Aromaticity of Osmaacenes in Their Lowest-Lying Singlet and Triplet States

Abstract

The aromatic character of a series of osmaacenes in their lowest-lying singlet and triplet states was thoroughly examined by means of the magnetically induced current densities and multicentre delocalization indices (MCIs). Both employed approaches agree that the osmabenzene molecule (OsB) in the S0 state exhibits dominant π-Hückel-type aromatic character, with a small but nonnegligible amount of π-Craig–Möbius aromaticity. Contrary to benzene, which is antiaromatic in the T1 state, OsB preserves some of its aromaticity in the T1 state. In higher members of the osmaacene series in the S0 and T1 states, the central Os-containing ring becomes nonaromatic, acting as a barrier between the two side polyacenic units which, on the other hand, exhibit a significant extent of π-electron delocalization.

Magnetic Antiaromaticity─Paratropicity─Does Not Necessarily Imply Instability

“…This concept not only proved to be useful for hydrocarbons in their electronic ground states but also has been vastly used to interpret characteristics of metallaaromatics , and metallic clusters − as well as hydrocarbons in their excited electronic states. , However, because aromaticity does not have a quantum mechanical operator, qualitative and quantitative assessment of aromaticity proved to be a controversial task. Aromaticity criteria, i.e., energetic, structural, electronic, and magnetic, , sometimes conflict with one another. − Even within a particular criterion of aromaticity, sometimes different tools show contradictory results or at least results that are open for discussion. − Even the validity of the Hückel rule and the role of active orbitals in the magnetic response of the aromatic species has been questioned recently . In a recent perspective article, my colleagues and I collected opinions of several experts in the field of aromaticity, hoping to reach an agreement on the topic .…”

“… 14 − 17 Even within a particular criterion of aromaticity, sometimes different tools show contradictory results or at least results that are open for discussion. 18 − 23 Even the validity of the Hückel rule and the role of active orbitals in the magnetic response of the aromatic species has been questioned recently. 14 In a recent perspective article, my colleagues and I collected opinions of several experts in the field of aromaticity, hoping to reach an agreement on the topic.…”

1,3-Dipolar Cycloadduct of Organometallic Allene with Dipolarophile─An Apparent Hückel–Möbius Aromatic Five-Membered Organometallic Carbene with Ambiphilic Reactivity

“…Since d yz has a nodal plane perpendicular to the five-membered ring, the π-molecular orbitals resulting from the interaction with d yz orbitals always have a nodal plane on the transition metals. Hence, the five-membered π-molecular orbitals resulting from d yz orbitals can be considered as Möbius-type delocalization. , Note that the actual Möbius type delocalization arises from the gradual twist of p-orbitals on sp 2 -hybridized carbon atoms. The molecular orbital analysis suggests that there are four occupied π-molecular orbitals with contributions from the d yz orbitals.…”