Adaptive aromaticity in the lowest singlet and triplet states is a rare property found among molecular systems. So far, only osmapentalene and osmapyridinium have been found to possess the adaptive aromaticity. Although it has been confirmed that the pattern of electron excitation is a key factor to achieve the adaptive aromaticity, further investigation of the metal center effect has not yet been made. Ruthenium, another Group 8 transition metal, can form metallacycles similar to the osmium counterparts.
The many manifestations of aromaticity have long fascinated both experimentalists and theoreticians. Due to their degenerate half-filled MOs, triplet [n]annulenes with 4n π-electrons are also aromatic, but the degree of their stabilization has been difficult to quantify. The isomerization stabilization energy (ISE) method has been applied to evaluate the triplet aromaticity. The reliability of this approach is indicated by the strong correlation of the ISE results with NICS(1)zz, a magnetic indicator of triplet state aromaticity.