{NH}_3$	$[\text{Co}(\text{NH}_3)_5\text{Cl}]\text{Cl}_2$	2	$1:2$	Purple
$\text{CoCl}_3 \cdot 4\text{NH}_3\ (\text{cis})$	$\text{cis}-[\text{Co}(\text{NH}_3)_4\text{Cl}_2]\text{Cl}$	1	$1:1$	Violet
$\text{CoCl}_3 \cdot 4\text{NH}_3\ (\text{trans})$	$\text{trans}-[\text{Co}(\text{NH}_3)_4\text{Cl}_2]\text{Cl}$	1	$1:1$	Green

Double Salts vs Coordination Complexes:
- Double Salts (e.g., Mohr's Salt $\text{FeSO}_4 \cdot (\text{NH}_4)_2\text{SO}_4 \cdot 6\text{H}_2\text{O}$ , Carnallite $\text{KCl} \cdot \text{MgCl}_2 \cdot 6\text{H}_2\text{O}$ ) completely dissociate into individual constituent ions in water.
- Coordination Complexes (e.g., $\text{K}_4[\text{Fe}(\text{CN})_6]$ ) maintain the integrity of the $[\text{Fe}(\text{CN})_6]^{4-}$ entity; no free $\text{Fe}^{2+}$ or $\text{CN}^-$ ions exist in solution.

2. Ligand Classifications & IUPAC Nomenclature

Ligand Classifications by Denticity:
- Unidentate: Binds through one donor atom (e.g., $\text{Cl}^-, \text{H}_2\text{O}, \text{NH}_3, \text{CN}^-$ ).
- Didentate: Binds simultaneously through two donor atoms (e.g., Ethane-1,2-diamine 'en', Oxalate $\text{C}_2\text{O}_4^{2-}$ 'ox', Glycinate 'gly').
- Polydentate: Contains multiple donor atoms (e.g., Diethylenetriamine 'dien' [tridentate], $\text{EDTA}^{4-}$ [hexadentate, 2 N + 4 O donors]).
- Ambidentate Ligands: Unidentate ligands possessing two distinct donor atoms, either of which can coordinate:
  - $\text{NO}_2^-$ : Nitrito-N ( $-\text{NO}_2$ ) vs Nitrito-O ( $-\text{ONO}$ )
  - $\text{SCN}^-$ : Thiocyanato-S ( $-\text{SCN}$ ) vs Thiocyanato-N ( $-\text{NCS}$ )
Essential IUPAC Rules:
1. Cation is named first, followed by the anion, regardless of whether the coordination entity is positively or negatively charged.
2. Ligands are listed alphabetically before the central metal name.
3. Multipliers: di-, tri-, tetra- for simple ligands; bis-, tris-, tetrakis- for complex ligands already containing numerical prefixes.
4. Anionic ligands end in -ido (e.g., chlorido, cyanido, oxalato); neutral ligands use specific names (ammine for $\text{NH}_3$ , aqua for $\text{H}_2\text{O}$ , carbonyl for $\text{CO}$ ).
5. In an anionic complex entity, the metal name ends with the suffix -ate (e.g., ferrate, cuprate, cobaltate, platinate, argentate).

3. Isomerism in Coordination Compounds

Figure 12.1: Structural and Stereochemical Isomerism Pathways in Coordination Chemistry

Key Geometrical Isomers:
- Square Planar $[\text{MA}_2\text{B}_2]$ (e.g., $[\text{Pt}(\text{NH}_3)_2\text{Cl}_2]$ ): Cis-isomer (chlorides adjacent at $90^\circ$ ) vs Trans-isomer (chlorides opposite at $180^\circ$ ).
- Octahedral $[\text{MA}_3\text{B}_3]$ (e.g., $[\text{Co}(\text{NH}_3)_3(\text{NO}_2)_3]$ ): Facial (fac, three identical ligands on one triangular octahedral face) vs Meridional (mer, three identical ligands positioned around a great meridian).
Optical Isomerism (Enantiomers):
- Complexes lacking an improper rotation axis ( $S_n$ , plane or centre of inversion) are non-superimposable on their mirror images.
- Common in tris-bidentate complexes (e.g., $[\text{Co}(\text{en})_3]^{3+}$ ) and cis-bis-bidentate complexes (e.g., $\text{cis}-[\text{CoCl}_2(\text{en})_2]^+$ ).
- $\text{trans}-[\text{CoCl}_2(\text{en})_2]^+$ is achiral and optically inactive due to an internal mirror plane.

4. Valence Bond Theory (VBT)

VBT views metal-ligand coordinate bonds as the overlap of vacant hybrid metal orbitals with ligand lone-pair orbitals.

