Coordination Compounds: Werner's Theory, Ligands, Coordination Number, and Isomerism

Werner's Theory of Coordination Compounds

Alfred Werner, in 1893, proposed a revolutionary theory to explain the structure and bonding in coordination compounds. This theory earned him the Nobel Prize in Chemistry in 1913 and laid the foundation for modern coordination chemistry. Before Werner's theory, the nature of bonding in these compounds was a mystery. Werner's key postulates are as follows:

• Two types of valencies: Metal atoms in coordination compounds exhibit two types of valencies: primary valency and secondary valency.
• Primary valency: This is equivalent to the oxidation state of the metal ion. Primary valencies are ionisable and are generally satisfied by negatively charged ions (anions). For example, in the complex [Co(NH₃)₆]Cl₃, the primary valency of cobalt is +3, which is satisfied by three chloride ions.
• Secondary valency: This is equivalent to the coordination number of the metal ion. Secondary valencies are non-ionisable and are directed towards specific positions in space around the metal ion. They are satisfied by neutral molecules or negatively charged ions (anions). In the same example, [Co(NH₃)₆]Cl₃, the secondary valency of cobalt is 6, which is satisfied by six ammonia molecules.
• Spatial arrangement: The secondary valencies have a definite direction and are arranged spatially around the central metal ion, leading to specific geometric shapes of the coordination compounds.

Werner's theory successfully explained the properties of numerous coordination compounds known at that time and predicted the existence of others. He used conductivity measurements and precipitation reactions to determine the number of ionisable and non-ionisable groups in a complex.

Example illustrating Werner's Theory:

Consider the complex diamminedichloroplatinum(II), [Pt(NH₃)₂Cl₂]. Werner proposed two possible structures based on the arrangement of ligands:

1. cis-isomer: Both chloride ligands are adjacent to each other, and both ammonia ligands are adjacent to each other.
2. trans-isomer: Chloride ligands are opposite to each other, and ammonia ligands are opposite to each other.

Werner synthesized these isomers and showed that they had different physical properties, thus confirming his theory about the spatial arrangement of secondary valencies.

Ligands in Coordination Compounds

Ligands are molecules or ions that donate at least one pair of electrons to a central metal atom or ion to form a coordination complex. These ligands bind to the central metal ion through coordinate covalent bonds. The nature of the ligand plays a crucial role in determining the properties of the coordination compound.

Classification of Ligands:

Ligands can be classified based on the number of donor atoms they possess:

• Monodentate Ligands: These ligands have only one donor atom that can bind to the central metal ion.
  • Examples: Halide ions (Cl^-, Br^-, I^-), cyanide ion (CN^-), hydroxide ion (OH^-), water molecule (H₂O), ammonia molecule (NH₃), carbon monoxide (CO), nitric oxide (NO⁺).
• Bidentate Ligands: These ligands have two donor atoms that can bind to the central metal ion. They form a ring structure with the metal ion, known as a chelate ring.
  • Examples: Ethane-1,2-diamine (en) - donates through two nitrogen atoms, oxalate ion (C₂O₄^2-) - donates through two oxygen atoms, acetylacetonate ion (acac) - donates through two oxygen atoms.

  The formation of chelate rings generally leads to increased stability of the coordination compound, a phenomenon known as the chelate effect.

• Polydentate Ligands: These ligands have more than two donor atoms.
  • Tridentate Ligands: Have three donor atoms. Example: Triethylenetetramine (trien).
  • Tetradentate Ligands: Have four donor atoms. Example: Ethylenediaminetetraacetate (EDTA) - can bind through two nitrogen and two oxygen atoms.
  • Hexadentate Ligands: Have six donor atoms. Example: EDTA, which can bind to a metal ion through two nitrogen and four oxygen atoms.

Ligands can also be classified based on their charge:

• Anionic Ligands: Negatively charged ligands. Examples: Cl^-, CN^-, OH^-, C₂O₄^2-.
• Cationic Ligands: Positively charged ligands. These are less common. Example: NO⁺.
• Neutral Ligands: Uncharged ligands. Examples: H₂O, NH₃, CO.

Ligands that bind to the metal through atoms of the same element are called homodentate ligands (e.g., NH₃). Ligands that bind through atoms of different elements are called heterodentate ligands (e.g., EDTA).

