Coordination Compounds | CBSE - Wyatt's Notes

sources:

• text: Standard textbook reference

Coordination compounds contain a central metal atom/ion bonded to ligands through coordinate bonds. They are fundamental to analytical chemistry, catalysis, and bioinorganic chemistry.

• Ligand: species donating electron pairs (monodentate, bidentate, polydentate)
• Coordination number: number of ligand donor atoms bonded to the central metal
• Oxidation state of metal: determined from charge of complex ion
• IUPAC naming: ligands first (alphabetical), then metal with oxidation state in Roman numerals
• Werner’s theory: primary valence (ionizable) and secondary valence (non-ionizable, coordination number)
• Crystal field theory: d-orbital splitting in ligand field
• Spectrochemical series: I$^-$ < Br$^-$ < Cl$^-$ < F$^-$ < OH$^-$ < H$_2$O < NH$_3$ < en < NO$_2$ < CN$^-$
• Strong field ligands (CN$^-$, CO): large splitting, low spin (paired electrons)
• Weak field ligands (I$^-$, Br$^-$): small splitting, high spin (unpaired electrons)

Problem: Give the IUPAC name for $[\text{Co(NH}_3)_4\text{Cl}_2]\text{Cl}$.

Solution:

Step 1: Identify the complex ion: $[\text{Co(NH}_3)_4\text{Cl}_2]^+$

Step 2: Ligands (alphabetical order):

• 4 NH$_3$: tetraammine
• 2 Cl: dichloro

Step 3: Metal with oxidation state: Co oxidation state: $x + 0 + 2(-1) = +1$ (charge of complex ion), so $x = +3$ Metal name: cobalt(III) (cation, so use element name)

Step 4: Counter ion: chloride

Full name: tetraamminedichloridocobalt(III) chloride

Common mistake: Forgetting alphabetical ordering of ligands. Ammine comes before chloro (a before c).

Problem: For $[\text{Fe(CN)}_6]^{3-}$, determine the number of unpaired electrons and whether it is paramagnetic.

Solution:

Fe$^{3+}$: $[\text{Ar}] 3d^5$ (5 d electrons)

CN$^-$ is a strong field ligand (high in spectrochemical series), causing large $\Delta_o$.

With large splitting, electrons pair up before occupying higher orbitals: $t_{2g}^5 \, e_g^0$

Unpaired electrons: 1 (one electron in each of the three $t_{2g}$ orbitals, plus one pair)

Wait — 5 electrons in 3 $t_{2g}$ orbitals: $\uparrow\downarrow, \uparrow\downarrow, \uparrow$ = 1 unpaired electron.

The complex is paramagnetic (has unpaired electrons).

Common mistake: Using high-spin filling for strong field ligands. Strong field ligands force low-spin configurations.

Problem: Identify the type of isomerism in $[\text{Co(NH}_3)_5\text{NO}_2]^{2+}$.

Solution:

This complex shows linkage isomerism. The NO$_2^-$ ligand can bond through:

• N (nitro): $-\text{NO}_2$ (nitro complex, yellow)
• O (nitrito): $-\text{ONO}$ (nitrito complex, red)

$[\text{Co(NH}_3)_5\text{(NO}_2)]^{2+} \quad \text{vs} \quad [\text{Co(NH}_3)_5\text{(ONO)}]^{2+}$

This occurs because NO$_2^-$ is an ambidentate ligand (can donate through two different atoms).

Common mistake: Confusing linkage isomerism with geometrical isomerism. Linkage involves the bonding atom, not the spatial arrangement.

1. Name $[\text{Pt(NH}_3)_2\text{Cl}_2]$.
2. For $[\text{CoF}_6]^{3-}$, determine the number of unpaired electrons.
3. Identify the type of isomerism in $[\text{Co(NH}_3)_4\text{Cl}_2]^+$.

Coordination compounds are used in analytical chemistry (complexometric titrations), medicine (cisplatin as anticancer drug), catalysis (Wilkinson’s catalyst), and biology (hemoglobin, vitamin B$_{12}$).

A central metal ion surrounded by electron-donating ligands: Think of a coordination compound as a central metal ion sitting in a cage made of ligands. Each ligand donates a pair of electrons to the metal, like hands gripping a ball from different directions. The more hands (ligands) and the stronger they grip, the more stable the complex. Crystal field theory adds that the d-orbitals of the metal split into different energy levels depending on the ligand arrangement — like how a ball squeezed from different sides deforms differently.

Why it matters: Coordination compounds are everywhere — hemoglobin carries oxygen using iron coordination, cisplatin fights cancer through platinum coordination, and industrial catalysts like Wilkinson’s catalyst rely on rhodium coordination. Without understanding these, modern medicine and industry wouldn’t exist.

The key insight: The spectrochemical series tells you whether electrons pair up in lower-energy d-orbitals (strong field) or spread out (weak field), which determines color, magnetism, and reactivity.

• IUPAC naming: alphabetical order, oxidation state in Roman numerals
• Strong field ligands produce low-spin complexes; weak field produce high-spin
• Paramagnetism: unpaired electrons attract to magnetic field
• Isomerism: linkage (ambidentate ligands), geometrical (cis/trans), optical (chirality)
• Spectrochemical series determines the magnitude of d-orbital splitting

Forgetting alphabetical order of ligands in IUPAC naming. Ligands must be listed in alphabetical order, not by charge or size. For example, ammine (NH3) comes before chloro (Cl) because ‘a’ precedes ‘c’. Prefixes like di-, tri- do not affect alphabetical ordering, so diamminodichlorido is correct.

Using high-spin filling for strong-field ligands. Strong-field ligands like CN- and CO cause large d-orbital splitting, forcing electrons to pair in lower orbitals before occupying higher ones (low-spin configuration). Students often apply the Aufbau principle without considering the ligand field strength, leading to incorrect electron configurations and magnetic moments.

Confusing linkage isomerism with geometrical isomerism. Linkage isomerism occurs when an ambidentate ligand (like NO2- or SCN-) bonds through different atoms. Geometrical isomerism involves different spatial arrangements (cis/trans) of ligands around the metal. These are fundamentally different types of isomerism with different causes.

• D-Block Elements: Coordination chemistry is primarily d-block chemistry — understanding transition metal properties explains why certain metals form stable complexes.
• P-Block Elements: Ligands like NH₃, CN⁻, and Cl⁻ are p-block compounds that coordinate to metal centers.
• Electrochemistry: Complex formation affects electrode potentials — the stability of coordination compounds influences redox behavior.
• Biomolecules: Biological coordination compounds like hemoglobin (Fe²⁺ with porphyrin) and vitamin B₁₂ (Co³⁺ with corrin) are essential for life.