Coordination Compounds

Definition

A Ligand is a molecule or ion that can donate a pair of electrons to the central metal atom to form a coordinate covalent bond. The metal acts as a Lewis Acid (accepts electrons), and the ligand acts as a Lewis Base (donates electrons).

Explanation

Imagine the metal ion is a person with empty hands, and the ligands are friends offering a handshake. The "handshake" is the coordinate bond.

• Unidentate: Donate one electron pair (e.g., $\text{Cl}^-$, $\text{H}_2\text{O}$).
• Bidentate: Donate two electron pairs (e.g., oxalate ion, ethylenediamine).
• Polydentate: Donate multiple pairs (e.g., EDTA).
• Chelation: When a polydentate ligand binds to a metal, it forms a ring-like structure called a "chelate," enhancing stability.

Examples

• $\text{NH}_3$ (Ammonia) $\rightarrow$ Unidentate
• $\text{en}$ (ethylenediamine) $\rightarrow$ Bidentate

Real-Life Application: Water Softening

EDTA (a polydentate ligand) wraps around $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions in hard water like a claw, trapping them and preventing them from reacting with soap.

Coordination Number (CN)

The total number of coordinate bonds formed between the central metal ion and the ligands.

• $\text{[Pt(NH}_3)_4\text{]}^{2+} \rightarrow$ CN = 4
• $\text{[Co(NH}_3)_6\text{]}^{3+} \rightarrow$ CN = 6

Definition

Alfred Werner proposed that metal ions in coordination compounds exhibit two types of valencies: Primary and Secondary.

Explanation

• Primary Valency: Corresponds to the oxidation state. It is ionizable and satisfied by negative ions. (Spherical arrangement).
• Secondary Valency: Corresponds to the coordination number. It is non-ionizable and defines the geometry of the molecule. (Fixed spatial arrangement).

Example

In $\text{CoCl}_3 \cdot 6\text{NH}_3$, Cobalt has a primary valency of 3 and a secondary valency of 6.

Real-Life Use: Analytical Chemistry

Allows chemists to predict how many ions a complex will release in water, helping in precipitation reactions with $\text{AgNO}_3$.

The Rules

1. Order: Name ligands first (alphabetical), then the metal.
2. Ligand Names: $\text{H}_2\text{O}$ (aqua), $\text{NH}_3$ (ammine), $\text{Cl}^-$ (chlorido), $\text{CN}^-$ (cyanido).
3. Prefixes: *di-, tri-, tetra-* for simple; *bis-, tris-* for complex ligands.
4. Metal Name: If the complex is an anion, metal ends in "-ate" (e.g., Ferrate).
5. Oxidation State: Written in Roman numerals in brackets, e.g., (III).

Examples

• $\text{[Co(NH}_3)_6\text{]Cl}_3 \rightarrow$ Hexaamminecobalt(III) chloride.
• $\text{K}_4\text{[Fe(CN)}_6\text{]} \rightarrow$ Potassium hexacyanidoferrate(II).

Definition

Isomers are compounds with the same chemical formula but different arrangements of atoms.

1. Structural Isomerism

• Linkage: Different binding atoms (e.g., $\text{SCN}^-$ vs $\text{NCS}^-$).
• Ionization: Exchange of ligands between sphere and counter ion.
• Hydrate: Water molecules inside vs outside the sphere.
• Coordination: Exchange between cationic and anionic entities.

2. Stereoisomerism

• Geometrical: *Cis* (same side) and *Trans* (opposite side).
• Optical: Non-superimposable mirror images (Enantiomers).

Real-Life Use: Medicine (Cisplatin)

Cisplatin (cis-isomer of $\text{[Pt(NH}_3)_2\text{Cl}_2\text{]}$) is a powerful anti-cancer drug. Its isomer, Transplatin, is completely ineffective.

Valence Bond Theory (VBT)

Explains hybridization based on coordination number:

• $sp^3$ $\rightarrow$ Tetrahedral
• $dsp^2$ $\rightarrow$ Square Planar
• $sp^3d^2$ or $d^2sp^3$ $\rightarrow$ Octahedral

Crystal Field Theory (CFT)

Suggests ligands split the five $d$-orbitals into two sets: $t_{2g}$ and $e_g$.

• Strong Field Ligands (SFL): Large splitting $\rightarrow$ Forced pairing $\rightarrow$ Low Spin.
• Weak Field Ligands (WFL): Small splitting $\rightarrow$ No pairing $\rightarrow$ High Spin.

Comparison Table: VBT vs. CFT

Real-Life Use: Gemstones

The red color of rubies is due to $\text{Cr}^{3+}$ ions. The $\Delta_o$ gap allows the crystal to absorb green-blue light and reflect red.

