Nitric acid electron configuration

Ferrocene is only one of a large number of compounds of transition metals with the cyclopentadienyl anion. Other metals that form sandwich-type structures similar to ferrocene include nickel, titanium, cobalt, ruthenium, zirconium, and osmium. The stability of metallocenes varies greatly with the metal and its oxidation state ferrocene, ruthenocene, and osmocene are particularly stable because in each the metal achieves the electronic configuration of an inert gas. Almost the ultimate in resistance to oxidative attack is reached in (C5H5)2Co , cobalticinium ion, which can be recovered from boiling aqua regia (a mixture of concentrated nitric and hydrochloric acids named for its ability to dissolve platinum and gold). In cobalticinium ion, the metal has the 18 outer-shell electrons characteristic of krypton. 

The oxidation of the iron(III) complexes [Fe(DAS)2X2][FeX4] (DAS = o-phenylenebis-dimethylarsine, X = Cl, Br) with concentrated nitric acid followed by addition of the appropriate anion led to the isolation of black [Fe(DAS)2X2]Y2 (Y = BF, CIO, ReO ). The magnetic properties indicate two unpaired electrons consistent with a low-spin configuration in a tetragonal trans halide) structure. Mossbauer quadrupole splittings are large ( 3.2mms ) and temperature independent. ... 

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