Coordination Compounds

Definition

Coordination compounds are complex chemical species consisting of a central metal atom or ion bonded to a fixed number of surrounding molecules or ions through coordinate covalent bonds.

Explanation

• According to Alfred Werner, coordination compounds are formed by the combination of simple stable compounds where the central metal exhibits two types of valency, namely primary and secondary valency.
• According to modern coordination chemistry, they are entities in which a metal atom is linked to a number of ions or neutral molecules by sharing electron pairs donated by those ions or molecules.
• When a transition metal is dropped into a solution containing molecules with lone pairs of electrons, the metal happily accepts those electron pairs into its empty d-orbitals.

Key Points

• Contains a central metal atom or ion (usually a transition metal).
• Contains ligands that donate electron pairs.
• Features coordinate covalent (dative) bonds.
• Written inside square brackets, for example, [Co(NH3)6]Cl3.

Examples

• Haemoglobin in our blood, which is an iron coordination complex responsible for carrying oxygen.
• Chlorophyll in green leaves, which is a magnesium coordination complex responsible for photosynthesis.

Real-Life Uses

• Medicine: Cisplatin, a platinum-based coordination compound, is widely used in chemotherapy to treat various types of cancer.
• Water Softening: EDTA forms stable coordination compounds with calcium and magnesium ions in hard water.

Definition

Ligands are ions or neutral molecules that donate a pair of electrons to the central metal atom or ion to form a coordinate covalent bond.

Explanation

• According to IUPAC, a ligand is a molecular entity bonded to a central atom in a coordination or organometallic entity.
• Textbooks define ligands as Lewis bases because they donate electron pairs to the Lewis acidic central metal ion.
• Ligands must possess at least one lone pair of electrons and are classified as monodentate, bidentate, or polydentate.

Key Points

• Act as Lewis bases.
• Can be neutral, anionic, or cationic.
• Chelation occurs when polydentate ligands form ring-like structures, increasing stability.

Examples

• Ammonia (NH3), a neutral monodentate ligand.
• Oxalate ion (C2O4 2-), a bidentate ligand.

Real-Life Uses

• Photography: Silver halides form soluble coordination complexes with sodium thiosulphate (hypo) as ligands.
• Extraction of Gold: Cyanide ligands are used in the cyanide process to extract gold and silver from their ores.

Definition

The coordination number (CN) of a metal ion in a coordination compound is the total number of ligand donor atoms directly attached to the central metal atom.

Explanation

• It represents the number of coordinate bonds formed by the central metal ion with the ligands.
• It represents the total number of sigma bonds between ligands and the central metal atom, independent of whether ligands are monodentate or polydentate.

Key Points

• Common coordination numbers are 2, 4, and 6.
• Directly dictates the spatial geometry of the complex.

Examples

• In [Cu(NH3)4]2+, the coordination number is 4.
• In [Fe(C2O4)3]3-, the coordination number is 6 (3 oxalates × 2 donor atoms each).

Real-Life Uses

• Industrial Catalysis: Helps chemists design catalysts with vacant coordination sites for reactant binding.

Definition

The characteristic colour exhibited by coordination compounds arises due to the absorption of light in the visible region, causing electronic transitions.

Explanation

• Explained by Crystal Field Theory as d-d transitions, where electrons jump from lower energy d-orbitals to higher energy d-orbitals by absorbing specific frequencies of visible light.
• The observed colour is the transmission of complementary light frequencies.

Key Points

• Depends on the presence of unpaired d-electrons (d1 to d9 systems).
• d0 and d10 configurations are generally colourless because d-d transitions are not possible.

Examples

• Anhydrous copper sulfate is white, but hydrated copper sulfate [Cu(H2O)4]SO4 is bright blue.
• Ruby gemstone gets its red colour from chromium(III) ions.

Real-Life Uses

• Gemstone Industry & Pigments: Utilization of rich colours in rubies, emeralds, synthetic paints, and textile dyes.

Definition

The magnetic property of a coordination compound describes its response to an external magnetic field, determined by the presence of unpaired electrons.

Explanation

• Paramagnetism is exhibited by substances with unpaired electrons that are attracted by an external magnetic field.
• Diamagnetism is exhibited by substances with all paired electrons that are weakly repelled by an external magnetic field.

Key Points

• Calculated using the spin-only formula: $\mu = \sqrt{n(n + 2)}$ Bohr Magnetons (BM).
• Strong field ligands cause electron pairing (low spin); weak field ligands leave more unpaired electrons (high spin).

