Transition Metals.

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Presentation transcript:

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. Coordination number: The number of atoms directly joined to a transition metal by a coordinate (dative covalent) bond.

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

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-.

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-).

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

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

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

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

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)

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-

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.

VO2+ → VO2+ → V(H2O)63+ → V(H2O)62+ 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.

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

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+).

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.

Cobalt Co(H2O)62+ → Co(NH3)62+ → Co(NH3)63+ CoCl42- Co(OH)2 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)

Nickel Ni(H2O)62+ Ni(NH3)62+ NiCl42- Ni(OH)2 Ni(CN)42- 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

Copper CuCl42- Cu(OH)2 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)

Silver Ag+(aq) Ag2O(s) Ag(NH3)2+ Cl- AgCl(s) 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)

Uses of transition metals and 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.