InTl_4-H⁺ (n = 0∼4): Tetracoordinate Hydrogen in a Planar Fashion?

The recent report of planar tetracoordinate hydrogen (ptH) in In₄H⁺ is very intriguing in planar hypercoordinate chemistry. Our high-level CCSD(T) calculations revealed that the proposed D₄-symmetric ptH In₄H⁺ is a first-order saddle point with an imaginary frequency in the out-of-plane mode of the hydrogen atom. In fact, at the CCSD(T)/aug-cc-pV5Z/aug-cc-pV5Z-PP level, the C₄ isomer, with the H atom located 0.70 Å above the In₄ plane, is 0.5 kcal/mol more stable than the D_4h isomer. However, given the small perturbation from planarity and essentially barrierless C₄ ↔ D₄ ↔ C₄ transition, the vibrationally averaged structure can still be considered as a planar. Extending our exploration to the InTl_4-H⁺ (n = 0-3) systems, we found all these ptH structures, except for In₂Tl₂H⁺, to be the putative global minimum. The single σ-delocalized interaction between the central hydrogen atom and InTl_4- ligand rings proves pivotal in establishing planarity and aromaticity and conferring substantial stability upon these rule-breaking ptH species.