[4] The COT2- rings are found to be planar with 8 equivalent C–C bonds of 1.385 Å length, and sit parallel in an eclipsed conformation.

[4] Neptunocene assumes a monoclinic crystal structure (P21/n space group) which is isomorphous to uranocene and thorocene but not to plutonocene.

[4] Neptunocene was first synthesised in 1970 by reacting neptunium(IV) chloride (NpCl4) with dipotassium cyclooctatetraenide (K2(C8H8)) in diethyl ether or THF:[1] The same reaction conditions have been routinely reproduced since then for the synthesis of the compound.

[3][4] The three actinocenes uranocene, neptunocene, and plutonocene share virtually identical chemistry: they do not react in the presence of water or dilute base, but are very air-sensitive, quickly forming oxides.

[1][2][3] All three are only slightly soluble (up to about 10−3 M concentrations) in aromatic or chlorinated solvents such as benzene, toluene, carbon tetrachloride or chloroform.