Coordination Number

The coordination number (CN) of a central metal atom or ion in a coordination complex is the total number of donor atoms attached to it. It is essentially the number of coordinate bonds formed between the central metal and the ligands.

• Monodentate Ligands: If a complex contains only monodentate ligands, the coordination number is equal to the number of ligand molecules or ions bound to the central metal.
  • Example: [Co(NH₃)₆]³⁺ has CN = 6. [PtCl₄]^2- has CN = 4.
• Bidentate Ligands: Each bidentate ligand contributes 2 to the coordination number.
  • Example: [Ni(en)₃]²⁺. Since 'en' is bidentate, the CN = 3 ligands × 2 donor atoms/ligand = 6.
• Polydentate Ligands: The coordination number is calculated by multiplying the number of ligand molecules by the number of donor atoms per ligand.
  • Example: [Co(EDTA)]^-. EDTA is usually hexadentate. So, CN = 1 ligand × 6 donor atoms/ligand = 6.
• Mixed Ligands: When a complex contains ligands of different denticities, the coordination number is the sum of contributions from each ligand.
  • Example: [Co(en)₂Cl₂]⁺. Here, 'en' is bidentate and there are two such ligands. Cl^- is monodentate and there are two such ligands. CN = (2 ligands × 2 donor atoms/ligand) + (2 ligands × 1 donor atom/ligand) = 4 + 2 = 6.

The most common coordination numbers observed in coordination compounds are 2, 3, 4, 5, and 6. Coordination number 4 can lead to either tetrahedral or square planar geometry, while coordination number 6 typically leads to octahedral geometry.

Isomerism in Coordination Compounds

Isomers are compounds that have the same molecular formula but differ in the arrangement of their atoms or ions. In coordination compounds, isomerism arises due to differences in the connectivity of ligands to the central metal ion or the spatial arrangement of ligands. Isomerism in coordination compounds is broadly classified into two main types: structural isomerism and stereoisomerism.

1. Structural Isomerism (Constitutional Isomerism)

Structural isomers have different connectivity between the central metal atom and the ligands or different compositions of the coordination sphere and the counter ions. The different types of structural isomerism are:

• Ionisation Isomerism: This occurs when the counter ion in the coordination compound can also act as a ligand and vice versa. The isomers differ in the ions they precipitate when treated with a precipitating agent.
  • Example: [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br.
  • In the first complex, SO₄^2- is the counter ion, and Br^- is the ligand. If treated with BaCl₂, a white precipitate of BaSO₄ will form.
  • In the second complex, Br^- is the counter ion, and SO₄^2- is the ligand. If treated with AgNO₃, a pale yellow precipitate of AgBr will form.
• Hydrate Isomerism (Solvate Isomerism): This is a special case of ionisation isomerism where water is the solvent molecule. The isomers differ in the number of water molecules present inside and outside the coordination sphere.
  • Example: [Cr(H₂O)₆]Cl₃ (hexa-aquachromium(III) chloride) contains 6 water molecules as ligands.
  • [Cr(H₂O)₅Cl]Cl₂·H₂O (penta-aqua-chloridochromium(III) chloride monohydrate) contains 5 water molecules as ligands and one water molecule in the crystal lattice.
  • [Cr(H₂O)₄Cl₂]Cl·2H₂O (tetra-aqua-dichloridochromium(III) chloride dihydrate) contains 4 water molecules as ligands and two water molecules in the crystal lattice.

  These isomers differ in the number of moles of AgCl precipitated on treatment with AgNO₃.

• Linkage Isomerism: This occurs when a ligand can coordinate to the central metal atom through two different donor atoms. Such ligands are called ambidentate ligands.
  • Example: The nitrite ion (NO₂^-) can coordinate either through the nitrogen atom (nitro-ligand, -NO₂) or through one of the oxygen atoms (nitrito-ligand, -ONO).
  • [Co(NH₃)₅NO₂]²⁺ can exist as:
    • Pentaammine-nitro-cobalt(III) ion ([Co(NH₃)₅(NO₂)]²⁺) where NO₂ is bonded through N.
    • Pentaammine-nitrito-cobalt(III) ion ([Co(NH₃)₅(ONO)]²⁺) where NO₂ is bonded through O.
  • Other ambidentate ligands include thiocyanate ion (SCN^-) which can coordinate through S (thiocyanato, -SCN) or N (isothiocyanato, -NCS), and cyanide ion (CN^-) which can coordinate through C or N.
• Coordination Isomerism: This type of isomerism occurs in compounds that contain both complex cation and complex anion. The isomers differ in the distribution of ligands between the complex cation and the complex anion.
  • Example: [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆].
  • In the first complex, cobalt is the central metal in the cation, and chromium is in the anion. In the second complex, the roles are reversed.

