The Molecular Orbital Picture of Cyclobutadiene

The pi molecular orbitals of cyclobutadiene. There are four MOs the lowest-energy bonding orbital, the highest-energy antibonding orbital, and two degenerate nonbonding orbitals. 

An electronic energy diagram of cyclobutadiene shows that two electrons are unpaired in separate nonbonding molecular orbitals. 

The localized structure of cyclobutadiene shows two double bonds, implying four pi electrons. Two electrons fill tti, the lowest-lying orbital. Once tT] is filled, there are two orbitals of equal energy available for the remaining two electrons. If the two electrons go into the same orbital, they must have paired spins and they must share the same region of space. Since electrons repel each other, less energy is required for the electrons to occupy different degenerate orbitals, with unpaired spins. This principle is another application of Hund s rule (Section 1-2). 

The pi electrons are filled into the orbitals in accordance with the aufbau principle (lowest-energy orbitals are filled first) and Hund s rule. 

The polygon rule gives you a fast way to draw an electronic configuration. It also provides a quick check on molecular orbitals you might draw, to see which are bonding, antibonding, and nonbonding. 

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Cyclobutadiene According to the molecular orbital picture, square planar cyclobn-tadiene shonld be a diradical (have two unpaired electrons). The fonr TT electrons are distribnted so that two are in the lowest energy orbital and, in accordance with Hund s rule, each of the two equal-energy nonbonding orbitals is half-filled. (Remember, Hnnd s rnle tells ns that when two orbitals have the same energy, each one is half-hlled before either of them reaches its full complement of two electrons.)... 

On the basis of this resonance picture only, organic chemists initially expected that cyclobutadiene, like benzene, would have a large resonance stabilization and would be especially stable. Yet cyclobutadiene proved to be an extraordinarily elusive compound. Many unsuccessful attempts were made to prepare this compound before it was finally synthesized at very low temperature in 1965. The compound is quite unstable and reacts rapidly at temperatures above 35 K. As we shall see, cyclobutadiene is a member of an unusual group of compounds that are actually destabilized by resonance. To understand why benzene is so stable while cyclobutadiene is so unstable, we must examine a molecular orbital picture for these compounds. 

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