Coordination Compounds: Unravelling Complex Chemistry for CBSE Class 12

What are Coordination Compounds? Key Definitions

Coordination Compound

A compound containing a central metal atom or ion (usually a transition metal) bonded to a cluster of molecules or ions, called ligands, via coordinate covalent bonds. These compounds retain their identity even in solution.

Central Metal Atom/Ion

The acceptor of electron pairs (Lewis acid) from ligands, usually a transition metal or inner transition metal, forming coordinate bonds.

Ligand

An atom, ion, or molecule capable of donating one or more electron pairs to the central metal atom or ion. Ligands can be monodentate (one donor atom), bidentate (two donor atoms), or polydentate (multiple donor atoms).

Coordination Number

The total number of donor atoms from the ligands directly attached to the central metal atom or ion in a complex. It represents the number of coordinate bonds formed.

Coordination Sphere

The central metal atom/ion and the ligands directly attached to it, enclosed in a square bracket [ ]. These entities act as a single unit and do not dissociate in solution.

Counter Ion

An ion present outside the coordination sphere that balances the charge of the complex ion. It dissociates in solution and is not directly bonded to the central metal.

IUPAC Nomenclature of Coordination Compounds

1. Step 1: Naming the Cation and Anion — Always name the cation first, followed by the anion. If the complex is neutral, this step is skipped. Remember, the entire coordination sphere can be a cation, an anion, or neutral.
2. Step 2: Naming Ligands — Within the coordination sphere, ligands are named first in alphabetical order (ignoring prefixes like di-, tri-). Anionic ligands end with '-o' (e.g., chloro, cyano, hydroxo). Neutral ligands usually retain their name (e.g., ammonia as 'ammine', water as 'aqua', carbon monoxide as 'carbonyl', nitric oxide as 'nitrosyl'). Cationic ligands are rare and end in '-ium'.
3. Step 3: Indicating Number of Ligands — Use prefixes 'di-', 'tri-', 'tetra-', 'penta-', 'hexa-' for simple ligands. For complex ligands (e.g., ethylenediamine, triphenylphosphine) or when the ligand name itself contains a numerical prefix, use 'bis-', 'tris-', 'tetrakis-' instead, and enclose the ligand name in parentheses.
4. Step 4: Naming the Central Metal Atom/Ion — If the complex is anionic, the name of the metal ends with the suffix '-ate' (e.g., ferrate for Fe, cuprate for Cu, argentate for Ag). If the complex is cationic or neutral, the metal name remains unchanged. For some metals, the Latin name is used with '-ate' (e.g., lead becomes plumbate, tin becomes stannate).
5. Step 5: Stating the Oxidation State — The oxidation state of the central metal atom/ion is indicated by a Roman numeral in parentheses immediately after the metal's name (without any space). This is a crucial step for correctly identifying the complex.

Isomerism in Coordination Compounds

Isomerism refers to the phenomenon where two or more compounds have the same chemical formula but different arrangements of atoms. Coordination compounds exhibit a wide variety of isomers, categorized into two main types: structural isomerism and stereoisomerism.

1. Structural Isomerism: These isomers have the same chemical formula but differ in the way the ligands are bonded to the central metal atom or in the composition of the coordination sphere.

• Ionisation Isomerism: Occurs when the counter ion in the complex salt is itself a potential ligand and can displace a ligand which then becomes the counter ion. For example, [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br. In the first, bromide is a ligand and sulfate is the counter ion; in the second, sulfate is a ligand and bromide is the counter ion.
• Hydrate Isomerism: A specific type of ionisation isomerism where water molecules are involved. Isomers differ in whether water is a ligand or a molecule of crystallisation (outside the coordination sphere). For example, [Cr(H₂O)₆]Cl₃ (violet), [Cr(H₂O)₅Cl]Cl₂·H₂O (blue-green), and [Cr(H₂O)₄Cl₂]Cl·2H₂O (dark green).
• Linkage Isomerism: Arises when an ambidentate ligand (a ligand that can bind to the central metal atom through two different atoms) coordinates through different atoms. Examples of ambidentate ligands include NO₂⁻ (can bind via N or O) and SCN⁻ (can bind via S or N). For instance, [Co(NH₃)₅(NO₂)]Cl₂ (nitrito-N, yellow) and [Co(NH₃)₅(ONO)]Cl₂ (nitrito-O, red).
• Coordination Isomerism: Occurs in compounds where both the cation and anion are complex ions, and the ligands are exchanged between the two complex entities. For example, [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

