Coordination Compounds Preparation and Properties
→ Coordination compounds are special class of compounds in which the central metal atom is surrounded by ions or molecules beyond their normal valency. These compounds are also referred to as complex compounds or simple complexes.
→ In the modern terminology, these compounds are called coordination compounds. These compounds are widely present in the minerals, plants and animals, and play many important functions. Many biologically important compounds are coordination compounds in which complicated organic species are bound to the metal ions.
→ The common examples are : haemoglobin, which is a coordination compound of iron, chlorophyll, which is a coordination compound of magnesium, Vitamin B12 which is a coordination compound of cobalt etc.
→ The coordination compounds are finding extensive applications in metallurgical processes, analytical chemistry and medicinal chemistry. Many complex metal oxides and sulphides which constitute minerals are solid-state coordination compounds.
Chapter Summary & Glossary
→ Coordination Compounds: A compound containing central metal atom or ion bonded to a fixed number of ions or molecules. These atoms or ion are called as ligands.
→ Ligands : An ion, atom or molecule of donating a pair of electrons to the central atom via a donor atom.
→ Unidentate Ligands : Ligands with only one donor atome.g.,NH3 Cl–, F– etc.
→ Bidentate Ligands : Ligands with two donor atoms eg, ethylenediamine, oxalate ion (C2O42-) etc.
→ Polydentate Ligands: Ligands with more than two donor atoma eg., EDTA, trien, dien etc.
→ Coordination Sphere : The central atom and the ligands which are directly attached to it are inclosed in square brackets and are collectively termed as coordination sphere.
→ Coordination Number : The number of coordinate bonds formed by the ligands with the metal atom i.e., number of unidentate ligands or double the number of bidentate ligands etc.
→ Chelating Ligands : Multidentate ligand simultaneously coordination to a metal ion through more than one site is called chelating ligand. Example: Ethylene diamine.
→ Coordination Polyhedron : The spatial arrangement of the ligands which are directly attached to the central atom.
→ Homolaptic Complexes : The complexes which contain only one type of ligands.
→ Heterolaptic Complexes : The complexes which contain more than one type of ligands.
→ Stereoisomers : The isomers which have the same position of atoms or groups of atoma but they differ in the spatial arrangement around the central metal atom.
→ The Valence Bond Theory (VBT): It explains the resonable success, the formation, magnetic behaviour and geometrical shapes of coordination compounds. It, however, fails to provide a quantitive interpretation of magnetic behaviour and has nothing to say about the optical properties of these compounds.
→ Crystal field Splitting : The conversion of five degenerate d-orbitals of the metal ion into different sets of orbitals having different energies in the presence of electrical field of ligands is called crystal
Constitution and Geometry :
→ The most important factors that govern the C.N. of a complex are :
- The size of the central metal atom or ion.
- The steric interaction between ligands.
- Electronic interactions between the central atom or ion and the ligands.
→ Based on above mentioned factors, bulky ligands result in low C.N.. Higher C.N. are most common on the left of a period and lower C.N. are found on the right of the d-block.
e.g. [Mo(CN)8]4- has high C.N. and [PtCl4]2- has low C.N..
→ Complex with C.N. = 2 : These are found for Cu+ and Ag+. Examples are [AgCl2]– and HgMe2. The geometry is linear.
→ Complexes with C.N. = 3 : These are very rare among metal complexes, but are found with bulky ligands. eg. [Pt(P(Ph)3))3] tricyclophenylphosphine platinium (0). The ligands are in trigonal arrangement.
→ Complexes with C.N. = 4 : Tetrahedral complexes are favored over higher coordinated complexes if the central atom is small or the ligands are large. Square planar complexes are typically observed for metals with a configuration. Four coordinated & and p block complexes without lone pair on the central atom, such as [BeCl4]2- or [BF4]– are always tetrahedral.
→ Complexes with C.N. = 5 : Square pyramidal five coordinated complexes are found in the biologically porphyrins. Another possible geometry for C.N. 5 is trigonal bipyramidal.
→ Complexes with C.N. = 6 : The majority of six coordinated complexes are octahedral. A coordination number of 6 is found for s, p, d and f metal coordination compounds. The deviations from octahedral symmetry are tetragonal, rhombic and trigonal distortions.
→ Higher C.N. : Large atoms particularly those of the f-block tend to form complexes with high C.N. Seven coordinated complexes are encountered in a few 3d complexes and many more 4d and 5d complexes. The geometries are pentagonal bipyramidal, capped octahedral, capped trigonal prismatic. Relatively large ions can act as host for nine coordinated complexes. e.g., [Nd(OH2)9]3+.