It is believed to exhibit resonance between its tetravalent and divalent carbon forms. There is some uncertainty about whether the oxidised manganese is tetravalent or trivalent. A few coordination compounds containing terbium in its tetravalent state are also known. KAmF. Tetravalent americium has also been observed in the aqueous phase. Uranium in its tetravalent (uranous) state is very important in different technological processes. The tetravalent state is common for the carbon group. Ti, a tetravalent ion, can replace Zr or Si in a temperature dependent mechanism. Lead, like carbon, is predominantly tetravalent in such compounds. Tetravalent manganese is used as an activator in red-emitting phosphors. In most organosilicon compounds, Si is tetravalent with tetrahedral molecular geometry. In the alkaline solution, dissolved oxygen will oxidize manganese(II) ions to the tetravalent state. Carbon is tetravalent but carbon free radicals and carbenes occur as short-lived intermediates.