Coordination CompoundsClass 12 Chemistry Notes

Coordination Compounds

Coordination compounds are central to modern inorganic chemistry and are found everywhere, from industrial processes to the very molecules that sustain life. Transition metals, in particular, form a vast number of these complex compounds where a central metal atom is bonded to several surrounding anions or neutral molecules.

Many vital biological substances are coordination compounds. For instance, chlorophyll (a magnesium compound) is essential for photosynthesis, haemoglobin (an iron compound) carries oxygen in our blood, and vitamin B₁₂ (a cobalt compound) is crucial for our health. They also have numerous applications in metallurgy, industry, and medicine.

Werner's Theory of Coordination Compounds

The foundational understanding of coordination compounds comes from the work of Swiss chemist Alfred Werner. He proposed that metal ions have two types of valencies:

1. Primary Valence: This corresponds to the oxidation state of the metal ion. It is ionisable and is satisfied by negative ions. For example, in CrCl₃, the primary valence of Cr is 3.
2. Secondary Valence: This corresponds to the coordination number of the metal ion, which is the number of groups directly bonded to it. It is non-ionisable and is satisfied by either negative ions or neutral molecules. This valence determines the geometry of the compound.

Werner's theory was developed by studying compounds like cobalt(III) chloride with ammonia. He observed that when silver nitrate (AgNO₃) was added to solutions of these compounds, different amounts of silver chloride (AgCl) precipitated.

• CoCl₃·6NH₃ (Yellow) produced 3 moles of AgCl.
• CoCl₃·5NH₃ (Purple) produced 2 moles of AgCl.
• CoCl₃·4NH₃ (Green or Violet) produced 1 mole of AgCl.

These results suggested that some chloride ions were tightly bound to the cobalt ion, while others were free to ionize and react. Werner proposed that the groups inside a square bracket [] form a single unit (the coordination sphere) that does not break apart in solution. The ions outside the bracket are counter ions.

Main Postulates of Werner's Theory:

• Metals in coordination compounds exhibit two types of valencies: primary and secondary.
• Primary valences are ionisable and satisfied by negative ions.
• Secondary valences are non-ionisable and are satisfied by neutral molecules or negative ions. The secondary valence is fixed for a metal and equals its coordination number.
• The groups bound by secondary linkages have specific spatial arrangements, leading to defined geometries (e.g., octahedral, tetrahedral, square planar). These arrangements are called coordination polyhedra.

Difference between a Double Salt and a Complex

Both double salts and complexes are formed from two or more stable compounds. However, their behavior in water is different.

• Double Salt: A double salt, like Mohr's salt (FeSO₄·(NH₄)₂SO₄·6H₂O), dissociates completely into its constituent ions when dissolved in water.
• Complex Compound: A complex compound, like potassium ferrocyanide (K₄[Fe(CN)₆]), contains a complex ion ([Fe(CN)₆]⁴⁻) that does not dissociate into its components (Fe²⁺ and CN⁻) in water.

Definitions of Some Important Terms

Coordination Entity

A coordination entity is a central metal atom or ion bonded to a fixed number of molecules or ions, called ligands. It is usually enclosed in square brackets.

• Examples: [CoCl₃(NH₃)₃], [Ni(CO)₄], [Fe(CN)₆]⁴⁻.

Central Atom/Ion

The central atom or ion is the atom/ion to which the ligands are attached in a definite geometric arrangement. Since it accepts electron pairs from ligands, it acts as a Lewis acid.

• Examples: Ni²⁺ in [NiCl₂(H₂O)₄], Co³⁺ in [CoCl(NH₃)₅]²⁺.

Ligands

Ligands are the ions or molecules that bind to the central atom/ion in a coordination entity. They are electron-pair donors, acting as Lewis bases.

Ligands are classified based on the number of donor atoms they use to bind to the central metal, a property called denticity.

