InnTl4–nH+(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 D4h-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 C4visomer, with the H atom located 0.70 Å above the In4plane, is 0.5 kcal/mol more stable than the D4hisomer. However, given the small perturbation from planarity and essentially barrierless C4v↔ D4h↔ C4vtransition, the vibrationally averaged structure can still be considered as a planar. Extending our exploration to the InnTl4–nH+(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 InnTl4–nligand rings proves pivotal in establishing planarity and aromaticity and conferring substantial stability upon these rule-breaking ptH species.