Cordination Compounds

COORDINATION COMPOUNDS: * FLEXIDENTATE CHARACTER OF POLYDENTATE LIGANDS; * Poly dentate ligands have flexi dentate character in the sense that all donors atoms may not form coordinate bonds with the central metal atom or ion. For example; EDTA which is the hexa dentate ligand also acts as tetra dentate or penta dentate in certain complexes. Similarly sulphate ion, which is a bi dentate ligand, also acts as a mono dentate ligand in certain complexes, e.g. in [Co(SO4)(NH3)5]Cl. * LABILE COMPLEXES; * A complex in which the ligands can be easily replaced by other ligands is called a labile complex. * GEOMETRY OF [CuX4]2- IONS; * The halide complex of Cu (2) shows two different stereo chemistries. In (NH4)2[CuCl4], [CuCl4]2- ion is square planar, but Cs2[CuCl4] and Cs2[CuBr4], the [CuX4]2- ions have a slightly squashed tetrahedral shape. Tetrahedral [CuCl4]2- ions are orange whereas, square planar [CuCl4]2- ions are yellow in colour. * GREATER STABILITY OF Co(+3) COMPLEXES THAN Co(+2) COMPLEXES; * Co+2 ions are very stable & are difficult to oxidise. Co+3 ions are less stable and are readily reduced by water to Co2+. In contrast Co(+2) are less stable and are readily oxidised to Co(+3) complexes, i.e. Co(3) are very stable. This is because CFSE of Co(+3) with d6 configuration is higher than Co(+2) with d7 configuration. * SIDGWICK THEORY OR EFFECTIVE ATOMIC NUMBER (EAN) RULE; * Sidgwick put forward a rule to explain the stability of complexes on the basis of effective atomic number as follows:-
EAN of metal in complex= atomic number –oxidation state+2× co-ordination number
A stable complex is formed if the EAN is equal to the atomic number of the next noble gas.

Thus, from the calculation given in the table below, [Fe(CN)6]4- is more stable than [Fe(CN)6]3-
Though this rule is found to be applicable in many cases, yet it fails in no. Of cases as illustrated by last two examples given in the table below:

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