Aromaticity of tropylium derivatives: When and why might captodative structures be preferred over the isomeric push-pull structures?

An intriguing question in the general problem of aromaticity is whether captodative aromatic systems with the donor and acceptor substituents at the same carbon of the CC bond can be more stable than the π-conjugated push-pull counterparts? The analysis of electronic, magnetic, and structural criteria of aromaticity showed that for conventional organic substituents XO, TfN, (NC)₂C, (NO₂)₂C, Tf₂C, the push-pull tropylidene derivatives [tropylium]⁺CHCHX⁻ are expectedly more stable than their captodative isomers [tropylium]⁺C(X⁻)CH₂, with the lowest ΔE for the most strong acceptor Tf₂C. A different behavior is observed for XMHlg₃ (MB, Al; HlgF, Cl). They are not only structurally and magnetically most aromatic in both series but show the inverse stability of the push-pull and captodative isomers, the latter being more stable by up to 10 kcal/mol (in gas).The difference between the MHlg₃ groups and conventional organic groups is that in the latter the electron density is transferred to the π-system of the substituent, while the former can accept it only to the σ*(CM) orbital. Thus, when the electron donor and acceptor effects are separated between the σ and π systems, captodative isomers can be more stable than their push-pull isomers with more extended conjugation.