Molecular orbital theory antibonding states

General. Nitric oxide readily forms complexes with transition metals and is in many ways similar to carbon monoxide. It has a single electron in a tt antibonding electron, which is easily lost. When NO is not bridging, molecular orbital theory would predict a linear M-NO+ moiety and that M-NO would be bent. In principal, this would seem to allow a ready indication of the metal oxidation state from an X-ray... 

The frontier molecular-orbital theory also explains exo selectivity in terms of the secondary antibonding interaction between the lobes on C-2 of cyclopentadiene and the carbonyl oxygen of propynal in the endo transition state (Fig. 10). 

As predicted by molecular orbital theory (see Topic C4I dioxygen has two unpaired electrons and some of its chemistry shows diradical characteristics in particular, it reacts readily with other radicals. Singlet oxygen is an excited state in which the two electrons in the n antibonding orbitals have paired spins. It is produced in some chemical reactions and has different chemical reactivity. 

For benzene, the molecular orbital theory states that the six p-orbitals combine to give six molecular orbitals. The three lower-energy molecular orbitals are bonding molecular orbitals, and these are completely filled by the six electrons (which are spin-paired). There are no electrons in the (higher-energy) antibonding orbitals, and hence benzene has a closed bonding shell of delocalised Jt-electrons. 

In photochemical reactions, it is desirable to classify the transition state according to its aromatic properties. In hydrocarbons, the excited states most likely to be involved are those in which an electron is transferred from a n-bonding orbital to a tt -antibonding orbital jt tt ), or in which an electron is transferred from a nonbonding orbital to a ff -antibonding orbital (n jt ). In principle, the excited states could be singlet or triplet states, but simple molecular orbital theory does not distinguish between the two. 

Problem 8.28 (a) Apply the MO theory to the allyl system (cf. Problem 8.26). Indicate the relative energies of the molecular orbitals and state if they are bonding, nonbonding, or antibonding, (b) Insert the electrons for the carbocation C,H, the free radical C,H, and the carbanion CjH, and compare the relative energies of these three species. 

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