Its replacements are other organometallic compounds, such as ferrocene and methylcyclopentadienyl manganese tricarbonyl (MMT). The redox active component is either sulfur, ferrocene, or manganese dioxide. Ruthenocene is analogous to ferrocene structurally, but exhibits distinctive redox properties. This compound on boiling under reflux in benzene is converted to ferrocene. In modern notation, this sandwich structural model of the ferrocene molecule is denoted as . Solid organometallocenes such as ferrocene, cobaltocene, nickelocene are also common catalysts. The first reported syntheses of ferrocene were nearly simultaneous. Two more modern alternative fuel additives are ferrocene and methylcyclopentadienyl manganese tricarbonyl. Oxidation of ferrocene gives the blue 17e species ferrocenium. The iron-containing deposits formed from ferrocene can form a conductive coating on spark plug surfaces. Cobaltocene is much more sensitive to oxidation than ferrocene. For non-aqueous work, IUPAC recommends the use of the ferrocene/ferrocenium couple as an internal standard. Cobaltocene is a structural analog to ferrocene, with cobalt in place of iron. For example, in ferrocene, iron(II), ferrous iron is present. A. Rich, who confirmed the structure of ferrocene by X-ray crystallography.