Examples

• [Fe(CN)6]3- contains 1 unpaired electron and is paramagnetic.
• [Fe(CN)6]4- contains 0 unpaired electrons and is diamagnetic.

Real-Life Uses

• MRI Scans: Gadolinium-based coordination compounds serve as contrast agents due to strong paramagnetism.

Definition

The shape or geometry of a coordination compound refers to the spatial arrangement of the ligand donor atoms around the central metal ion.

Explanation

• Geometries are predicted by Valence Bond Theory and VSEPR principles based on coordination number and hybridization.
• Ligand electron pairs repel each other to achieve a minimum repulsion state in symmetrical geometric patterns.

Key Points

• Coordination Number 4: Tetrahedral (sp3) or Square Planar (dsp2).
• Coordination Number 6: Octahedral (d2sp3 or sp3d2).

Examples

• [Ni(CO)4] is tetrahedral.
• [PtCl4]2- is square planar.

Real-Life Uses

• Drug Design: The precise square planar shape of cisplatin is crucial for interacting with DNA base pairs.

Definition

IUPAC nomenclature is the systematic, internationally accepted set of rules used to name chemical coordination compounds unambiguously.

Explanation

• Cationic parts, ligands in alphabetical order, central metal, and anionic parts are designated in a specific sequence.
• If the complex is an anionic sphere, the suffix "-ate" is added to the metal's Latin name.

Key Points

• Cation is always named before the anion.
• Ligands are named alphabetically (neutral: aqua, ammine; anionic: ending in "-o").
• Oxidation state is written in Roman numerals in parentheses.

Examples

• [Co(NH3)6]Cl3 is Hexaamminecobalt(III) chloride.
• K4[Fe(CN)6] is Potassium hexacyanidoferrate(II).

Real-Life Uses

• Global Chemical Communication: Ensures safety and clarity across research and pharmaceutical manufacturing.

Definition

Isomerism occurs when two or more coordination compounds share the exact same chemical formula but have different arrangements of atoms or spatial orientations.

Explanation

• Structural isomerism arises from differences in atom connectivity (Linkage, Coordination, Ionization, Solvate).
• Stereoisomerism arises from different spatial arrangements (Geometrical cis/trans and Optical d/l).

Key Points

• Octahedral complexes of type [Ma2b4] show cis-trans isomerism.
• Tetrahedral complexes rarely show optical isomerism due to planes of symmetry.

Examples

• [Co(NH3)5(NO2)]Cl2 and [Co(NH3)5(ONO)]Cl2 are linkage isomers.
• Cis-platin is an active anticancer drug, whereas trans-platin is inactive and toxic.

Real-Life Uses

• Pharmacology: Demonstrates how 3D spatial arrangement dictates medicinal efficacy versus toxicity.

Definition

Bonding theories explain how and why the central metal ion and ligands bond together, accounting for magnetic, spectral, and structural properties.

Explanation

• Werner's Theory: Primary valencies (ionizable) and secondary valencies (non-ionizable).
• Valence Bond Theory (VBT): Hybridization of metal orbitals overlapping with ligand orbitals.
• Crystal Field Theory (CFT): Electrostatic model treating ligands as point charges splitting d-orbitals.

Key Points

• VBT accounts for magnetic moments and coordination geometries.
• CFT explains colours, magnetic properties, and ligand field splitting energy ($\Delta_o$).

Examples

• [CoF6]3- is outer orbital, high spin, paramagnetic.
• [Co(NH3)6]3+ is inner orbital, low spin, diamagnetic.

Real-Life Uses

• Display Technology: Crystal field splitting principles assist in designing materials for LED screens.

Definition

Coordination compounds play indispensable roles in biological systems, industrial catalysis, metallurgy, and medicine.

Explanation

• Applications span analytical chemistry, biological systems, and industrial extraction processes.

Key Points

• Biological roles: Haemoglobin, chlorophyll, Vitamin B12.
• Analytical chemistry: Hardness estimation using EDTA, metal ion detection using DMG.
• Metallurgy: Gold and silver extraction via cyanide complexation.

Examples

• Vitamin B12 is a cobalt coordination compound vital for human nervous system function.
• Nickel is purified industrially using the Mond process via [Ni(CO)4].

Real-Life Uses

• Cancer treatment (Cisplatin) and water treatment (EDTA).