• Unidentate Ligand: Binds through a single donor atom. Examples: Cl⁻, H₂O, NH₃.
• Didentate Ligand: Binds through two donor atoms. Examples: ethane-1,2-diamine (en or H₂NCH₂CH₂NH₂), oxalate (C₂O₄²⁻).
• Polydentate Ligand: Binds through several donor atoms. An important example is Ethylenediaminetetraacetate ion (EDTA⁴⁻), which is a hexadentate ligand.
• Chelate Ligand: A di- or polydentate ligand that binds to a single metal ion, forming a ring-like structure. The resulting complex is called a chelate complex, which tends to be more stable than complexes with unidentate ligands.
• Ambidentate Ligand: A ligand that can bind through two different donor atoms. Examples:
  • The NO₂⁻ ion can bind through Nitrogen (-NO₂) or Oxygen (-ONO).
  • The SCN⁻ ion can bind through Sulphur (-SCN) or Nitrogen (-NCS).

Coordination Number

The coordination number (CN) is the number of ligand donor atoms to which the central metal is directly bonded. It is determined by the number of sigma (σ) bonds formed between the metal and ligands.

• Examples: In [PtCl₆]²⁻, the CN of Pt is 6. In [Ni(NH₃)₄]²⁺, the CN of Ni is 4. In [Co(en)₃]³⁺, the CN of Co is 6 (since en is didentate and there are three en ligands, $3 \times 2 = 6$).

Coordination Sphere

The coordination sphere consists of the central metal atom/ion and the ligands attached to it. It is written inside square brackets []. The ionisable groups written outside the brackets are called counter ions.

• Example: In K₄[Fe(CN)₆], the coordination sphere is [Fe(CN)₆]⁴⁻ and the counter ion is K⁺.

Coordination Polyhedron

The coordination polyhedron is the spatial arrangement of the ligand atoms directly attached to the central atom/ion. Common shapes include octahedral, square planar, and tetrahedral.

Oxidation Number of Central Atom

The oxidation number is the charge the central atom would have if all ligands were removed along with the electron pairs they shared. It is written as a Roman numeral in parentheses after the name of the metal.

• Example: In [Cu(CN)₄]³⁻, the oxidation number of copper is +1, written as Cu(I).

Homoleptic and Heteroleptic Complexes

• Homoleptic complexes are those in which the metal is bonded to only one kind of ligand. Example: [Co(NH₃)₆]³⁺.
• Heteroleptic complexes are those in which the metal is bonded to more than one kind of ligand. Example: [Co(NH₃)₄Cl₂]⁺.

Nomenclature of Coordination Compounds

An unambiguous system of naming is essential, especially for isomers. The International Union of Pure and Applied Chemistry (IUPAC) provides the rules.

Formulas of Mononuclear Coordination Entities

1. The central atom is listed first.
2. Ligands are listed alphabetically, regardless of their charge.
3. The entire coordination entity is enclosed in square brackets [].
4. Formulas of polyatomic ligands (and abbreviations) are enclosed in parentheses ().
5. There is no space between the parts inside the brackets.
6. The charge of the entity is written as a superscript outside the bracket (e.g., [Co(CN)₆]³⁻).
7. The charge of the counter ions balances the charge of the coordination sphere.

Naming of Mononuclear Coordination Compounds

1. The cation is named first, whether it is the complex ion or the counter ion.
2. Ligands are named alphabetically before the central metal.
3. Ligand Names:
   • Anionic ligands end in -o. (e.g., Cl⁻ is chlorido, CN⁻ is cyanido).
   • Neutral ligands generally use their common names, with some exceptions:
     • H₂O: aqua
     • NH₃: ammine
     • CO: carbonyl
     • NO: nitrosyl
4. Prefixes for Ligands: Prefixes like di-, tri-, tetra- indicate the number of each ligand. If the ligand's name already contains a numerical prefix (like ethane-1,2-diamine), use bis- (for 2), tris- (for 3), tetrakis- (for 4), and enclose the ligand name in parentheses.
   • Example: [NiCl₂(PPh₃)₂] is named dichlorido**bis(triphenylphosphine)**nickel(II).
5. Oxidation State: The oxidation state of the central metal is written in Roman numerals in parentheses immediately after its name.
6. Name of the Metal:
   • If the complex ion is a cation or is neutral, the metal is named as the element (e.g., cobalt, platinum).
   • If the complex ion is an anion, the metal's name ends in -ate (e.g., cobaltate). For some metals, Latin names are used (e.g., Fe becomes ferrate).
7. A neutral complex molecule is named like a complex cation.

