Notes On Important Terms Pertaining To Coordination Compounds - CBSE Class 12 Chemistry
The atom (or) ion to which one (or) more neutral molecules (or) anions are attached in a definite geometrical arrangement around it is called the central atom (or) ion. Any atom, ion (or) molecule capable of donating a pair of electrons to the central atom is called a coordination group (or) ligand. As ligands are electron pair donors, they act as Lewis bases. The number of atoms in a ligand that bind to the central atom in a complex is referred to as denticity. When a ligand is bound to a metal ion through a single donor atom, is said to be unidentate. Ex: Halide ions (F-, Cl-, Br-, I-) Ligands that contain two donor atoms are known as bidentate or didentate. Ligands with several donor atoms are called polydentate or multidentate ligands. Ligands that have a lone pair of electrons on more than one atom and are capable of binding to the central metal atom (or) ion through more than one coordinate covalent bond are called chelating ligands. Ligands that have two different donor atoms but utilise only one to form coordinate covalent bonds with the metal atom (or) ion are called ambidentate ligands. Ex: The total number of ligand donor atoms to which the metal is directly bonded is known as the coordination number of that ion. The central metal atom and the ligands attached to it are called the coordination sphere (or) coordination entity. The solid figure defined by the position of the ligand atoms directly attached to the central atom (or) ion is known as the coordination polyhedron. The electric charge that the central metal atom would carry if all the ligands were removed along with the electron pairs that are shared with the central atom is known as the oxidation number of that metal atom. Example:                                                 k4[Fe(CN)6]                K4[Fe(CN)6]    →     4K+  + [Fe(CN)6]-4 let oxidation number of central metal atom = x                         Monodentate cyanide ion = CN-                                 Charge on complex = -4       Number of monodentate negative ions = 6           Oxidation number of Iron : x + 6(-1) = -4                                                       x - 6 = -4                                                            x = -4 + 6                                                            x = +2                                                               Fe(II)

#### Summary

The atom (or) ion to which one (or) more neutral molecules (or) anions are attached in a definite geometrical arrangement around it is called the central atom (or) ion. Any atom, ion (or) molecule capable of donating a pair of electrons to the central atom is called a coordination group (or) ligand. As ligands are electron pair donors, they act as Lewis bases. The number of atoms in a ligand that bind to the central atom in a complex is referred to as denticity. When a ligand is bound to a metal ion through a single donor atom, is said to be unidentate. Ex: Halide ions (F-, Cl-, Br-, I-) Ligands that contain two donor atoms are known as bidentate or didentate. Ligands with several donor atoms are called polydentate or multidentate ligands. Ligands that have a lone pair of electrons on more than one atom and are capable of binding to the central metal atom (or) ion through more than one coordinate covalent bond are called chelating ligands. Ligands that have two different donor atoms but utilise only one to form coordinate covalent bonds with the metal atom (or) ion are called ambidentate ligands. Ex: The total number of ligand donor atoms to which the metal is directly bonded is known as the coordination number of that ion. The central metal atom and the ligands attached to it are called the coordination sphere (or) coordination entity. The solid figure defined by the position of the ligand atoms directly attached to the central atom (or) ion is known as the coordination polyhedron. The electric charge that the central metal atom would carry if all the ligands were removed along with the electron pairs that are shared with the central atom is known as the oxidation number of that metal atom. Example:                                                 k4[Fe(CN)6]                K4[Fe(CN)6]    →     4K+  + [Fe(CN)6]-4 let oxidation number of central metal atom = x                         Monodentate cyanide ion = CN-                                 Charge on complex = -4       Number of monodentate negative ions = 6           Oxidation number of Iron : x + 6(-1) = -4                                                       x - 6 = -4                                                            x = -4 + 6                                                            x = +2                                                               Fe(II)

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