 MTO also catalyses the conversion of aldehydes and diazoalkanes into an alkene.A distinctive derivative of rhenium is nonahydridorhenate, originally thought to be the "rhenide" anion, Re−, but actually containing the anion in which the oxidation state of rhenium is +7.Rhenium is one of the rarest elements in Earth's crust with an average concentration of 1 ppb; other sources quote the number of 0.5 ppb making it the 77th most abundant element in Earth's crust.