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Transition metals Form coloured ions Form complexes Have variable oxidation states Show catalytic activity Colour of ions When a colour change occurs in the reaction of a transition metal ion, there is a change in at least one of the following:- Oxidation state Co-ordination number Ligand Changing oxidation states [Fe(H2O)6]2+ [Fe(H2O)6]3+ GREEN VERY PALE VIOLET Changing oxidation states [Cr(H2O)6]3+ [Cr(H2O)6]2+ RUBY BLUE Changing ligands. Changing ligand can usually change co-ordination number and colour. [Cu(H2O)6]2+ [CuCl4]2- BLUE GREEN OCTAHEDRAL TETRAHEDRAL Changing ligands [Co(H2O)6]2+ [CoCl4]2- PINK BLUE OCTAHEDRAL TETRAHEDRAL Ligand change but no co-ordination change [Cr(H2O)6]3+ [Cr(NH3)6]3+ RUBY PURPLE OCTAHEDRAL OCTAHEDRAL Colour changes arising as a result of all three changes [Mn(H2O)6]2+ [MnO4]- VERY PALE PINK PURPLE OCTAHEDRAL TETRAHEDRAL Complex formation Complex compounds contain a central atom surrounded by ions or molecules called ligands. Ligands donate an electron pair to a metal forming a co-ordinate bond. The number of ligands bonded to the metal ion is called the co-ordination number. The co-ordination number exceeds its oxidation state. Example CuSO4(s) + excess H2O [Cu(H2O)6]2+ White Blue Co-ordination number of Cu= 6 Oxidation state of Cu = 2 Each water molecule is donating an electron pair (one of its lone pairs) The bonds within the complex, i.e. the Cu-O and the O-H are covalent. Unidentate ligands Ligands which have only one atom which can donate a pair of electrons is said to be unidentate. They include the following:- H2O, NH3, Cl-, OH - and CN- Bidentate ligands Ligands which contain two donor atoms they bond to the metal ion through two atoms Bidentate ligands include:- 1,2-diaminoethane NH2CH2CH2NH2 Ethanedioate(oxalate) ion COO - COO - Multidentate Contain many donor atoms Example is bis[di(carboxymethyl)amino]ethane commonly known as EDTA. EDTA is able to bond to metal ions from six donor atoms. EDTA The EDTA4- anion has the formula -OOCCH CH2COO - 2 NCH2CH2N -OOCCH CH2COO - 2 with six donor sites(4O and 2N)and forms a 1:1 complex with metal(II) ions Variable oxidation states Oxidation state is the formal number of positive charges which must be given to the metal to satisfy electroneutrality . E.g [Co(NH3)6]3+ NH3 no charge, overall charge = +3 so Co is in +3 oxidation state. When transition metal ions undergo oxidation - reduction reactions the metal changes its oxidation state. Its time to wake up!!!!! Work out the oxidation state of the metal in each ion below. Vanadium VO4 3-, [VO(H2O)5]2+, [V(H2O)6]3+ Chromium CrO42- , [Cr(H2O)6]3+ Iron [Fe(H2O)6]3+ Catalysis Homogeneous Heterogeneous Haber process uses Iron Contact process uses vanadium(V)oxide Contact process Sulphur dioxide oxidised to sulphur by vanadium (v) oxide SO2 + V2O5 SO3 + V2O4 The vanadium has been reduced to vanadium(IV). It is oxidised back to the starting vanadium (V) oxide by oxygen 2V2O4 + O2 2V2O5 the vanadium(v)oxide is unchanged at the end of the reaction.
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