Important Compounds of Transition Elements

Having covered the general trends of the 3d transition series in section 4.3, this section turns to specific, named compounds of transition elements, beginning with a general survey of transition-metal oxides and oxoanions before examining two industrially and analytically central compounds -- potassium dichromate and potassium permanganate -- in full detail.

Transition-metal oxides are, in general, formed by the direct reaction of a transition metal with molecular oxygen (O₂) at elevated (high) temperature. With the single exception of the very first member of the 3d series, scandium (for which no simple, stable ionic oxide analogous to those of its neighbours is well characterised), every other 3d transition element forms genuinely ionic metal oxides. Across the accessible range of oxidation states for transition-metal oxides -- from +2 up to +7 -- there is a clear, systematic trend: as the oxidation number of the metal within the oxide increases, the ionic character of the metal-oxygen bonding correspondingly DECREASES (equivalently, covalent character increases). At the very highest oxidation numbers, the bonding becomes predominantly covalent rather than ionic; manganese heptoxide, Mn₂O₇ (manganese in the extreme +7 state), is the textbook example of an essentially covalent transition-metal oxide. …