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Marvellous Metals Nyholm Lecture 2002 Professor Tony Baker & Dr Linda Xiao Faculty of Science, UTS Sir Ronald Nyholm 1917-1971 Coordination Chemist Inspiring Chemical Educator Leader of the Profession Sponsorship The Royal Australian Chemical Institute (RACI) www.chem.unsw.edu.au/raci Crown Scientific APS Marvellous Metals: the Lecture Redox Chemistry Spectra and Spectroscopy Coordination Chemistry Redox Chemistry • Many reactions can be classified as redox reactions. • These are reactions in which the oxidation numbers of the elements involved change Example: Redox Chemistry • An acidified solution of permanganate ions reacts with hydrogen peroxide to give dioxygen gas: 2 MnO4- + 6 H+ + 5 H2O2 2 Mn2+ + 8 H2O + 5 O2 Mn +7 +2; O (in peroxide) –1 0 Vanadium • Vanadium is a transition element that displays a maximum oxidation state of +5 (eg in the oxide V2O5). • Named after Vanadis, the Norse goddess of beauty because of the beautiful colours in solution • Used in high strength steels Vanadium reduction: demo Initial: solid NH4VO3 Acidification: VO3- + 2 H+ VO2+ + H2O Reduction (Zn as reductant): VO2+ + 2 H+ + e- VO2+ + H2O VO2+ + 2 H+ + e- V3+ + H2O V3+ + e- V2+ Vanadium Application • Sulfuric Acid Manufacture: SO2 (g) + ½ O2 (g) SO3 (g) • Vanadium(V) oxide catalysts are used in this process. • Sulfuric acid: 150 million tonnes produced each year. Other redox processes The rusting of iron Batteries Electrolysis to purify metals Using reductants to liberate metals from ores Photoreduction: Blueprint • Blueprints (an early form of copying) were first made around 1840 2 [Fe(C2O4)3]3- 2 Fe2+ + 2 CO2 + 5 C2O42(K+ +) Fe2+ + [Fe(CN)6]3- Prussian Blue • The pigment Prussian Blue has been known since 1704 More on Prussian Blue Fe3+ + [Fe(CN)6]4- Prussian Blue Fe2+ + [Fe(CN)6]3- Turnbull’s Blue Found to have same spectra / XRD. Colour arises from charge transfer: Fe3+ + e Fe2+ (lmax 700nm). Probable formula: Fe(III)4[Fe(II)(CN)6]3.15H2O Spectra and Spectroscopy • Spectrum: solar spectrum, rainbow • Plot of radiation intensity vs. wavelength / frequency • May be absorption or emission Uses of Spectroscopy • Identification • Quantification • Study bonding / energy levels X-ray: inner shell electrons UV-Vis: outer shell electrons IR: molecular vibrations Microwave: rotations Vanadium check-up VO2+ VO2+ yellow blue V3+ green V2+ violet Emission Spectra E2 h E1 Emission Flame tests Lithium Sodium Potassium Calcium Strontium Barium Copper Flame tests • The thermal energy is enough to shift electrons to higher energy levels (excited state). • The electron returns to a lower energy level with emission of visible radiation. Absorption spectra E2 h E1 Absorption Absorption: demonstration 1 Absorbance 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 500 550 600 650 700 Wavelength (nm) 750 800 850 Absorption and colour • The copper solution appears blue and absorbs red light. • Under white light illumination some wavelengths are absorbed and some are reflected / transmitted. • The object / solution has the complementary colour to the radiation absorbed. Atomic absorption • Atoms in the ground state will absorb radiation that promotes electrons to an excited state. • The amount of radiation absorbed is proportional to the the number of atoms present. • This concept is the basis of Atomic Absorption Spectroscopy (AAS). AAS: schematic diagram Light source Flame E2 E2 h E1 h E1 Detector AAS: Australia’s contribution • Alan Walsh had worked on emission spectra and molecular spectroscopy. • Demonstrated possibility of AAS in early 1952. • Developed commercially by CSIRO and Australian instrument manufacturers AAS: application • AAS was long considered the best technique for trace metal analysis. • Detection Limits (ppb): Cd 1 Cr 3 Cu 2 Pb 10 V 20 Vanadium: one more time VO2+ VO2+ yellow blue V3+ green V2+ violet Coordination Chemistry ….it is correct to say that modern inorganic chemistry is, especially in solution, the study of complex compounds. Nyholm, The Renaissance of Inorganic Chemistry, 1956 Dissolution of a salt • Water binds to ions at edges of lattice • When bonds to water are stronger than bonds to ions, the ion enters solution H + O H Na O H H Examples • Nickel(II) ions in solution: Ni2+(aq). • Species in solution is [Ni(H2O)6]2+. OH2 H2O OH2 2+ Ni H2O OH2 OH2 • Other examples would include [Cu(H2O)6]2+, [Fe(H2O)6]3+, etc. Shapes of Complexes 6-coordinate: Octahedral 4-coordinate: Tetrahedral Demonstration: [Co(H2O)6]2+ + 4 Cl- [CoCl4]2- + 6 H2O Changing shapes: demo [Co(H2O)6]2+ + 4 Cl- [CoCl4]2- + 6 H2O pink blue OH2 OH2 OH2 2+ Co OH2 2- Cl Co Cl OH2 OH2 OCTAHEDRAL Cl Cl TETRAHEDRAL Coordinate Bond • Many molecules and ions have lone pairs of electrons (eg NH3) and can act as electron pair donors (Lewis bases). • Transition metal ions can have vacant orbitals and can accept electron pairs (Lewis acids). Ligands • The molecules or ions that bind to a metal ion are known as ligands. • Many ligands are known ranging from monoatomic ions such as chloride to huge protein molecules. • Examples include NH3, H2O, NH2CH2CH2NH2 (diaminoethane, a chelating ligand), SCN(thiocyanate) Nickel(II) Complexes: Demo [Ni(H2O)6]2+ [Ni(NH3)6]2+ green blue [Ni(NH2CH2CH2NH2)3]2+ blue-purple [Ni(dmg)2] red Colours of Metals Complexes • In an octahedral complex, the d orbitals are split into two energy levels separated by a gap Do. • The size of Do depends on the nature of the ligand. eg Do t2g Differing interactions • Different metals react in different ways with the same ligand. • One example is the difference in interaction of Ni2+ and Co2+ with SCN-. • In the case of cobalt a stable complex ion is formed [Co(SCN)4]2which is soluble in some organic solvents. Demonstration • A mixture of Ni2+ and Co2+ is treated with excess SCN-. • 2-Butanone (CH3COCH2CH3) is used to extract the reaction mixture. • Nickel ions remain in the aqueous phase and cobalt ions (as [Co(SCN)4]2-) are extracted into the organic phase. Application • Many extractive metallurgical processes depend on different metals interacting in different ways with ligands. • Copper can be purified through a solvent extraction technique. • Treatment of 107 tonnes per year of low grade tailings (1%) recovers a further 105 tonnes of copper. Thermite: Return to Redox • The thermite reaction can be used for such applications as welding in remote locations and depends on the activity of aluminium. • Aluminium powder and iron oxide are mixed together and the reaction is started with burning Mg ribbon. • Highly exothermic reaction! Thermite Thermodynamics Reaction DH (kJ mol-1) 2 Al(s) + 3/2 O2(g) Al2O3(s) Fe2O3(s) 2 Fe(s) + 3/2 O2(g) 2Al(s) + Fe2O3(s) Al2O3(s)+ 2Fe(s) -1676 824 -852