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Transition Metals

Transition Metals. Definitions. Complex: A metal ion surrounded by ligands. Ligand: an electron pair donor i.e. a molecule or ion joined onto the metal ion by a dative covalent bond to the metal.

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Transition Metals

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  1. Transition Metals

  2. Definitions • Complex: A metal ion surrounded by ligands. • Ligand: an electron pair donor i.e. a molecule or ion joined onto the metal ion by a dative covalent bond to the metal. • Coordination number: The number of atoms directly joined to a transition metal by a coordinate (dative covalent) bond.

  3. Typical ligands • Unidentate H2O: :NH3 :Cl- • Bidentate C2O42- H2NCH2CH2NH2 • Multidentate EDTA4- • Watch out for the chelate effect.

  4. Shapes • Six-coordinate complexes are octahedral.eg Cu(H2O)62+ Co(NH3)63+ • Four-coordinate complexes are usually tetrahedral eg CoCl42- but a few are square planar eg Ni(CN)42-. • Two-coordinate complexes are linear eg AgCl2-.

  5. Colour • Most transition metal colours are due to d-d electron transitions. The energy gap between the split d-orbitals corresponds to visible light (E=hf). • Some of the really strong colours are due to charge transfer (such as in MnO4-).

  6. Changing colour • Anything that changes the energy difference between the d-orbitals causes a change in colour: • Oxidation state • Ligand • Coordination number

  7. Measuring concentration by colour A measurement of colour intensity (using a colorimeter) can measure the concentration of a solution. Sometimes a special complex is formed so that the colour is more intense. This is known as UV/visible spectroscopy. eg Fe(H2O)63+ + SCN-Fe(H2O)5(SCN)2+ + H2O

  8. Reaction types • The reactions of the transition metals come under three headings (but sometimes more than one occurs at the same time!). • Ligand substitution • Hydrolysis (the acidity reaction) • Redox

  9. Ligand substitution • One kind of ligand is replaced by another. For example, Cu(H2O)62+ + 4Cl-→ CuCl42- + 6H2O

  10. Hydrolysis (the Acidity reaction) • One or more hydrogen ions is removed. For example, Cu(H2O)62+(aq) + 2OH-(aq)→ Cu(H2O)4(OH)2(s) + 2H2O(l)

  11. Redox The oxidation state of the metal is changed by adding or removing one or more electrons For example, Co(NH3)62+ → Co(NH3)63+ + e-

  12. Titanium • Mainly known for its chloride, TiCl4, as part of the Kroll process for extraction of titanium from its ores. • The chloride is covalently bonded. • Titanium oxide, TiO2 , is used as a white pigment.

  13. Vanadium • Most stable form is V(IV) eg as VO2+. • V(V) can be reduced in acidic conditions (eg Zn / HCl) VO2+→ VO2+→ V(H2O)63+→ V(H2O)62+ V(H2O)62+ is unstable in air since it is oxidised by oxygen.

  14. Chromium • Cr(H2O)63+ Cr(H2O)3(OH)3 Cr(OH)63- >>>>Increasingly alkaline conditions>> Cr2O72- CrO42- <<<<Increasingly alkaline conditions<<<<

  15. Manganese • Mn(H2O)62+ MnO2 MnO42- MnO4- II IV VI VII v. pale pink brown deep purple MnO4- is a very strong oxidising agent. Use acidic conditions. For titrations, make sure you add sufficient sulfuric acid to take Mn all the way to Mn2+. Otherwise you get incomplete reduction (the brown solid, MnO2 is formed, rather than Mn2+).

  16. Iron • Fe(H2O)62+ is non-acidic in water (green). • Pure Fe(H2O)63+ is a lilac colour but on contact with water goes rusty brown. • Fe(H2O)62+ forms Fe(OH)2 (a green solid) with NaOH but it goes brown (forming Fe(OH)3 on standing in air.

  17. Cobalt add ammonia solution add hydrogen peroxide (or air) Co(H2O)62+→ Co(NH3)62+ → Co(NH3)63+ pink brown / yellow orange CoCl42- Co(OH)2 blue blue-grey Add conc HCl Add NaOH or NH3 (aq)

  18. Nickel add ammonia solution Ni(H2O)62+ Ni(NH3)62+ green blue NiCl42- Ni(OH)2 Ni(CN)42- blue pale green a square planar complex! Add NaOH or NH3 (aq) Add NaCN (aq) Add conc HCl

  19. Copper add ammonia solution Cu(H2O)62+ Co(NH3)4(H2O)22+ blue deep blue / violet CuCl42- Cu(OH)2 yellow-green pale blue Add conc HCl Add NaOH or NH3 (aq)

  20. Silver Small quantity of NaOH, NH3(aq) excess NH3 (aq) Ag+(aq) Ag2O(s) Ag(NH3)2+ Cl- AgCl(s) excess Cl- AgCl2- Ag(S2O3)23- Ag(CN)2- NaCN(aq) Na2S2O3(aq)

  21. Uses of transition metalsand their complexes • V2O5 is used as a catalyst in the Contact Process (manufacture of sulfuric acid). • Cis-platin (a platinum complex) is a very effective anti-cancer drug. • Fe2+ is an important part of haemoglobin. • Ag(NH3)2+ is Tollen’s reagent (test for aldehydes). • Ag(S2O3)23- is formed during photographic processing. • Ag(CN)2- is used in electroplating.

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