Solution

(a) The coordination sphere contains cobalt (Co), four ammine (NH₃), one aqua (H₂O), and one chlorido (Cl) ligand. Ligands are listed alphabetically: ammine, aqua, chlorido. The formula is [Co(NH₃)₄(H₂O)Cl]. The oxidation state of Co is +3. Charges: Co(+3) + NH₃(0) + H₂O(0) + Cl(-1) = +2. To balance this +2 charge, two chloride counter ions are needed. Final Answer: [Co(NH₃)₄(H₂O)Cl]Cl₂

(b) The complex is an anion (name ends in -ate). Central atom is zinc (Zn) with four hydroxido (OH) ligands. The formula is [Zn(OH)₄]. Oxidation state of Zn is +2. Charges: Zn(+2) + 4OH(-1) = -2. To balance this -2 charge, two potassium (K⁺) cations are needed. Final Answer: K₂[Zn(OH)₄]

(c) The complex is an anion. Central atom is aluminum (Al) with three oxalato (C₂O₄) ligands. The formula is [Al(C₂O₄)₃]. Oxidation state of Al is +3. Charges: Al(+3) + 3C₂O₄(-2) = -3. Three potassium (K⁺) ions are needed. Final Answer: K₃[Al(C₂O₄)₃]

(d) The complex is a cation. Central atom is cobalt (Co) with two chlorido (Cl) and two ethane-1,2-diamine (en) ligands. The formula is [CoCl₂(en)₂]. Oxidation state of Co is +3. Charges: Co(+3) + 2Cl(-1) + 2en(0) = +1. Final Answer: [CoCl₂(en)₂]⁺

(e) The complex is neutral. Central atom is nickel (Ni) with four carbonyl (CO) ligands. Oxidation state of Ni is 0. Final Answer: [Ni(CO)₄]

Solution

(a) The complex is neutral. Ligands are ammine, chlorido, and nitrito. NO₂ is an ambidentate ligand, and here it's specified as nitrito-N (binding through nitrogen). Alphabetical order: ammine, chlorido, nitrito-N. Metal is platinum. Let oxidation state of Pt be x. $2(0) + (-1) + (-1) + x = 0 \implies x = +2$. Final Answer: diamminechloridonitrito-N-platinum(II)

(b) The cation is potassium. The anion is the complex sphere. Ligand is oxalate (C₂O₄²⁻). Since there are three, the prefix is tri-. The metal is chromium, and since it's in an anion, it becomes chromate. Let oxidation state of Cr be x. $3(+1) + x + 3(-2) = 0 \implies x = +3$. Final Answer: potassium trioxalatochromate(III)

(c) The cation is the complex sphere. Ligands are chlorido and ethane-1,2-diamine (en). There are two of each. The prefix for en is bis- because its name contains di-. Alphabetical order: chlorido, ethane-1,2-diamine. Metal is cobalt. The counter ion is chloride. Let oxidation state of Co be x. $x + 2(-1) + 2(0) + (-1) = 0 \implies x = +3$. Final Answer: dichloridobis(ethane-1,2-diamine)cobalt(III) chloride

(d) The cation is the complex sphere. Ligands are ammine and carbonate (CO₃²⁻). Alphabetical order: ammine, carbonato. Metal is cobalt. Counter ion is chloride. Let oxidation state of Co be x. $x + 5(0) + (-2) + (-1) = 0 \implies x = +3$. Final Answer: pentaamminecarbonatocobalt(III) chloride

