Coordination Compounds Class 12 Notes: Quick Exam Revision

Essential Vocabulary & Exam Terms

Coordination Entity

A central metal atom or ion bonded to a fixed number of ions or molecules in a defined spatial arrangement.

Ligand

An ion or molecule bound to the central metal atom/ion in a coordination entity by sharing/donating its lone pair of electrons (acting as a Lewis base).

Coordination Number

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

Denticity

The number of donor atoms sharing electron pairs with a single central metal atom/ion in a coordination complex.

Chelate Ligand

A di- or polydentate ligand that uses two or more donor atoms simultaneously to bind to a single metal ion, forming a stable cyclic ring structure.

Homoleptic Complex

Complexes in which the central metal is bound to only one kind of donor group (e.g., [Co(NH3)6]3+).

Heteroleptic Complex

Complexes in which the central metal is bound to more than one kind of donor group (e.g., [Co(NH3)4Cl2]+).

Must-Remember Key Revision Points

• Werner's Theory: Metals exhibit primary valency (ionizable, corresponds to oxidation state) and secondary valency (non-ionizable, corresponds to coordination number, directional).
• Spectrochemical Series order: I- < Br- < SCN- < Cl- < S2- < F- < OH- < C2O4(2-) < H2O < NCS- < EDTA(4-) < NH3 < en < CN- < CO.
• Strong-field ligands (e.g., CN-, CO, en) cause pairing of electrons (forming low-spin complexes), whereas weak-field ligands (e.g., F-, Cl-, H2O) do not cause pairing (forming high-spin complexes).
• Isomerism: Broadly classified into Structural isomerism (linkage, coordination, ionization, hydrate) and Stereoisomerism (geometrical and optical).
• Magnetic Moment Formula: Spin-only magnetic moment mu = sqrt(n(n+2)) BM, where n is the number of unpaired electrons.
• Valence Bond Theory (VBT) coordinates hybridization with molecular geometry: sp3 (Tetrahedral), dsp2 (Square Planar), sp3d2 (Outer orbital octahedral), and d2sp3 (Inner orbital octahedral).
• Crystal Field Splitting: For octahedral complexes, five d-orbitals split into a lower t2g set (dxy, dyz, dzx) and a higher eg set (dx2-y2, dz2) due to axial ligand electrostatic repulsion.

Crystal Field Theory (CFT) and d-orbital Splitting

Crystal Field Theory (CFT) is an electrostatic model which considers the metal-ligand bond to be ionic, arising purely from electrostatic interactions. In an isolated gaseous metal atom/ion, all five d-orbitals are degenerate (have equal energy). When ligands approach, an asymmetric negative field is created, lifting this degeneracy.

In octahedral complexes, ligands approach along the axes. The $d_{x^2-y^2}$ and $d_{z^2}$ orbitals ($e_g$ set) point directly at the ligands and experience greater repulsion, raising their energy. Conversely, the $d_{xy}, d_{yz}, d_{zx}$ orbitals ($t_{2g}$ set) lie between the axes and experience less repulsion, staying at a lower energy state. The difference in energy between these two split sets is called the crystal field splitting energy ($\Delta_o$). If $\Delta_o > P$ (pairing energy, typical of strong-field ligands), electrons pair up in $t_{2g}$ orbitals, forming low-spin complexes. If $\Delta_o < P$ (weak-field ligands), electrons enter the $e_g$ orbitals without pairing, yielding high-spin complexes. In tetrahedral complexes, the approach of ligands occurs between the axes, reversing the splitting pattern where the $e$ set becomes lower in energy than the $t_2$ set, with the splitting energy $\Delta_t \approx \frac{4}{9} \Delta_o$.

Comparison: Double Salts vs. Coordination Compounds

Step-by-Step IUPAC Nomenclature Rules

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Worked Revision Examples

• {"title":"Example 1: IUPAC Naming of [Co(NH3)5(CO3)]Cl","bodyMarkdown":"Problem: Write the IUPAC name for $[Co(NH_3)_5(CO_3)]Cl$.\n\nSolution: \n1. Find the oxidation state of Cobalt ($x$): $x + 5(0) + (-2) + (-1) = 0 \\implies x = +3$.\n2. Identify ligands: Ammine (five times -> pentaamine) and Carbonato (one time).\n3. Arrange alphabetically: pentaamine before carbonato.\n4. Cationic sphere, so the metal name is 'cobalt'.\n5. IUPAC Name: Pentaaminecarbonatocobalt(III) chloride."}
• {"title":"Example 2: VBT and Magnetic Properties of [CoF6]3-","bodyMarkdown":"Problem: Predict the hybridization and magnetic properties of $[CoF_6]^{3-}$ using VBT.\n\nSolution:\n1. Central ion is $Co^{3+}$ with electronic configuration $[Ar] 3d^6$.\n2. $F^-$ is a weak-field ligand, meaning it cannot force pairing of the 3d electrons.\n3. The five 3d orbitals remain as: $\\uparrow\\downarrow, \\uparrow, \\uparrow, \\uparrow, \\uparrow$ (4 unpaired electrons).\n4. Six ligands require six empty hybrid orbitals, forcing the use of outer $4s$, $4p$, and $4d$ orbitals ($sp^3d^2$ hybridization).\n5. Structure is octahedral (outer orbital, high-spin complex).\n6. Magnetic Moment: $\\mu = \\sqrt{4(4+2)} = \\sqrt{24} \\approx 4.90$ BM (highly paramagnetic)."}

Board Exam Traps & Marking Cues

1. The Ammine Trap: In IUPAC spelling, always write 'ammine' with a double 'm' for $NH_3$ complexes. Spelling it 'amine' will lead to mark deduction in boards.
2. Valency Matching: In Werner's theory questions, remember that primary valencies match the oxidation state, while secondary valencies match the coordination number.
3. Isomerism Checks: Tetrahedral complexes do not show geometrical isomerism because all four positions are adjacent to one another. Do not draw cis/trans isomers for $sp^3$ structures!
4. Magnetic Formula Step: Always write down the formula $\mu = \sqrt{n(n+2)}$ BM explicitly before calculating. Writing only the final number of unpaired electrons can lose you step-marking points.

Quick Revision Check

• Why are low-spin tetrahedral complexes rarely observed? Because the crystal field splitting energy in tetrahedral fields (Delta_t) is small (about 4/9 of Delta_o) and almost always less than the electron pairing energy (P). Thus, electrons prefer to occupy higher energy orbitals rather than pairing up.
• What is linkage isomerism? Give an example. It occurs in coordination compounds containing ambidentate ligands (ligands that can bind through two different atoms). For example: [Co(NH3)5(ONO)]2+ (bound via Oxygen) and [Co(NH3)5(NO2)]2+ (bound via Nitrogen).
• Calculate the spin-only magnetic moment of [Fe(H2O)6]2+. Fe2+ has a 3d6 configuration. H2O is a weak-field ligand, so no electron pairing occurs, leaving 4 unpaired electrons (n = 4). Using the formula mu = sqrt(4*(4+2)), the spin-only magnetic moment is sqrt(24) = 4.90 BM.
• What is the hybridization and magnetic nature of [Ni(CN)4]2-? Ni2+ has a 3d8 configuration. CN- is a strong-field ligand that forces pairing of the 3d electrons, leaving one d-orbital vacant. The hybridization is dsp2, resulting in a square planar geometry. Since all electrons are paired, it is diamagnetic.

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