Complex	Metal Ion & Config.	Hybridisation	Geometry	Unpaired $e^-$	Magnetic Character
$[\text{Co}(\text{NH}_3)_6]^{3+}$	$\text{Co}^{3+}\ (3d^6)$	$d^2sp^3$ (Inner)	Octahedral	0	Diamagnetic (Low-spin)
$[\text{CoF}_6]^{3-}$	$\text{Co}^{3+}\ (3d^6)$	$sp^3d^2$ (Outer)	Octahedral	4	Paramagnetic (High-spin)
$[\text{Ni}(\text{CN})_4]^{2-}$	$\text{Ni}^{2+}\ (3d^8)$	$dsp^2$	Square Planar	0	Diamagnetic
$[\text{NiCl}_4]^{2-}$	$\text{Ni}^{2+}\ (3d^8)$	$sp^3$	Tetrahedral	2	Paramagnetic
$[\text{Ni}(\text{CO})_4]$	$\text{Ni}^0\ (3d^{10})$	$sp^3$	Tetrahedral	0	Diamagnetic

5. Crystal Field Theory (CFT): Octahedral & Tetrahedral Splitting

CFT models metal-ligand interactions as electrostatic point-charge interactions, removing the degeneracy of the five $d$ -orbitals:

Figure 12.2: Crystal Field Splitting Profiles for Octahedral (Left) vs Inverted Tetrahedral (Right) Geometries

Octahedral Entities: Ligands approach along the $x, y, z$ axes. Axial orbitals ( $d_{x^2-y^2}, d_{z^2}$ ) experience greater repulsion and rise to the higher $e_g$ set ( $+0.6\Delta_o$ ). Inter-axial orbitals ( $d_{xy}, d_{yz}, d_{zx}$ ) drop into the lower $t_{2g}$ set ( $-0.4\Delta_o$ ).
Tetrahedral Entities: Ligands approach between the Cartesian axes. Splitting is inverted into $e$ (lower) and $t_2$ (higher), without the ' $g$ ' parity subscript due to the lack of an inversion centre: $\Delta_t = \frac{4}{9}\Delta_o$

6. Colour Origins, Spectrochemical Series & CFSE Calculations

Spectrochemical Series: Ligands arranged in order of increasing crystal field splitting energy: $\text{I}^- < \text{Br}^- < \text{SCN}^- < \text{Cl}^- < \text{S}^{2-} < \text{F}^- < \text{OH}^- < \text{C}_2\text{O}_4^{2-} < \text{H}_2\text{O} < \text{NCS}^- < \text{edta}^{4-} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$
Formula for Octahedral CFSE: $\text{CFSE} = \left[-0.4 \cdot n(t_{2g}) + 0.6 \cdot n(e_g)\right]\Delta_o + mP$ Where $n$ represents the number of electrons in each sub-level, $P$ is pairing energy, and $m$ is the number of extra electron pairs formed relative to the free gaseous ion.
Mechanism of Colour: Absorption of a photon promotes an electron between split $d$ $d$ -levels ( $d-d$ $d - d$ transition). The observed colour is the complementary colour of the wavelength absorbed:
- $[\text{Ti}(\text{H}_2\text{O})_6]^{3+}$ : absorbs blue-green ( $498\text{ nm}$ ), transmits violet.
- $[\text{Cu}(\text{H}_2\text{O})_4]^{2+}$ : absorbs red ( $600\text{ nm}$ ), transmits blue.

7. Synergic Bonding in Metal Carbonyls & Biological Roles

Synergic Mechanism in Metal Carbonyls:
- $\sigma$ -Bond: Carbonyl carbon donates its lone pair into an empty hybrid orbital of the metal atom ( $\text{OC} \rightarrow \text{M}$ ).
- $\pi$ -Back-Bond: Filled metal $d$ -orbitals donate electron density into vacant $\pi^*$ antibonding molecular orbitals of $\text{CO}$ ( $\text{M} \rightarrow \text{CO}$ ).
- This mutual reinforcement strengthens the $\text{M-C}$ bond while reducing the $\text{C-O}$ bond order.

Figure 12.3: Synergic Bonding Architecture in Metal Carbonyls Showing Mutual Bond Reinforcement

Vital Coordination Complexes in Medicine and Biology:
- Chlorophyll: Photosynthetic porphyrin complex containing central Magnesium ( $\text{Mg}^{2+}$ ).
- Haemoglobin: Oxygen-transport complex of blood containing central Iron ( $\text{Fe}^{2+}$ ).
- Vitamin $\text{B}_{12}$ (Cyanocobalamin): Anti-pernicious anaemia factor containing Cobalt ( $\text{Co}^{3+}$ ).
- cis-Platin ( $\text{cis}-[\text{PtCl}_2(\text{NH}_3)_2]$ ): Potent chemotherapeutic agent inhibiting cancer cell proliferation.
- Wilkinson's Catalyst ( $[(\text{PPh}_3)_3\text{RhCl}]$ ): Homogeneous catalyst for the hydrogenation of alkenes.
- EDTA Therapy: Treats acute lead poisoning via water-soluble excretion of the $[\text{Pb}(\text{edta})]^{2-}$ chelate.