(e) The cation is mercury (Hg). The anion is the complex sphere. The ligand is thiocyanate (SCN⁻), which is ambidentate. Here it is tetrathiocyanato-S (binding through sulfur). The metal is cobalt, and in an anion it becomes cobaltate. Let oxidation state of Co be x. The charge on [Co(SCN)₄] is -1 if Hg is +1, or -2 if Hg is +2. Let's assume Hg is +1. Then $x + 4(-1) = -1 \implies x = +3$. A +3 oxidation state for Co is reasonable. If Hg is +2, $x + 4(-1) = -2 \implies x = +2$. Let's check the source example, it implies Mercury(I) and Cobalt(III). Final Answer: mercury(I) tetrathiocyanato-S-cobaltate(III)

Isomerism in Coordination Compounds

Isomers are compounds that have the same chemical formula but different arrangements of atoms, leading to different physical or chemical properties. There are two main types of isomerism in coordination compounds.

1. Stereoisomerism: Isomers have the same chemical bonds but different spatial arrangements.
   • Geometric Isomerism
   • Optical Isomerism
2. Structural Isomerism: Isomers have different chemical bonds.
   • Linkage Isomerism
   • Coordination Isomerism
   • Ionisation Isomerism
   • Solvate Isomerism

Geometric Isomerism

This isomerism arises in heteroleptic complexes due to different possible geometric arrangements of the ligands. It is common in square planar and octahedral complexes.

• Square Planar Complexes: In a complex of the formula [MX₂L₂], the two X ligands can be:

  • cis: adjacent to each other (90° apart).
  • trans: opposite to each other (180° apart).

• Octahedral Complexes: In a complex of the formula [MX₂L₄], the two X ligands can also be arranged in cis or trans positions.

• In octahedral complexes of the formula [Ma₃b₃], two isomers are possible:

  • facial (fac) isomer: The three 'a' ligands occupy the corners of one face of the octahedron.
  • meridional (mer) isomer: The three 'a' ligands occupy positions around the meridian (an imaginary semicircle) of the octahedron.

Optical Isomerism

Optical isomers are non-superimposable mirror images of each other, like your left and right hands. These molecules are called chiral, and the pair of isomers are called enantiomers.

• They rotate the plane of polarized light in opposite directions.
  • dextro (d): rotates light to the right.
  • laevo (l): rotates light to the left.
• This type of isomerism is common in octahedral complexes with didentate ligands, such as [Co(en)₃]³⁺.
• For a complex like [PtCl₂(en)₂]²⁺, only the cis-isomer is chiral and shows optical activity. The trans-isomer is symmetrical and not optically active.

Solution

To determine if a molecule is chiral, we check if it is superimposable on its mirror image. (a) The cis-isomer lacks a plane of symmetry and is not superimposable on its mirror image. Therefore, it is chiral. (b) The trans-isomer has a plane of symmetry, making it superimposable on its mirror image. Therefore, it is not chiral (it is achiral).

Final Answer (a) cis-[CrCl₂(ox)₂]³⁻ is chiral (optically active).

Linkage Isomerism

This occurs when a complex contains an ambidentate ligand, which can bind to the metal through different atoms.

• Example: The complex [Co(NH₃)₅(NO₂)]Cl₂ exists in two forms:
  • A red form, where the nitrite ligand binds through oxygen (-ONO).
  • A yellow form, where it binds through nitrogen (-NO₂).

Coordination Isomerism

This arises from the interchange of ligands between the cationic and anionic coordination entities in a complex salt.

• Example: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆] are coordination isomers. In the first, NH₃ is with Co and CN⁻ is with Cr; in the second, they are swapped.

Ionisation Isomerism

This occurs when the counter ion in a complex salt is itself a potential ligand and can displace a ligand from the coordination sphere. The isomers give different ions in solution.

• Example: [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄.
  • The first gives Br⁻ ions in solution (precipitates with AgNO₃).
  • The second gives SO₄²⁻ ions in solution (precipitates with BaCl₂).

