Uranocene – The Trend Setting Aromatic Actinide Sandwich Compound

Chemistry, Technology
Uranocene, U(C₈H₈)₂, or biscyclooctatetraenyl uranium, is a most interesting aromatic organometallic sandwich compound.1 Notice its dumbbell-like shape. By no means is its shape uranocene's only claim to fame. But before we get into that, it would be helpful to explain terms. We can guess why it's an organometallic compound, just as we can guess why it's called a sandwich compound. Let us, however, discuss what we mean by the term aromatic. Aromaticity in a Nutshell An aromatic structure is a molecule or ion that is cyclic and flat and that incorporates 4n+2 conjugated (alternating) sp2 (double) bonds, where n is some small integer. It displays increased stability over non-aromatic structures. Despite possessing double bonds, aromatic structures tend to undergo substitution rather than addition reactions. It requires considerable energy to disrupt…
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Why Cyclodecapentaene is Not an Aromatic Hydrocarbon

[caption id="attachment_16073" align="alignright" width="440"] Fig 1. Cyclodecapentaene - all cis form.[/caption] There are aromatic hydrocarbons and there are non-aromatic hydrocarbons. For basic mono-ring structures that have alternating carbon-carbon double bonds, there is a rule called Hückel’s rule that defines whether a given hydrocarbon can be aromatic or not. It might seem, at first that cyclodecapentaene (C10H10 or [10]-annulene) should be aromatic. Yet, cyclodecapentaene is not an aromatic hydrocarbon. Let’s find out why not. A Few Simple Rules for Aromatic Behavior  The double bonds in the ring must be conjugated (alternating). This allows for the electrons to be delocalized [in conceptualization be flipped in either a clockwise or counterclockwise direction]. This allows for the presence of what is called a ring current.  The ring must be flat.  The mono-ring must feature…
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