Cyclobutadiene electronic configuration

For some systems a single determinant (SCFcalculation) is insufficient to describe the electronic wave function. For example, square cyclobutadiene and twisted ethylene require at least two configurations to describe their ground states. To allow several configurations to be used, a multi-electron configuration interaction technique has been implemented in HyperChem. 

With the MO predictions of zero delocalization energy and an electronic configuration with unpaired electrons, we should not be surprised that cyclobutadiene readily dimerizes to give 26 even at very low temperatures. 

Several transition-metal complexes of cyclobutadiene have been prepared, and this is all the more remarkable because of the instability of the parent hydrocarbon. Reactions that logically should lead to cyclobutadiene give dimeric products instead. Thus, 3,4-dichlorocyclobutene has been de-chlorinated with lithium amalgam in ether, and the hydrocarbon product is a dimer of cyclobutadiene, 5. However, 3,4-dichlorocyclobutene reacts with diiron nonacarbonyl, Fe2(CO)9, to give a stable iron tricarbonyl complex of cyclobutadiene, 6, whose structure has been established by x-ray analysis. The 7r-electron system of cyclobutadiene is considerably stabilized by complex formation with iron, which again attains the electronic configuration of krypton. 

The electronic configuration in Figure 16-7 indicates that cyclobutadiene should be unstable. Its highest-lying electrons are in nonbonding orbitals (ir2 and ir3) and are therefore very reactive. According to Hund s rule, the compound exists as a diradical (two unpaired electrons) in its ground state. Such a diradical is expected to be extremely reactive. Thus, molecular orbital theory successfully predicts the dramatic stability difference between benzene and cyclobutadiene. 

Figure 16-8 shows that the first 3 pairs of electrons are in three bonding molecular orbitals of cyclooctatetraene. Electrons 7 and 8, however, are located in two different nonbonding orbitals. As in cyclobutadiene, a planar cyclooctatetraene is predicted to be a diradical, a particularly unstable electron configuration. 

The Mj" (M = Li, Na, K, Rb, Cs) anions are examples of a-antiaromatic systems with 4 o-electrons [100]. Singlet Mj" anions at the symmetry have the la/ le electron configuration and their triangular structures undergo Jahn-Teller distortion. Indeed, Mj anions are linear. Two valence MOs can be localized into two 2c-2e bonds and the linear structure of Mj" can be considered as a classical structure. That is similar to the antiaromatic cyclobutadiene, where canonical MOs can be localized into two double and two single carbon-carbon bonds, producing a classical structure. 

Use group theory to solve for the energy levels of cyclobutadiene. Calculate values for DE. ., py, and q.. Use Hand s rule (p. 4 ) to determine the proper electronic configuration. 

Application of the simple molecular orbital theory to cyclobutadiene (cf Exercise 4-6) leads to prediction of four one-electron energy levels a + 2p, a, a, and a 2p. Use of Hund s rule leads to the following electronic configuration for the four tt electrons ... 

The two lowest orbitals are occupied in the 1,3-diene molecule, therefore the electron configuration is ij/lij/l. Cyclobutadiene and trimethylenemethane have electron configuration and thus are biradicals.Table 8.1 gives symmetries of orbitals... 

The reaction between the tetraphenylcyclobutadienepalladium compound (Ph4C4PdBr2)2 and CFa-S Ag gives the monomeric compound (136), which appears to be the first example of a cyclobutadiene metal complex with a 16-electron configuration. ... 

Probably the most academically dramatic use of metal Tc-complex formation was demonstrated by the trapping of the cyclobutadiene-silver nitrite complex. Many subsequent cyclobutadiene derivatives were isolated and proved to be rather stable complexes. For example, tetraphenylcyclo-butadieneiron tricarbonyl melts without decomposition at 234° and tetra-phenylcyclobutadiene 7r-cyclopentadienyl cobalt melts at 256°C under nitrogen. Upon examination of the electronic configuration the stability of these complexes often can be predicted. 

For square cyclobutadiene, the lowest lying singlet state is described by the electron configuration 12.15, in which the degenerate orbitals are singly occupied with opposite spins. The alternative singlet state configurations 12.16 and 12.17,... 

Cyclobutadiene complexes afford a classic example of the stabilization of a ligand by coordination lo a metal and, indeed, were predicted theoretically on this basis by H. C. Longuei-Higgins and L, E, Orgel (1956) some 3y before the first examples were synthesized, In the (hypothetical) free cyclobutadiene molecule 2 of the 4 rr-electrons would occupy t /i and there would be an unpaired electron m each of the 2 degenerate oibilals 2, Coordination to a metal provides funhei interactions and avoids this unstable configuration, See also the discussion on ferra-boranes (p. 174). 

Craig, D. P., Proc. Roy. Soc. [London) A202, 498, Electronic levels in simple conjugated systems. I. Configuration interaction in cyclobutadiene. (ii) All the interelectron repulsion integrals, three- and four-centered atomic integrals, are included. 

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