Monday, 9 April 2012

Ligand Substitution Reactions and Shape of Complexes

There are 4 sets of reactions we need to learn regarding ligand substitution, with an example for each, so here there are:

1. Chloro ligand is larger than most other uncharged unidentate ligands. Therefore only 4 chloro ligands can fit around a transition metal ion so this results in a co-ordination number change to 4 and the formation of a tetrahedral shape

[Cu(H20)6]2+ + 4Cl- à [Cu(Cl)4]2- + 6H20

2. Not all ligand substitution reactions are complete

[Cu(H20)6]2+ + 4NH3 à [Cu(H20)2(NH3)4]2+ + 4H20

3. Ligands H20 and NH3 are of a similar size and both uncharged. Therefore ligand substitution occurs without change of co-ordination number or charge on the complex

[Co(H20)6]2+ + 6NH3 à [Co(4NH3)6]2+ + 6H20

4. Substition of unidentate with multidentate ligands leads to the formation of more stable complexes. Formation of complexes containing a multidentate ligand is known as CHELATION

[Cu(H20)6]2+ + 3H2NCH2CH2NH2 à [Cu(H2NCH2CH2NH2)3]2+ + 6H20

[Cu(H20)6]2+ + edta4- à [Cu(edta4-)]2- + 6H20

[Cu(H2NCH2CH2NH2)3]2+ + edta4- à [Cu(edta4-)]2- + 3H2NCH2CH2NH2

All of these reactions involve an increase in enthalpy as there are more entities on the RHS of the reaction. This makes it feasible and multidentate ligands more stable = CHELATE EFFECT
An interesting application of such an effect, using eggshells...

There are also four possible shapes of complexes that we need to be aware of:
1. Octahedral
    Co-ordination number = 6
     e.g hexa ammine cobalt (II) ion

2. Tetrahedral
     Co-ordination number = 4
     e.g tetra chloro cobalt (II) ion
     # this complex has only four ligands as Cl- ions are a larger ligand, therefore fewer can fit around the central metal ion

3. Square-Planar
    Co-ordination number = 4
    e.g Cis-platin (platinium (II) complex)
    # cis-platin is an anti-cancer drug, It works by bonding to DNA to stop it unwindingm and thereby preventing cell replication. Therefore, the main danger of using this drug is that it could stop cell replication all together and prevent an organisms growth

4. Linear
    Co-ordination number = 2
    e.g di ammine silver (I) ion
    # solution containing this complex is called Tollen's reagent (think unit 4 organic tests!)which is used in organic chemistry to distinguish between aldehydes and ketones; aldehydes will reduce the silver in the complex to Ag(solid) to form a 'silver mirror'

I hope the Chemistry revision is going well - I have to admit, it is not my favourite subject to revise for so if you have any tips on how to make it more interesting anf effective I would love to here them!!!

Thursday, 5 April 2012

Transition Metals - the basics

With the written ISA coming up soon after we return, I thought it best to cover Transition Metals first, seeing as this is likely to be the basis of the paper.....

TRANSITION METAL = a metal that can form one or more stable ions with a partially filled d-subshell

- A d-subshell can fit 10 electrons in, so transition metals must form an ion that has between 1 and 9 electrons in this subshell
- It is this incomplete d-subshell that creates the key characteristics that define a transition metal
---> Scandium only forms one ion Sc3+, which has an empty d-subshell. Electron configuration for Sc is[Ar]3d14s2 so for Sc3+it is just [Ar].
---> Zinc also only forms one ion, Zn2+, which has a full d-subshell. Electron configuration for Zn is
[Ar]3d104s2so for Zn2+is just [Ar]3d10.
Transition Metals have four important characteristics in common:
1. Complex Formation
---> complex ion is formed when a transition metal is surrounded by ions or other molecules
2. Formation of Coloured Ions
---> majority of metal ions are coloured
3. Variable Oxidation States
---> have more than one oxidation state in their compounds and therefore take place in many redox reactions
4. Catalytic Activity
---> affect the rate of reaction without being used up or chemically changed themselves
LIGAND = an ion or molecule with a lone pair of electrons that can form a dative covalent bond
COMPLEX = central metal ion surrounded by ligands (2, 4 or 6)
CO-ORDINATION NUMBER = the number of dative covalent bonds formed
Unidentate = 1 dative covalent bond            e.g [Cu(CN)6]4-          [Co(OH)2(H20)2]
Bidentate = 2 dative covalent bonds            e.g [Co(en)3]2+         [Cr(C2O4)]3-
Hexadentate = 6 dative covalent bonds        e.g [Cu(edta)]1-