Cordination Compounds

* 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;

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* 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:

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

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