2. Stereoisomerism

Stereoisomers have the same molecular formula and the same connectivity but differ in the spatial arrangement of ligands around the central metal atom. Stereoisomerism is further divided into geometric isomerism and optical isomerism.

• Geometric Isomerism: This arises due to the different possible spatial arrangements of ligands around the central metal atom in a coordination compound, especially when the ligands are identical or similar. It is most common in complexes with coordination numbers 4 and 6.
  • In Coordination Number 4:
  • Square Planar Complexes (MA₂B₂ type): Two types of isomers are possible: cis and trans. In the cis isomer, similar ligands are adjacent to each other (at 90°), while in the trans isomer, they are opposite to each other (at 180°).
    • Example: [Pt(NH₃)₂Cl₂]. The cis isomer is yellow, and the trans isomer is pale yellow and has a higher melting point.
  • Tetrahedral Complexes (MA₂B₂ type): Geometric isomerism is generally not observed in tetrahedral complexes because all positions are equivalent relative to each other.
  • In Coordination Number 6 (Octahedral Complexes):
  • MA₄B₂ type: Two isomers are possible: cis and trans. In the cis isomer, the two 'B' ligands are adjacent (at 90°), while in the trans isomer, they are opposite (at 180°).
    • Example: [Co(NH₃)₄Cl₂]⁺. The cis isomer is violet, and the trans isomer is green.
  • MA₃B₃ type: Two isomers are possible: fac (facial) and mer (meridional). In the fac isomer, the three identical ligands occupy one face of the octahedron. In the mer isomer, the three identical ligands lie in a plane passing through the central metal atom.
    • Example: [Co(NH₃)₃Cl₃].
  • M(A-A)₂B₂ type: Where (A-A) is a bidentate ligand. This can lead to cis and trans isomers.
    • Example: [Co(en)₂Cl₂]⁺. The cis isomer is violet, and the trans isomer is green.
  • M(A-A)₃ type: No geometric isomerism is possible as all ligands are identical and symmetrically arranged.
    • Example: [Co(en)₃]³⁺.
• Optical Isomerism: Optical isomers are stereoisomers that are non-superimposable mirror images of each other. They rotate the plane of plane-polarized light in opposite directions (dextrorotatory (+) and levorotatory (-)). A compound that exhibits optical isomerism is said to be chiral.
  • Optical isomerism is observed in coordination compounds that lack a plane of symmetry and a center of inversion. Such compounds are called chiral.
  • Coordination Number 4: Square planar complexes with the type MA₂B₂, MABX₂, or MABCD do not exhibit optical isomerism if they possess a plane of symmetry. However, complexes of the type MABCD, where all ligands are different, can be chiral if the geometry is not planar. Tetrahedral complexes of the type MABCD are chiral.
    • Example: [Zn(NH₃)₂Br₂] (tetrahedral) does not show optical isomerism. However, if it were MABCD tetrahedral complex, it would be chiral.
  • Coordination Number 6 (Octahedral Complexes): Optical isomerism is commonly observed.
    • M(A-A)₃ type: Complexes like [Co(en)₃]³⁺ are chiral and exhibit optical isomerism. They exist as a pair of enantiomers (non-superimposable mirror images).
    • M(A-A)₂B₂ type: The cis isomer of complexes like [Co(en)₂Cl₂]⁺ is chiral and exhibits optical isomerism. The trans isomer is achiral because it has a plane of symmetry.
    • M(A-A)₂BC type: These complexes are also chiral and exhibit optical isomerism.
    • M(AA)(BB) type: Where AA and BB are bidentate ligands, can also exhibit optical isomerism.

  A racemic mixture is an equimolar mixture of two enantiomers, which is optically inactive.

Summary of Key Concepts