Nitrenes molecular orbitals

Scheme 17.34 Molecular orbital analysis of putative metallo-nitrene.

It is informative to compare methylene and nitrene in terms of simple molecular orbital theory and then to extend this comparison to their aromatic derivatives phenyl carbene and phenyl nitrene. Hoffman et al. [28] have described the molecular orbitals of CH2 in the following way. Methylene has two nonbonding molecular orbitals, one is an in-plane, a type, hybrid orbital, the other is an out-of-plane pure p type n orbital. Singlet methylene has a bond angle of 105° and the in-plane, sigma, orbital is doubly occupied. Triplet methylene has both a singly occupied n and a orbital and a bond angle of 135°. 

Figure 5.1 Molecular orbitals of nitrene NH. The lo-orbital, which is not shown, is the Is AO on nitrogen.

Zink has applied a molecular orbital approach to photochemical assignments of excited states of iridium(III) azide complexes. The observed products of 250-400 nm irradiation of [Ir(N3)(NH3)5] are [Ir(NH3)5(NH2Cl)] and N2, the same products expected on the basis of populating the LL (ligand-localized Jt o - n ) excited state. It was suggested, therefore, that the photoreaction originates in the LL excited state. The formation of a coordinated nitrene intermediate has been ascribed to a combination of LMCT and LL states rather than to the LL state alone. 

Computed atomic charges and binding energies from an INDO molecular orbital study of a-heteroatom nitrenes are consistent with their known reactivity towards olefins to give aziridines. Molecular orbital calculations on the reaction of propylene oxide and isobutylene oxide with hydrogen chloride and ammonia predict an orientation of addition in agreement with experiment. 

In the context of this chapter, the electronic structure and spectroscopy of the simplest nitrene, NH, are very important because they will be useful in the analysis of more complicated nitrenes. The electronic structure of NH can be understood on the basis of elementary molecular orbital considerations (Figure 11.1). 

However, on bending the carbene or nitrene 10° out of the molecular plane -as required for rearrangement — both methods give identical answers the carbene or nitrene lone-pair conjugates with the ring. The HOMO and LUMO orbitals in phenylnitrene are shown below. The more electrophilic the ring, the more pronounced... 

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