Comments on “Planar Tetracoordinate Hydrogen: Pushing the Limit of Multicentre Bonding”

In a recent communication ( Angew. Chem. Int. Ed . 2024 , 63 , e202317312), Kalita et al. studied In 4 H + system within the frame of single‐reference approximation (SRA) and found that the global energy minimum ( 1 a ) adopted the singlet state and a planar tetracoordinate hydrogen (ptH), while the second lowest isomer ( 1 b ) located 3.0 kcal/mol above 1 a and adopted the triplet state as well as non‐planar structure with a quasi‐ptH. They assessed the reliability of SRA by checking the T1‐diagnostic values of coupled cluster calculations. However, according to our multi‐configurational second‐order perturbation theory calculations at the CASPT2(12,13)/aug‐cc‐pVQZ (aug‐cc‐pVQZ‐PP for In) level, both 1 a and 1 b exhibit obvious multi‐referential characters, as reflected by their largest reference coefficients of 0.928 (86.1 %) and 0.938 (88.0 %), respectively. Moreover, 1 b is 5.05 kcal/mol lower than 1 a at this level, that is, what can be observed in In 4 H + system is the quasi‐ptH.