2. Stereoisomerism: These isomers have the same chemical formula and the same bonding, but differ in the spatial arrangement of the ligands around the central metal atom.

• Geometrical Isomerism (cis-trans Isomerism): Arises due to different possible spatial arrangements of ligands around the central metal ion. It is common in square planar (e.g., MA₂B₂ type) and octahedral complexes (e.g., MA₄B₂ type). In cis isomers, identical ligands are adjacent; in trans isomers, they are opposite. For octahedral complexes of type MA₃B₃, facial (fac) isomers have identical ligands on one face of the octahedron, while meridional (mer) isomers have identical ligands along a meridian.
• Optical Isomerism (Enantiomerism): Occurs when a complex and its mirror image are non-superimposable. Such complexes are called enantiomers and are chiral. Octahedral complexes are more likely to show optical isomerism, especially those containing bidentate ligands, such as [Co(en)₃]³⁺ (where 'en' is ethylenediamine).

Bonding in Coordination Compounds: VBT and CFT

Understanding how ligands bond to metal ions and what gives coordination compounds their unique properties requires specific theories. The two main theories we study are Valence Bond Theory (VBT) and Crystal Field Theory (CFT).

Valence Bond Theory (VBT):
Developed by Linus Pauling, VBT explains the formation of coordinate covalent bonds by considering the overlap of filled ligand orbitals with vacant metal ion orbitals. Key aspects include:

• Hybridisation: The central metal atom undergoes hybridisation of its atomic orbitals (s, p, d) to form a set of equivalent hybrid orbitals that can accommodate electron pairs from ligands. Common hybridisations and their corresponding geometries are: sp³ (tetrahedral), dsp² (square planar), sp³d² (outer octahedral), and d²sp³ (inner octahedral).
• Magnetic Properties: By knowing the electron configuration of the central metal ion and its hybridisation, we can determine if the complex is diamagnetic (all electrons paired) or paramagnetic (unpaired electrons present). Ligands are classified as strong-field (cause electron pairing, forming inner orbital complexes) or weak-field (do not cause pairing, forming outer orbital complexes). For example, [Ni(CN)₄]²⁻ (dsp² hybridisation, diamagnetic) versus [NiCl₄]²⁻ (sp³ hybridisation, paramagnetic).
• Limitations: VBT doesn't explain the colour of coordination compounds, the relative stabilities of complexes, or the quantitative magnetic properties.

Crystal Field Theory (CFT):
CFT provides a more refined explanation, treating the metal-ligand bond as purely electrostatic, arising from the attraction between the positively charged metal ion and the negative charge (or dipole) of the ligands. Key aspects include:

• d-orbital Splitting: In the presence of ligands, the degenerate d-orbitals of the central metal ion split into different energy levels. For octahedral complexes, the five d-orbitals split into two sets: two higher energy orbitals (e_g) and three lower energy orbitals (t₂_g). The energy difference between these sets is called crystal field splitting energy (Δ₀ or 10 Dq).
• Spectrochemical Series: Ligands are arranged in a series based on their ability to cause d-orbital splitting (strength of the crystal field). Examples: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < CN⁻ < CO. Strong-field ligands (e.g., CN⁻, CO) cause large splitting, while weak-field ligands (e.g., Cl⁻, H₂O) cause small splitting.
• Colour: The colour of coordination compounds is explained by the absorption of light (d-d transitions) as electrons move between the split d-orbitals. The absorbed colour is complementary to the observed colour.
• Magnetic Properties: CFT explains magnetic properties by considering the distribution of electrons in the split d-orbitals, taking into account pairing energy (P) and crystal field splitting energy (Δ₀). If Δ₀ > P, electrons pair up in t₂_g orbitals (low spin); if Δ₀ < P, electrons occupy e_g orbitals before pairing (high spin).
• Merits: Successfully explains colour, magnetic properties, and to some extent, stability and geometry.

