Cyclopropenium cations orbitals

This difference between the a - and rf -P-containing rings may be rationalized by the nature of the orbitals involved in 7t-delocalization. In diphosphirenylium ions I with dicoordinate phosphorus atoms, these are one 2p(C) and two 3p(P) orbitals. In ions of type IV one 2p(C), one 3p(P), and one a (PX) orbital have to be considered for the cyclic dilocalization of two 7t-electrons. It is obvious that the Hiickel-type three-orbital two-electron interaction typical for the cyclopropenium cation is more favored in diphosphirenylium ions I than in the related diphosphir-enium systems IV. 

Figiire 4.4. The ji-orbitals of the cyclopropenium cation, CT I/, (a) The filled TT-bonding MO can act as a source of electrons for metal-carbon bonding (b) the empty re-antibonding MOs can receive electronic charge from metal pd hybrid AOs. 

The valence orbitals of the chain, cyclic, and cluster structures constructed from the isolobal groups also have the similar shapes. For example, the valence orbitals of Co2(CO)6 are similar to those of acetylene C2H2, the orbitals of the cyclic clusters Fe2(CO)i2 and Fe3(CO)9 are analogous to the MO s of the cyclopropane and cyclopropenium cation. Some other examples of isolobally related organic and organometallic compounds are represented by the structures... 

Aspects of electronic theory like aromaticity (Figure 4) (and studies of orbital symmetry control, see later) have been probed through the synthesis of molecules like cyclobutadiene 44, 45 ), bridged armulenes ( 46), the cyclopropenium cation (47), and the cyclooctadiene dianion 48). 

To synthesize a new electronic system, tri-heteroatom-substituted cyclopropenium, we thought it useful to examine beforehand the stability of the proposed trisubstituted cyclopropenium system by quantum mechanical methods (or, in other ways). A simple Hiickel molecular orbital calculation for the tri-heteroatom-substituted cyclopropenimn cation 3) gave the results shown in Table 1, where qc and -pc-c denote the n electron density on the ring carbon atom and the n bond order between ring carbon atoms, respectively. Ai it (additional resonance energy) as the measure of stability of 3) is a value obtained as a resonance energy difference between 3 a) and 3 b). 

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