Cordination Compounds Essay

* 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+2Ãâ€" 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: complex| Oxidation state| Atomic number | Co-ordination number| EAN| [Co(NH3)6]3+| +3| 27| 6| 27-3+2Ãâ€"6=36, i.e. [Kr]| [Fe(CN)6]4-| +2| 26| 6| 26-2+2Ãâ€"6=36, i.e. [Kr]| [PtCl6]2-| +4| 78| 6| 78-4+2Ãâ€"6=86, i.e. [Rn]| [Fe(CN)6]3-| +3| 26| 6| 26-3+2Ãâ€"6=35| [Ag(NH3)2]1+| +1| 47| 2| 47-1+2Ãâ€"2=50|