Solvate Isomerism

This is similar to ionisation isomerism but involves solvent molecules. When water is the solvent, it's called hydrate isomerism. Isomers differ by whether a solvent molecule is directly bonded to the metal or is present as a free molecule in the crystal lattice.

• Example: [Cr(H₂O)₆]Cl₃ (violet) and its solvate isomer [Cr(H₂O)₅Cl]Cl₂·H₂O (grey-green).

Bonding in Coordination Compounds

Valence Bond Theory (VBT)

VBT explains the formation and structure of coordination compounds based on hybridization of the metal's atomic orbitals.

Key Concepts:

• The central metal ion provides a number of empty orbitals for the formation of coordinate bonds with ligands.
• These empty orbitals hybridize to form a set of new, equivalent orbitals with a definite geometry (e.g., tetrahedral, square planar, octahedral).
• The type of hybridization determines the shape of the complex.

Inner vs. Outer Orbital Complexes (for Octahedral Geometry):

• Inner Orbital Complex: Uses inner (n-1)d orbitals for hybridization (d²sp³). These are often low spin complexes because electrons are forced to pair up to free the inner d-orbitals.
• Outer Orbital Complex: Uses outer nd orbitals for hybridization (sp³d²). These are often high spin complexes because inner d-electrons remain unpaired.

Examples:

• [Co(NH₃)₆]³⁺ (Octahedral, Diamagnetic):

  • Co³⁺ has a 3d⁶ configuration.
  • NH₃ is a strong ligand, so the 3d electrons pair up.
  • This frees two 3d orbitals, leading to d²sp³ hybridization.
  • Since all electrons are paired, the complex is diamagnetic. It is an inner orbital (low spin) complex.

• [CoF₆]³⁻ (Octahedral, Paramagnetic):

  • Co³⁺ is 3d⁶.
  • F⁻ is a weak ligand, so the 3d electrons do not pair up.
  • Hybridization involves outer orbitals: one 4s, three 4p, and two 4d, leading to sp³d² hybridization.
  • There are four unpaired electrons, so the complex is paramagnetic. It is an outer orbital (high spin) complex.

• [Ni(CN)₄]²⁻ (Square Planar, Diamagnetic):

  • Ni²⁺ has a 3d⁸ configuration.
  • CN⁻ is a strong ligand, forcing one of the unpaired 3d electrons to pair up.
  • This leaves one 3d orbital empty, leading to dsp² hybridization.
  • All electrons are paired, so the complex is diamagnetic.

• [NiCl₄]²⁻ (Tetrahedral, Paramagnetic):

  • Ni²⁺ is 3d⁸.
  • Cl⁻ is a weak ligand and does not cause pairing of 3d electrons.
  • Hybridization uses one 4s and three 4p orbitals, leading to sp³ hybridization.
  • There are two unpaired electrons, so the complex is paramagnetic.

Limitations of Valence Bond Theory

• It involves many assumptions.
• It does not provide a quantitative interpretation of magnetic data.
• It fails to explain the color of coordination compounds.
• It does not distinguish between weak and strong ligands.

Crystal Field Theory (CFT)

CFT is an electrostatic model that treats ligands as point charges or dipoles. It focuses on the effect of the ligand's electric field on the energies of the d-orbitals of the central metal ion.

Key Concepts:

• In an isolated, gaseous metal ion, all five d-orbitals are degenerate (have the same energy).
• When ligands approach the metal ion, their negative electric field repels the electrons in the d-orbitals. This repulsion is not uniform.
• The degeneracy of the d-orbitals is lifted, and they split into different energy levels. This is called crystal field splitting.

Crystal Field Splitting in Octahedral Complexes:

• In an octahedral arrangement, ligands approach along the x, y, and z axes.
• The d(x²-y²) and d(z²) orbitals (the e_g set), which point directly at the axes, experience more repulsion and are raised in energy.
• The d(xy), d(yz), and d(xz) orbitals (the t₂g set), which lie between the axes, experience less repulsion and are lowered in energy.
• The energy difference between the e_g and t₂g sets is the crystal field splitting energy (Δo).

