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Co-Ordination Compounds

Coordination compound: A coordination compound has a central metal atom or ion surrounded by a number of molecules or ions with opposing charges. These ions or molecules formed a coordinate bond with the metal atom or ion once more.

A coordination entity: It is a group of ions or molecules with a central metal atom or ion bound to a predetermined number of ions or molecules. Example: [Fe(CN)6]4 is a coordination entity in K4[Fe(CN)6].

The central atom or ion: In a coordination entity is the atom or ion to which a predetermined number of ions or groups are bound in a predetermined geometric arrangement. Fe2+ is the main metal ion in K4[Fe(CN)6], for instance.

Ligands: A ligand is any molecule, ion, or group that is connected to a metal atom or ion in a complex or coordination compound by a coordinate bond. It could be positively, negatively, or neutrally charged. For instance, H2O, CN, NO+, etc.

Atom donor: The donor atom is a member of the ligand that is directly bonded to the metal. Example: The donor atom in the complex K4[Fe(CN)6] is carbon.

Coordination number: The quantity of ligand donor atoms to which a metal ion in a complex is directly bonded is known as the coordination number (CN). Example: The coordination number of Fe in the compound K4[Fe(CN)6] is 6.

Coordination sphere: The term "coordination sphere" refers to the grouping of the central atom/ion and the ligands that are bound to it. The coordination sphere in the complex K4[Fe(CN)6] is [Fe(CN)6]4.

Counter ions: These are the ions that are present outside of the coordination sphere. K+ is the counter ion in the complex K4[Fe(CN)6].

Coordination polyhedron: A coordination polyhedron is defined by the spatial arrangement of the ligand atoms that are directly attached to the central atom or ion. The octahedral, square planar, and tetrahedral coordination polyhedra are the most prevalent. Examples include the tetrahedral Ni(CO)4, [PtCl4]2's square planar structure, and [Cu(NH3)6]. Octahedral 3+

Coordination sphere: The grouping of the central atom/ion and the ligands that are bound to it is referred to as the "coordination sphere." [Fe(CN)6]4 serves as the coordination sphere in the complex K4[Fe(CN)6].

The ions that are present outside of the coordination sphere are known as counter ions. In the complex K4[Fe(CN)6], K+ serves as the counterion.

Coordination polyhedron: The spatial arrangement of the ligand atoms that are directly connected to the main atom or ion characterizes a coordination polyhedron. The most common coordination polyhedra are the octahedral, square planar, and tetrahedral varieties. Examples include the square planar structure of [PtCl4]2, the tetrahedral Ni(CO)4, and [Cu(NH3)6]. 3+ Octahedral is.

Polydentate ligand: A ligand is referred to as a polydentate ligand when it contains multiple donor atoms. Example: The ligand in N(CH2CH2NH2)3 is referred to as polydentate. An essential hexadentate ligand is the ion ethylenediaminetetraacetate (EDTA4-). It has the ability to bind to a central metal ion through two nitrogen and four oxygen atoms.

A chelate is an inorganic metal complex with a tightly bound ring of atoms that results from the attachment of a ligand to a metal atom at two different locations. One such instance is the complex ion produced when the cupric ion [Cu(NH2CH2NH2)2]2+ and ethylene diamine interact.

Ambidentate ligands are substances that can bind (link) through two different atoms that are present in them.

Common Multidentate (Chelating) Ligands:

Werner’s coordination theory: Werner was able to explain the type of bonding in complexes using his coordination theory. The Werner theory's tenets are

Primary valence and secondary valence are the two types of valencies that metal exhibits.

  • The secondary linkages that bind the ions or groups to the metal have distinctive spatial configurations that correspond to various coordination numbers.

  • The octahedral, square planar, and tetrahedral geometrical shapes found in coordination compounds are the most frequent.

Oxidation number of the central atom: The charge that the central atom in a complex would have if all of the ligands and the electron pairs that it shares with other atoms were removed is known as the oxidation number of the central atom.

Homoleptic complexes are those in which only one type of donor atom is coordinately bonded to a metal or ion. Consider [Co(NH3)6] 3+

Complexes known as heteroleptics are those in which a metal or ion is coordinately bonded to various types of donor atoms. For instance, [CoCl2(NH3)4]+ and [Co(NH3)5Br]


Isomers: The term "isomer" refers to two or more compounds with the same chemical formula but a different atom arrangement.

Types of isomerism:

  • Structural isomerism

  • Linkage isomerism

  • Solvate isomerism or hydrate isomerism

  • Ionisation isomerism

  • Coordination isomerism

  • Stereoisomerism

  • Geometrical isomerism

  • Optical isomerism

Structural isomerism: The difference in the structures of coordination compounds gives rise to structural isomerism. Constitutional isomerism, also known as structural isomerism, is a type of isomerism in which atoms in molecules with the same molecular formula are bonded in various ways.

Ionization isomerism: This type of isomerism occurs when a complex salt's counter ion is also a potential ligand, and it has the ability to displace a ligand that could otherwise become the counter ion.

Co(NH3)5Br SO4 and Co(NH3)5 SO4 are two examples. Br

Solvate isomerism: This type of isomerism uses the solvent as the ligand. Hydrate isomerism is the term used when water serves as the solvent.

An illustration would be [Cr(H2O)6]Cl3 and [CrCl2(H2O)4] Cl2.2H2O.

Linkage isomerism: A coordination compound with an ambidentate ligand will exhibit linkage isomerism. A ligand can link up with a metal in an isomerism through various atoms.

Co(NH3)5ONO]Cl2 and Co(NH3)5NO2 are two examples.

Coordination isomerism: The exchange of ligands between cationic and anionic entities of various metal ions present in a complex gives rise to the isomerism known as coordination. Examples include [Cr(NH3)6], [Co(NH3)6], and [Cr(C2O4)3]. [Co(C2O4)3]

Stereoisomerism: This kind of isomerism results from a difference in how space is arranged.

Geometric isomerism: It develops in heteroleptic complexes as a result of various potential geometrical configurations of ligands.

optical isomerism: The term "optical isomerism" refers to isomerisms that do not have mirror images that can be superimposed.

valence bond theory:

According to the valence bond theory, a metal atom or ion can use its (n 1)d, ns, np or ns, np or nd orbitals for hybridization under the influence of ligands to produce a set of equivalent orbitals with definite geometry, such as octahedral, tetrahedral, and square planar.

The ligand orbitals that can provide electron pairs for bonding are allowed to overlap with these hybridized orbitals.

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