Following on from ferrocene—the first sandwich compound with a central Fe atom coordinated to two parallel cyclopentadienyl rings—names for compounds with similar structures such as osmocene and vanadocene are in common usage. The recommendation is that the name-ending ocene should be restricted to compounds where there are discrete molecules of bis(η-cyclopentadienyl)metal (and ring-substituted analogues), where the cyclopentadienyl rings are essentially parallel, and the metal is in the d-block. The terminology does NOT apply to compounds of the s- or p-block elements such as Ba(CH) or Sn(CH).
Examples of compounds that meet the criteria are:
* vanadocene, [V(η-CH)]
* chromocene, [Cr(η-CH)]
* cobaltocene, [Co(η-CH)]
* rhodocene, [Rh(η-CH)]
* nickelocene, [Ni(η-CH)]
* ruthenocene, [Ru(η-CH)]
* osmocene, [Os(η-CH)]
* manganocene, [Mn(η-CH)]
* rhenocene, [Re(η-CH)].
Examples of compounds that should not be named as metallocenes are:
* CHTi
* [Ti(η-CH)Cl] is properly named dichloridobis(η-cyclopentadienyl)titanium NOT titanocene dichloride