Spectrochemical Series: Ligands can be arranged in order of their ability to cause d-orbital splitting. This is the spectrochemical series.

• Weak-field ligands (e.g., I⁻, Br⁻, Cl⁻, F⁻) cause small splitting (Δo is small). They form high-spin complexes because it's energetically easier for electrons to occupy the higher e_g orbitals than to pair up in the t₂g orbitals (Δo < P, where P is pairing energy).
• Strong-field ligands (e.g., CN⁻, CO, en, NH₃) cause large splitting (Δo is large). They form low-spin complexes because pairing up in the t₂g orbitals is energetically more favorable than moving to the high-energy e_g orbitals (Δo > P).

Series (increasing strength): I⁻ < Br⁻ < SCN⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO

Crystal Field Splitting in Tetrahedral Complexes:

• The splitting pattern is inverted compared to octahedral complexes. The t₂ set is higher in energy, and the e set is lower.
• The splitting energy (Δt) is much smaller than in octahedral fields: Δt = (4/9)Δo.
• Because Δt is small, pairing rarely occurs, and tetrahedral complexes are almost always high-spin.

Colour in Coordination Compounds

The color of transition metal complexes is explained by CFT.

1. When white light passes through a solution of a complex, it absorbs light of a specific wavelength (color).
2. This absorbed energy promotes an electron from a lower-energy d-orbital (t₂g) to a higher-energy d-orbital (e_g). This is called a d-d transition.
3. The color we see is the complementary color—the light that is transmitted, not absorbed.

Limitations of Crystal Field Theory

• It treats ligands as point charges, failing to explain why neutral ligands like CO can cause large splitting.
• It does not account for any covalent character in the metal-ligand bond.

Bonding in Metal Carbonyls

Metal carbonyls are compounds with carbon monoxide (CO) as the only ligand.

Structure:

• Ni(CO)₄: Tetrahedral
• Fe(CO)₅: Trigonal bipyramidal
• Cr(CO)₆: Octahedral

The bond between the metal and carbon in metal carbonyls has both σ and π character, which creates a special strengthening effect called synergic bonding.

1. M-C σ bond: A lone pair of electrons from the carbon atom of CO is donated into a vacant d-orbital of the metal.
2. M-C π bond (Back-bonding): A pair of electrons from a filled d-orbital of the metal is donated back into the vacant antibonding π* orbital of the CO molecule.

This synergic effect strengthens the metal-carbon bond.

Importance and Applications of Coordination Compounds

Coordination compounds play crucial roles in many fields:

• Analytical Chemistry: Used for detection and estimation of metal ions. For example, EDTA is used to determine the hardness of water by forming stable complexes with Ca²⁺ and Mg²⁺.
• Metallurgy: Used in the extraction of metals. Gold is extracted by forming the complex [Au(CN)₂]⁻. Nickel is purified via the formation and decomposition of [Ni(CO)₄].
• Biological Systems:
  • Chlorophyll (Mg complex) in photosynthesis.
  • Haemoglobin (Fe complex) for oxygen transport.
  • Vitamin B₁₂ (Co complex).
• Industrial Catalysts: Wilkinson's catalyst, [(Ph₃P)₃RhCl], is used for the hydrogenation of alkenes.
• Electroplating: Smooth and even plating of silver and gold is achieved using solutions of their cyanide complexes, [Ag(CN)₂]⁻ and [Au(CN)₂]⁻.
• Photography: In black and white photography, undeveloped AgBr is removed from the film by washing with "hypo" solution, which forms the complex ion [Ag(S₂O₃)₂]³⁻.
• Medicinal Chemistry:
  • Chelate therapy is used to treat heavy metal poisoning. EDTA is used to remove lead.
  • Cis-platin, a platinum complex, is an effective anti-cancer drug.