Worked Examples: Applying the Concepts

• Example 1: Naming a Complex Name the complex: K₃[Fe(CN)₆] Step 1: Identify Cation and Anion. K₃ is the cation (Potassium), and [Fe(CN)₆]³⁻ is the anionic complex. Step 2: Name Ligands. The ligand is CN⁻, which is 'cyano'. There are six of them, so 'hexacyano'. Step 3: Name Central Metal and Oxidation State. Since the complex is anionic, the metal iron (Fe) takes the suffix '-ate', becoming 'ferrate'. Let the oxidation state of Fe be x. The charge of K is +1, and CN is -1. 3(+1) + x + 6(-1) = 0 3 + x - 6 = 0 x - 3 = 0 x = +3 The oxidation state of iron is (+3). Step 4: Combine to form the full name. Cation first: Potassium. Then the complex: hexacyanoferrate(III). Final Answer: Potassium hexacyanoferrate(III)
• Example 2: Determining VBT Hybridisation, Geometry, and Magnetic Nature Determine the hybridisation, geometry, and magnetic nature of [Ni(CN)₄]²⁻. Step 1: Determine Oxidation State of Central Metal. Let the oxidation state of Ni be x. CN⁻ is a monodentate ligand with a -1 charge. x + 4(-1) = -2 x - 4 = -2 x = +2. So, the central metal is Ni²⁺. Step 2: Write Electronic Configuration of Metal Ion. Atomic number of Ni = 28. Electronic configuration: [Ar] 3d⁸ 4s². For Ni²⁺: [Ar] 3d⁸ (4s⁰ 4p⁰ 4d⁰) Step 3: Consider Ligand Strength and Hybridisation. CN⁻ (cyanide) is a strong field ligand. It causes pairing of electrons in the d-orbitals. Original 3d⁸: ↑↓ ↑↓ ↑↓ ↑_ ↑_ (two unpaired electrons) After pairing (due to strong field ligand): ↑↓ ↑↓ ↑↓ ↑↓ __ (all electrons paired, one 3d orbital is vacant) Now, for hybridisation, we need vacant orbitals for the four CN⁻ ligands to donate electron pairs. One vacant 3d orbital, one 4s orbital, and two 4p orbitals combine: d + s + p + p = dsp² hybridisation. Step 4: Determine Geometry and Magnetic Nature. dsp² hybridisation corresponds to Square Planar geometry. Since all electrons are paired after hybridisation, the complex is Diamagnetic. Final Answer: Hybridisation: dsp², Geometry: Square Planar, Magnetic nature: Diamagnetic

YoLearn.ai Exam Tip: Mastering Coordination Compounds

Coordination Compounds can be tricky, but a few key focus areas can help you ace your exams:

1. IUPAC Nomenclature: This is a guaranteed question. Practice naming at least 20-30 complexes, both simple and with ambidentate/complex ligands. Pay close attention to the order of naming, prefixes (di/tri vs. bis/tris), oxidation state in Roman numerals, and the '-ate' suffix for anionic complexes. A common mistake is forgetting to use '-ate' for anionic complexes or miscalculating the oxidation state.
2. Isomerism: Understand the distinctions between all types of structural and stereoisomerism. Drawing structures, especially for geometrical and optical isomers, is crucial for clarity. Remember that optical isomers are non-superimposable mirror images. Practice identifying which type of isomerism is shown by given pairs of compounds.
3. VBT and CFT: While VBT gives a basic idea of bonding and magnetic properties, CFT offers a more detailed explanation of colour and better magnetic property predictions. For VBT, remember to check ligand strength (strong field vs. weak field) to determine if pairing occurs. For CFT, focus on drawing d-orbital splitting diagrams for octahedral and tetrahedral complexes, and relate Δ₀ and P to high-spin/low-spin complexes and colours. Don't confuse inner orbital and outer orbital complexes.

