Coordination Compounds – I Coordination compounds (complexes) are the compounds in which the central metal atom is linked to a number of ions or neutral molecules by coordinate bonds. On the basis of the nature of charge, coordination compounds may be Cationic complex, Anionic complex or Neutral complex. There are some basic terms, which are commonly used to describe the coordination compounds. These are Coordination entity, Ligands, Coordination number, Coordination sphere and Coordination polyhedron. A coordination entity consists of a central metal atom or ion bonded to a fixed number of ions or molecules. Ligands are the ions or molecules bonded to the central metal atom or ion. Ligands can be Monodentate (Unidentate) such as H2O or NH3, Didentate such as C2O42– or Polydentate such as EDTA. When a polydentate ligand attaches itself to a central metal ion through two or more donor atoms it forms a five or six membered ring with the central metal ion. These ligands are called chelates. Ambidentate ligands can ligate through two different atoms (one at a time). The number of coordinate bonds formed with the central metal atom or ion is called coordination number. Coordination sphere consists of the central metal atom or ion and the ligands bonded to it. The spatial arrangement of the ligands, which are directly attached to the central atom or ion, is called coordination polyhedron around that central atom or ion. Oxidation number is the charge that the central atom or ion in a complex would carry when all the ligands are removed along with the electron pairs that are shared with the central atom. Charge on the complex ion is equal to the oxidation state of the metal plus the charges on the ligands (with signs). In homoleptic complexes, a metal is bound to only one kind of donor groups while in heteroleptic complexes, a metal is bound to more than one kind of donor groups. The name of coordination compounds is assigned according to the rules of IUPAC system. Compounds, which have the same molecular formulae but differ in their structures, are called isomers and the phenomenon is called isomerism. Coordination compounds exhibit both types of isomerism, i.e., structural isomerism as well as stereoisomerism. Structural isomerism is of four types–ionisation isomerism, solvate or hydrate isomerism, linkage isomerism, and coordination isomerism. Stereoisomerism is of two types, geometrical isomerism (cis and trans) and optical isomerism. Tetrahedral complexes do not show geometrical isomerism because all ligands are neighbours and different spatial arrangements are not possible. Only square planar complexes show geometrical isomerism. Due to optical isomerism, the compound can rotate the plane of polarized light either towards the right (Dextro) or the left (Laevo). It is common in octahedral complexes with coordination number 6, involving 1, 2 or 3 symmetrical didentate ligands. Coordination Compounds – II To explain bonding in coordination compounds, Werner’s theory, Valence bond theory and crystal field theory are used. According to Werner’s theory of coordination compounds, the metals in complexes have two types of valencies- primary valencies and secondary valencies. Primary valencies are also referred as oxidation states while the secondary valencies are involved in the formation of complex ions. These are known as coordination numbers. Werner's theory was unable to explain a number of properties of the coordination compounds. Later on bonding in coordination compounds was explained with the help of Valence Bond Theory (VBT) and Crystal Field Theory (CFT). Valence Bond Theory (VBT) also referred as Pauling’s theory. Ligand-metal coordinate bond is formed by overlapping of vacant hybrid orbitals of the central metal atom or ion with the orbitals of ligands containing lone pair of electrons. A paramagnetic complex has unpaired electrons, while a diamagnetic complex has all electrons paired. The coordination complexes can exhibit different types of hybridisation which give rise to different shapes of the molecules. Crystal Field Theory is based on the assumption that the metal ion and the ligands act as point charges and the interaction between them are purely electrostatic. The crystal field splitting depends upon the geometry of the complex, i.e., Octahedral complexes and Tetrahedral complexes. Crystal field stabilizing energy (CFSE) is the difference between the energies of two sets of d-orbitals. It is represented by 0. For a given cation, magnitude of 0 depends upon the nature of ligands. Some ligands produce strong fields & hence large splitting whereas some produce weak fields & hence small splitting of d-orbitals. Spectrochemical series is the arrangement of ligands in the order of their increasing field strength (0). The value of 0 is compared with energy required for electron pairing (called pairing energy P) to find whether it is a weak field ligand or strong field ligand. Transition metal complexes absorb visible light and excite electrons from the lower energy d-orbitals into the higher ones. As a result, complex gives the colour complementary to that absorbed by them. The metal-carbon bonds in metal carbonyls possess both s and p character. Synergic bonding is responsible for the stability of metal carbonyls. The stability of a complex in solution means the degree of association between the metal ion and the ligand involved in the state of equilibrium. Coordination compounds are of great importance, they find extensive applications in biological, metallurgical processes, analytical and medicinal chemistry.
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