The transition elements are d-block elements that form at least one stable ion with an incomplete d sub-shell. Across the first row, from titanium to copper, they share a set of characteristic properties.Like other metals they are good conductors of heat and electricity. They have relatively high densities and high melting and boiling points compared with the s-block metals, because many 4s and 3d electrons are delocalised, giving strong metallic bonding.They show variable oxidation states, because the 4s and 3d electrons are close in energy and different numbers can be lost. For example, iron forms Fe2+ and Fe3+, and manganese ranges from +2 up to +7 in MnO4-.They form coloured compounds and ions, such as blue Cu2+(aq) and pale green Fe2+(aq). They also form complex ions, in which ligands bond to the central metal ion by coordinate bonds.Transition metals and their compounds are good catalysts, because they can change oxidation state and can provide a surface for reactions. Examples include iron in the Haber process, nickel in the hydrogenation of alkenes and manganese(IV) oxide in the decomposition of hydrogen peroxide.