Color

Arises from d-d transitions. If $\text{d}^0$ or $\text{d}^{10}$, the compound is usually colorless.

Magnetic Moment ($\mu$)

($n = \text{unpaired electrons}$, $\text{BM} = \text{Bohr Magneton}$)

• MRI Contrast Agents: Gadolinium ($\text{Gd}^{3+}$) complexes are used for high paramagnetism.
• Catalysis: Wilkinson’s Catalyst $\text{[RhCl(PPh}_3)_3\text{]}$ for alkene hydrogenation.
• Waste Water: Chelating agents remove heavy metals like $\text{Pb}^{2+}$ and $\text{Hg}^{2+}$.

Hemoglobin contains an $\text{Fe}^{2+}$ ion coordinated to a porphyrin ring. One site is open for $\text{O}_2$ to bind.

The Conflict: Carbon Monoxide ($\text{CO}$) is a strong field ligand. It binds to $\text{Fe}^{2+}$ nearly 200 times more tightly than $\text{O}_2$ does.

The Result: $\text{CO}$ locks the hemoglobin, forming carboxy-hemoglobin and preventing oxygen transport, which leads to lethal poisoning. This demonstrates how ligand strength dictates biological survival.

1. Which of the following is a bidentate ligand?
   Ans: (c) $\text{ox}^{2-}$
2. The coordination number of $\text{Co}$ in $\text{[Co(en)}_3\text{]Cl}_3$ is:
   Ans: (b) 6
3. Which theory explains the color of coordination complexes?
   Ans: (c) CFT
4. A complex with $dsp^2$ hybridization is usually:
   Ans: (c) Square Planar
5. Which ligand is the strongest?
   Ans: (d) $\text{CN}^-$
6. Optical isomerism is possible in:
   Ans: (b) $\text{[Co(en)}_3\text{]}^{3+}$
7. The magnetic moment of a $\text{d}^3$ system is approximately:
   Ans: (b) 3.87 BM
8. In $\text{[Pt(NH}_3)_2\text{Cl}_2\text{]}$, the *trans* isomer is:
   Ans: (b) Ineffective in cancer treatment
9. Which of the following is a "low spin" complex?
   Ans: (b) $\text{[Fe(CN)}_6\text{]}^{4-}$
10. The IUPAC name for $\text{[Ni(CO)}_4\text{]}$ is:
    Ans: (a) Tetracarbonylnickel(0)

1. Q: Distinguish between a double salt and a complex salt.
   Ans: Double salts dissociate completely into simple ions in water; complex salts do not.
2. Q: Why are most coordination compounds colored?
   Ans: Due to d-d transitions caused by the splitting of d-orbitals (CFT).
3. Q: Explain the "Chelate Effect."
   Ans: Polydentate ligands form ring structures which are significantly more stable than unidentate complexes.
4. Q: Difference between *fac* and *mer* isomers?
   Ans: *Facial (fac)* ligands occupy one face of the octahedron; *meridional (mer)* ligands are around the meridian.
5. Q: Predict geometry of $\text{[Ni(CN)}_4\text{]}^{2-}$.
   Ans: Square Planar ($dsp^2$) and diamagnetic.
6. Q: Why is $\text{[Ti(H}_2\text{O)}_6\text{]}^{3+}$ colored but $\text{[Sc(H}_2\text{O)}_6\text{]}^{3+}$ is colorless?
   Ans: $\text{Ti}^{3+}$ is $\text{d}^1$ (transition possible), while $\text{Sc}^{3+}$ is $\text{d}^0$ (no electrons).
7. Q: Define Ambidentate ligands.
   Ans: Ligands that can bind via two different atoms, e.g., $\text{NO}_2^-$ (N or O).
8. Q: Effect of oxidation state on $\Delta_o$?
   Ans: Increasing oxidation state increases $\Delta_o$ due to stronger ligand attraction.
9. Q: IUPAC name of $\text{[Co(NH}_3)_5\text{Cl]Cl}_2$.
   Ans: Pentaamminechloridocobalt(III) chloride.
10. Q: Why do strong field ligands lead to low spin?
    Ans: Large $\Delta_o$ makes pairing energy lower than the jump to $e_g$ orbital.

• Spectrochemical Series: *"**I** **Br**ing **Cl**ean **F**lowers, **O**h $\text{H}_2\text{O}$ $\text{NH}_3$ **en** $\text{CN}$ $\text{CO}$"* (Weakest $\rightarrow$ Strongest).
• VBT Geometry Check:
  • CN 4 + SFL = Square Planar
  • CN 4 + WFL = Tetrahedral
  • CN 6 = Octahedral