Coordination Compounds (Introduction)

Werner’S Theory Of Coordination Compounds

Coordination Compounds: Coordination compounds are compounds that contain a central metal atom or ion bonded to a surrounding array of atoms, ions, or molecules known as ligands. These compounds have distinct properties that differ from those of the individual components.

Historical Context: Before the early 20th century, the nature of bonding in coordination compounds was not well understood. The simple formulas and observed properties (like color, conductivity, and reactivity) of compounds like cobalt-ammonia complexes could not be explained by classical valency theories.

Alfred Werner's Contribution: Alfred Werner, in the late 19th and early 20th centuries, laid the foundation for modern coordination chemistry through his pioneering work and theory.

Werner's Postulates (Theory): Werner proposed that metal atoms in coordination compounds exhibit two types of valencies:

1. Primary Valency:
• This is the ionizable valency of the metal atom, which corresponds to the charge on the metal ion.
• These valencies are satisfied by simple ions that are outside the coordination sphere (represented by square brackets in the formula).
• Werner showed that these primary valencies are equivalent and directed towards the corners of geometric shapes (like octahedron, tetrahedron, square plane).
2. Secondary Valency:
• This is the non-ionizable valency of the metal atom, which corresponds to the coordination number of the metal ion.
• These valencies are satisfied by ligands directly attached to the central metal atom within the coordination sphere (enclosed in square brackets).
• The number of secondary valencies is usually equal to the coordination number of the metal ion.
• Ligands satisfying secondary valency are directly bonded to the metal atom.

Key Concepts from Werner's Theory:

• Coordination Sphere: The central metal atom/ion and the ligands directly bonded to it form the coordination sphere. This part of the compound does not dissociate into ions in solution.
• Coordination Number: The number of secondary valencies or the number of ligands directly attached to the central metal atom. For most common complexes, the coordination number is 4 or 6.
• Ionization Sphere: The ions that are outside the coordination sphere and satisfy the primary valency. These ions dissociate in solution and contribute to the conductivity of the solution.
• Relationship between Primary and Secondary Valencies: Werner proposed that the primary and secondary valencies are held together by forces of different types. Primary valencies are satisfied by ions, while secondary valencies are satisfied by ligands directly bonded to the metal. He also suggested that primary valencies are ionizable, while secondary valencies are non-ionizable.

Werner's Explanations: Werner's theory successfully explained the observed properties of many coordination compounds by proposing specific structures and the concept of primary and secondary valencies.

• For example, considering the complex $CoCl_3 \cdot 6NH_3$: Werner proposed it to be $[Co(NH_3)_6]Cl_3$. Here, $Co^{3+}$ is the central metal ion, $NH_3$ are the ligands satisfying secondary valency (coordination number 6), and $Cl^-$ ions are outside the coordination sphere satisfying primary valency (charge +3). This structure explained why all three chloride ions could be precipitated by $AgNO_3$.
• Similarly, for $CoCl_3 \cdot 5NH_3$, he proposed $[Co(NH_3)_5Cl]Cl_2$, explaining why two chloride ions were ionizable.

Successes of Werner's Theory:

• Explained the existence of coordination compounds with different properties.
• Predicted the geometry of complexes (e.g., octahedral for coordination number 6).
• Successfully explained conductivity, color, and isomerism in complexes.

Limitations of Werner's Theory:

• Did not explain the nature of the metal-ligand bond.
• Did not explain the stability and color of coordination compounds.
• Could not explain why certain metals form complexes while others do not.

These limitations were later addressed by modern theories like Valence Bond Theory (VBT), Crystal Field Theory (CFT), and Ligand Field Theory (LFT).

Definitions Of Some Important Terms Pertaining To Coordination Compounds

Coordination chemistry involves a specialized vocabulary to describe the structure, bonding, and properties of coordination compounds.

Definitions Of Important Terms

1. Complex Ion/Compound: A compound in which a central metal atom or ion is bonded to a surrounding array of other atoms, ions, or molecules called ligands.
2. Central Metal Atom/Ion: The atom or ion in the center of the coordination compound, to which the ligands are bonded. It is usually a transition metal atom or ion.
3. Ligands:
• Definition: An atom, ion, or molecule that has at least one lone pair of electrons and can donate this pair to the central metal atom/ion to form a coordinate covalent bond.
• Classification based on donor sites:
• Monodentate Ligands: Ligands that donate only one lone pair of electrons (e.g., $NH_3$, $H_2O$, $Cl^-$, $CN^-$).
• Bidentate Ligands: Ligands that donate two lone pairs of electrons (e.g., ethylenediamine ($en$), oxalate ($ox^{2-}$), acetylacetonate ($acac^-$)).
• Polydentate Ligands: Ligands that donate more than two lone pairs (e.g., EDTA$^4-$ is hexadentate).
• Chelation: When a polydentate ligand binds to a central metal ion through multiple donor atoms, it forms a ring-like structure called a chelate. The ligand is called a chelating agent. Ligands that form chelates are called chelating ligands.
• Classification based on charge: Ligands can be cationic (e.g., $NO^+$), neutral (e.g., $NH_3$, $H_2O$, $en$), or anionic (e.g., $Cl^-$, $CN^-$, $SO_4^{2-}$).
4. Coordination Sphere: The central metal atom/ion and the ligands directly bonded to it constitute the coordination sphere. This unit is enclosed in square brackets in the formula of a coordination compound. (e.g., in $[Co(NH_3)_6]Cl_3$, the coordination sphere is $[Co(NH_3)_6]^{3+}$).
5. Coordination Number: The number of donor atoms (or ligands) that are directly bonded to the central metal atom/ion in the coordination sphere. It is equal to the number of coordinate covalent bonds formed.

Examples:

• In $[Co(NH_3)_6]Cl_3$, the coordination number of Co is 6.
• In $[Ni(en)_2Cl_2]$, the coordination number of Ni is 4 (en is bidentate, so 2 $\times$ 2 = 4 donor atoms).
• In $[Fe(CO)_5]$, the coordination number of Fe is 5.
6. Oxidation State of Central Metal Atom/Ion: The oxidation state of the central metal atom/ion is determined by the charge of the complex ion and the charges of the ligands. The sum of the oxidation states of the metal and the ligands equals the charge of the complex ion.

Example: In $[Co(NH_3)_6]Cl_3$, the charge on the complex ion is +3 (to balance the three $Cl^-$ ions). Since $NH_3$ is neutral, the oxidation state of Co is +3.

In $[Ni(en)_2Cl_2]$, the complex ion is neutral. $en$ is neutral, and $Cl$ is -1. So, $Ni + 2(0) + 2(-1) = 0 \Rightarrow Ni = +2$. The oxidation state of Ni is +2.

7. Counter Ions/Counter Ligands: Ions or molecules present outside the coordination sphere that balance the charge of the complex ion. They are ionizable. (e.g., $Cl^-$ ions in $[Co(NH_3)_6]Cl_3$).
8. Ligand Dissociation: The ligands satisfying the primary valency are ionizable and dissociate in solution.
9. Coordination Number: The number of secondary valencies or the number of ligands directly attached to the central metal atom.