Halides Transition metals react with halogens at high temperatures to form halides. These reactions have very high activation energies; therefore, high temperatures are required to start these reactions. However, once the reaction starts, the heat liberated is sufficient to sustain the reaction. The order of reactivity of different halogens decreases as: Fluorides are usually ionic whereas chlorides, bromides and iodides are predominantly covalent. The covalent character increase in atomic mass of the halogen. For example, CuF2 is ionic while CuCl2 and CuBr2 are covalent consisting of infinite chains. Co-ordination number of copper in these compounds is six. Fluorine brings about the highest oxidation state of the metal. This is due its small size and high electronegativity. Halides of metals in higher oxidation states are relatively unstable and get hydrolysed easily. For example, TiCl4 + 2H2O TiO2 + 4HCl Sulphides Sulphides of transition metals are prepared either by direct combination of sulphur with metals or by the reaction of H2S or Na2S with the solution of their salts. In sulphides, the oxidation state of metal is generally low because sulphur is a weak oxidising agent. These sulphides are generally coloured or black. For example, NiS, CoS and CuS are black, MnS is light pink and CdS is yellow. They are insoluble in water. Example: classify the following oxides as acidic, basic and amphoteric: MnO, MnO2, Mn2O3, Mn3O4, Mn2O7. Solution: Acidic oxide : Mn2O7 Basic oxides : MnO, Mn2O3 Amphoteric oxides: Mn3O4, MnO2.