Delocalized open chain cations

Bis(ethylenediamine)gold(III) chloride reacts with /3-diketonates in aqueous base via Schiff base condensation to form complexes of gold(III) with a 14-membered macrocyclic tetraaza ligand such as (292).1711-1713 The X-ray structure showed the cation to be nearly planar. Delocalization of -electrons within the six-membered /3-diketonate rings was indicated by the observed pattern of C—C and C—N distances. Open-chain tetraaza ligand complexes in which condensation of only one /3-diketonate has occurred, can be isolated as intermediates in this reaction. They may be used for further condensation with a different /3-diketonate. Oxidation of this complex with trityl tetrafluoroborate introduces a double bond in position C2C3.1712,1714... 

Nonplanar carbenium ions, such as the 1-adamantyl cation, display shielded C+ carbon nuclei compared with open-chain trialkylcarbenium ions (e.g. the triethylcarbenium ion in Table 4.76). Deshielding of a and jt carbons relative to the parent adamantane is attributed to charge delocalization via a bonds [493]. 

Let us exemplify the method with two systems, benzene and allyl radical which are prototypical conjugated and delocalized species of the cyclic and open-chain varieties. The a frame of a conjugated molecule is simply the cation that one gets after complete removal of all tt electrons, leaving an effective charge on each carbon (or hetero) atom as represented below for benzene and allyl in 1 and 2... 

One way to evaluate whether a cyclic compound is stabilized by delocalization of tt electrons through its ring is to compare it with an open-chain compound having the same number of tt electrons. This approach is particularly useful because it furnishes us with models not only for annulenes but for aromatic cations and anions, as well. (Corrections need to be made, of course, when the cyclic system is strained.)... 

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