Problem 1

Calculate the spin-only magnetic moment of a divalent metal ion in aqueous solution if its atomic number is 25.

Step-1: Identify element with atomic number 25 -> Manganese (Mn).
Step-2: Neutral Mn configuration: [Ar] 3d5 4s2.
Step-3: Divalent ion (Mn2+) configuration: [Ar] 3d5.
Step-4: Number of unpaired electrons (n) = 5.
Step-5: Formula: μ = √(n(n + 2))
Step-6: Calculation: μ = √(5(7)) = √35 ≈ 5.92 BM.
Answer: 5.92 Bohr Magnetons.
        

Problem 2

A coordination compound has the formula CoCl3.5NH3. When treated with excess silver nitrate solution, 2 moles of silver chloride precipitate per mole of compound. Write its structural formula and IUPAC name.

Step-1: 1 mole compound gives 2 moles AgCl precipitate -> 2 Cl- ions are outside the bracket.
Step-2: Remaining 1 Cl- and 5 NH3 are inside the coordination sphere.
Step-3: Structural formula: [Co(NH3)5Cl]Cl2.
Step-4: Coordination number = 6.
Step-5: IUPAC Name: Pentaamminechloridocobalt(III) chloride.
        

• Spin-only magnetic moment: $\mu = \sqrt{n(n + 2)}$ BM
• Crystal field splitting energy for octahedral complexes: $\Delta_o$
• Relation between wavelength and energy: $E = \frac{hc}{\lambda}$

• Spectrophotometers: Measure absorbance spectra to determine complex concentrations.
• Computational Chemistry Software: Model molecular orbitals and predict ligand binding affinities.
• Ion-Exchange Resins: Industrial water purification systems removing heavy metal pollutants.

Cancer remains one of the most challenging diseases in modern medicine, but a simple coordination compound discovered in the 1960s changed oncology forever: Cisplatin.

Dr. Barnett Rosenberg studied electric field effects on E. coli division using platinum electrodes and discovered that a platinum coordination compound, cis-diamminedichloroplatinum(II), halted cell division by forming giant filaments.

Cisplatin features a square planar geometry where two chlorine ligands and two ammonia molecules sit adjacent to each other. Inside the cell, chloride ligands are displaced by water molecules. The complex binds strongly to adjacent guanine bases on DNA, causing a bend in the double helix that triggers apoptosis in cancer cells. The trans-isomer is medically ineffective, demonstrating how 3D spatial arrangement impacts pharmacology.

1. Q1. What is the coordination number of iron in [Fe(EDTA)]-?
   A) 2
   B) 4
   C) 6
   D) 8
   Answer: C) 6
   Explanation: EDTA is a hexadentate ligand donating 6 electron pairs.


2. Q2. Which of the following complexes is diamagnetic?
   A) [Fe(CN)6]3-
   B) [Fe(CN)6]4-
   C) [CoF6]3-
   D) [Mn(H2O)6]2+
   Answer: B) [Fe(CN)6]4-
   Explanation: Strong field cyanide ligand causes all 6 d-electrons to pair up ($n=0$).


3. Q3. What is the IUPAC name of K4[Fe(CN)6]?
   A) Potassium hexacyanoferrate(II)
   B) Potassium hexacyanidoferrate(III)
   C) Tetrapotassium hexacyanoiron(II)
   D) Potassium hexacyano iron(II)
   Answer: A) Potassium hexacyanoferrate(II)
   Explanation: Cation named first, followed by anionic complex with iron in +2 state.


4. Q4. Which of the following species is expected to be colourless?
   A) [Ti(H2O)6]3+
   B) [Cu(NH3)4]2+
   C) [Sc(H2O)6]3+
   D) [Co(NH3)6]3+
   Answer: C) [Sc(H2O)6]3+
   Explanation: Scandium(III) has a d0 configuration, preventing d-d transitions.


5. Q5. The geometry of [Ni(CO)4] is:
   A) Square planar
   B) Tetrahedral
   C) Octahedral
   D) Linear
   Answer: B) Tetrahedral
   Explanation: Undergoes sp3 hybridization due to strong carbonyl ligands.


6. Q6. Which type of isomerism is exhibited by [Co(NH3)5(NO2)]Cl2?
   A) Geometrical isomerism
   B) Hydrate isomerism
   C) Linkage isomerism
   D) Ionization isomerism
   Answer: C) Linkage isomerism
   Explanation: Ambidentate ligand NO2- coordinates through N or O.


