Cyclic molecules with 4n + 2 or 4n electrons are aromatic in the lowest
singlet state (S0) or the lowest triplet state (T1) according to Hückel
and Baird’s rules. Thus, the design of aromatic species in
both the S0 and T1 states (termed as adaptive
aromaticity) is particularly challenging. In this work, we demonstrate
that metallasilapentalynes show adaptive aromaticity supported by
structural, magnetic, and electronic indices, in sharp contrast to
metallapentalynes, which exhibit aromaticity in the S0 state
only. The sharp difference in aromaticity could be caused by the distinctive
excitation pattern. Specifically, the T1 state in metallasilapentalyne
is formed by the excitation from the π to σ* molecular
orbital, whereas it is excited from π to π* molecular
orbital in metallapentalyne. In addition, a series of metallasilapentalynes
with adaptive aromaticity are predicted by tuning the ligands. Our
findings enrich the family of adaptive aromatics, inviting experimental
chemists’ examination.