Practice Questions with Solutions

• Q: Write the IUPAC name for [Co(NH₃)₅Cl]Cl₂. A: Step 1: Identify cation and anion. Cation is [Co(NH₃)₅Cl]²⁺, anion is Cl⁻. Step 2: Name ligands. Ammine (NH₃) and Chloro (Cl⁻). Ammine comes before Chloro alphabetically. There are five ammine ligands (pentaammine) and one chloro ligand. Step 3: Name central metal and oxidation state. The complex is cationic, so Cobalt is used. Let oxidation state of Co be x. x + 5(0) + 1(-1) = +2 => x - 1 = +2 => x = +3. So, Cobalt(III). Step 4: Combine. Pentaamminechlorocobalt(III) chloride. Final answer: Pentaamminechlorocobalt(III) chloride
• Q: What type of isomerism is shown by [Pt(NH₃)₂Cl₂] and [Pt(NH₃)₃Cl]Cl? A: Step 1: Analyze the formulas. The two compounds have different chemical formulas. [Pt(NH₃)₂Cl₂] is a neutral complex, while [Pt(NH₃)₃Cl]Cl is an ionic complex with a complex cation and a simple anion. Step 2: Check the definition of isomerism. Isomers must have the same chemical formula but different arrangements. Since these compounds have different formulas, they are not isomers of each other. Final answer: These compounds do not exhibit isomerism with respect to each other as their chemical formulae are different. [Pt(NH₃)₂Cl₂] can show geometrical isomerism (cis-trans).
• Q: Predict the hybridisation, geometry, and magnetic nature of [Fe(CN)₆]⁴⁻. (Atomic number of Fe = 26). A: Step 1: Determine oxidation state of Fe. Let oxidation state of Fe be x. x + 6(-1) = -4 => x - 6 = -4 => x = +2. So, Fe²⁺. Step 2: Write electronic configuration of Fe²⁺. Fe: [Ar] 3d⁶ 4s². Fe²⁺: [Ar] 3d⁶. Step 3: Consider ligand strength. CN⁻ is a strong field ligand, causing pairing of electrons. Original 3d⁶: ↑↓ ↑_ ↑_ ↑_ ↑_ (4 unpaired electrons) After pairing: ↑↓ ↑↓ ↑↓ __ __ (all electrons paired, two 3d orbitals vacant) Step 4: Determine hybridisation. To accommodate 6 CN⁻ ligands, Fe²⁺ will use two 3d, one 4s, and three 4p orbitals. This leads to d²sp³ hybridisation. Step 5: Determine geometry and magnetic nature. d²sp³ hybridisation corresponds to Octahedral geometry. Since all electrons are paired, the complex is Diamagnetic. Final answer: Hybridisation: d²sp³, Geometry: Octahedral, Magnetic nature: Diamagnetic
• Q: Give one example each of linkage isomerism and coordination isomerism. A: Step 1: Linkage Isomerism. This involves ambidentate ligands. Example: [Co(NH₃)₅(NO₂)]Cl₂ (Pentaamminecobalt(III) nitrite-N chloride, where NO₂⁻ binds via N) and [Co(NH₃)₅(ONO)]Cl₂ (Pentaamminecobalt(III) nitrito-O chloride, where NO₂⁻ binds via O). Step 2: Coordination Isomerism. This involves exchange of ligands between complex cationic and anionic parts. Example: [Co(NH₃)₆][Cr(CN)₆] (Hexaamminecobalt(III) hexacyanochromate(III)) and [Cr(NH₃)₆][Co(CN)₆] (Hexaamminechromium(III) hexacyanocobaltate(III)). Final answer: Linkage Isomerism: [Co(NH₃)₅(NO₂)]Cl₂ and [Co(NH₃)₅(ONO)]Cl₂. Coordination Isomerism: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].

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