7. Q7. What is the spin-only magnetic moment of a d5 ion with strong field ligands?
   A) 1.73 BM
   B) 2.83 BM
   C) 3.87 BM
   D) 5.92 BM
   Answer: A) 1.73 BM
   Explanation: Electrons pair up leaving 1 unpaired electron ($n=1$, $\mu = \sqrt{3} \approx 1.73$ BM).


8. Q8. Which of the following is a bidentate ligand?
   A) Ammonia
   B) Water
   C) Ethylene diamine
   D) Cyanide
   Answer: C) Ethylene diamine
   Explanation: Ethylene diamine has two donor nitrogen atoms.


9. Q9. Which vitamin contains a cobalt coordination center?
   A) Vitamin A
   B) Vitamin C
   C) Vitamin B12
   D) Vitamin D
   Answer: C) Vitamin B12
   Explanation: Cobalamin contains a central cobalt ion.


10. Q10. According to Werner's theory, secondary valency corresponds to:
    A) Oxidation state
    B) Coordination number
    C) Number of counter ions
    D) Number of charges on the complex
    Answer: B) Coordination number
    Explanation: Secondary valency is non-ionizable and corresponds to coordination number.

1. Q1. Explain why [Ti(H2O)6]3+ is coloured while [Sc(H2O)6]3+ is colourless.
   Answer: Titanium(III) has a d1 configuration allowing d-d electronic transitions in visible light. Scandium(III) has a d0 configuration with no d-electrons, rendering it colourless.


2. Q2. What is chelation? Give one biological example.
   Answer: Chelation is the formation of ring structures when a polydentate ligand attaches to a metal ion through multiple donor sites, yielding high stability. Example: Haemoglobin contains an iron porphyrin ring system.


3. Q3. Using Crystal Field Theory, write the electronic configuration of d4 in terms of t2g and eg for octahedral complexes when Delta_o > P and Delta_o < P.
   Answer: When $\Delta_o > P$, $t_{2g}^4 e_g^0$. When $\Delta_o < P$, $t_{2g}^3 e_g^1$.


4. Q4. Write the IUPAC name of [Co(en)2Cl2]Cl.
   Answer: Dichloridobis(ethylenediamine)cobalt(III) chloride.


5. Q5. Explain linkage isomerism with a suitable example.
   Answer: Occurs with ambidentate ligands. Example: [Co(NH3)5(NO2)]Cl2 exists in a yellow nitro form and red nitrito form.


6. Q6. Calculate the spin-only magnetic moment of [Fe(H2O)6]2+.
   Answer: Fe2+ has a d6 configuration with weak field water ($t_{2g}^4 e_g^2$), giving 4 unpaired electrons ($n=4$). $\mu = \sqrt{24} \approx 4.90$ BM.


7. Q7. Differentiate between double salts and coordination compounds with examples.
   Answer: Double salts dissociate completely into simple ions in water (e.g., Mohr's salt). Coordination compounds retain their complex entity in solution (e.g., K4[Fe(CN)6]).


8. Q8. Why are square planar complexes more common for d8 metal ions with strong field ligands?
   Answer: Maximizes crystal field stabilization energy and minimizes ligand-ligand repulsion in a 2D plane.


9. Q9. Give the formula and medicinal application of Cisplatin.
   Answer: Formula: cis-[Pt(NH3)2Cl2]. Application: Anticancer drug treating testicular, ovarian, and bladder cancers.


10. Q10. What are ambidentate ligands? Give two examples.
    Answer: Monodentate ligands with two different donor atoms capable of coordinating through either. Examples: Nitrite ion (NO2-) and Thiocyanate ion (SCN-).

• Coordination compounds contain metal-centered complexes bound by dative bonds.
• Ligands act as Lewis bases donating electron pairs.
• Coordination number counts direct attachment points, not total ligands.
• Colour arises from d-d electronic transitions in d1-d9 systems.
• Paramagnetism requires unpaired electrons; diamagnetism requires all paired electrons.
• Cisplatin (cis-geometry) is a life-saving anticancer drug.

Mnemonic for Spectrochemical Series (Weak to Strong Field):
*“I Brought Some Chocolate From Our Warehouse, Never Did I Prepare Cyanide Carbonyl”*
(I-, Br-, SCN-, Cl-, S2-, F-, OH-, C2O4 2-, H2O, NCS-, EDTA, NH3, en, NO2-, CN-, CO)