In<sub><i>n</i></sub>Tl<sub>4–<i>n</i></sub>H<sup>+</sup> (<i>n</i> = 0∼4): Tetracoordinate Hydrogen in a Planar Fashion?
Li-juan Cui, Xinbo Liu, Hui-yu Zhang, Bing Yan, Mesías Orozco‐Ic, Sudip Pan, Zhong‐hua Cui
Abstract
The recent report of planar tetracoordinate hydrogen (ptH) in In 4 H + is very intriguing in planar hypercoordinate chemistry. Our high-level CCSD(T) calculations revealed that the proposed D 4 h -symmetric ptH In 4 H + 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 C 4 v isomer, with the H atom located 0.70 Å above the In 4 plane, is 0.5 kcal/mol more stable than the D 4h isomer. However, given the small perturbation from planarity and essentially barrierless C 4 v ↔ D 4 h ↔ C 4 v transition, the vibrationally averaged structure can still be considered as a planar. Extending our exploration to the In n Tl 4– n H + ( n = 0–3) systems, we found all these ptH structures, except for In 2 Tl 2 H +, to be the putative global minimum. The single σ-delocalized interaction between the central hydrogen atom and In n Tl 4– n ligand rings proves pivotal in establishing planarity and aromaticity and conferring substantial stability upon these rule-breaking ptH species.