Chemistry Explosives Overview Of Metal Chemistry

Overview of Metal Chemistry Angus Paul Wilkinson School of Chemistry and Biochemistry, Georgia Institute of Technology Topics to be covered ‹ An overview of the properties of the metallic elements ‹ s - metals ‹ d - metals ‹ p - block metals ‹ f - block (lanthanides and actinides) The occurrence of metallic elements Enthalpies of vaporization Reactivity ‹ Most metals are reactive towards reagents such as oxygen ‹ The noble metals, Au, Pt show no tendency to react with oxygen and are generally of low reactivity ‹ Reaction of a metal with a reagent can produce compounds that still display metal-metal bonding Examples of metal-metal bonding ‹ Rb9O2, Cs4O ‹ ReCl3, MoCl2, ZrCl ‹ Fe2(CO)9 Occurrence of cluster formation S - block metals ‹ Very reactive ‹ Compounds tend to be ionic – much of their chemistry can be explained using the ionic model ‹ Do not form a wide variety of complexes ‹ Their chemistry is predominantly that of species in the group oxidation state Complexes of s-block metals ‹ Complexes of alkali and alkaline earth cations are restricted to polydentate ligands – crown ethers – cryptands – EDTA and its relatives Redox properties of s-block ‹ Standard electrode potentials are quite uniform down the groups as the decrease in vaporization and ionization enthalpies is counterbalanced by a decrease in hydration enthalpy Note that lithium is widely used in the fabrication of batteries. Why? Crown ethers and cryptands The macrocylic chelate effect ‹ The efficient binding of chelating ligands is typically argued to be a consequence of entropic effects – Chelating ligand displaces several monodentate ligands ‹ With macrocyclic ligands there is a thermodynamic advantage over more open chelating ligands that is enthalpic in origin Size selectivity ‹ Crown ethers and cryptands show quite high selectively for ions that fit the ligand well Suboxides ‹ A range of alkali metal oxides can be prepared that have the metal in oxidation states of less than one. They are often electrically conducting and can be viewed as materials where O2- is occupying holes in a metallic structure Rb9O2 Cs11O3 Liquid ammonia solutions ‹ Dilute solutions of Na in liquid ammonia are blue ‹ Na(s) --(NH3)---> Na+(am) + e-(am) ‹ The color is due to solvated electrons ‹ More concentrated solutions take on a metallic appearance ‹ The solutions are metastable Solvated electrons can also be produced in water based glasses by irradiation, but they are not as stable Color centers ‹ Electrons can also be metastably trapped in solid matrices – Called color centers ‹ Irradiation of salts with x-rays or other ionizing radiation produces colored defects. The color of the defect depends on the nature of the host lattice KCl KBr NaCl In each case the color is from a trapped electron. The color can be rationalized by using electron in a box arguments. As the “box” gets smaller the energy levels get further apart so the absorption moves further towards the blue Alkalide ions ‹ If alkali metals are dissolved in alkyamines alkalide ions (M-) are formed – The color of the solution in only dependent upon M-. That cation does not mater. ‹ Alkalides can be isolated if the counter cation is complexed with a cryptand – Na(2.2.2)+ Na- Electrides ‹ It is possible, using macrocyclic, ligands to prepare electrides from solutions of alkali metals – Cs(18-C-6)2+ e- ‹ An electride is an ionic solid where the anion is an electron