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Density functional theory (DFT) calculations using BHH X = Cl, Br; n = 2–4). The purpose of the study was to investigate the abnormal Tc 2 X 8 n − system, where the Tc–Tc bond length unexpectedly shortened from Tc 2 X 8 2− to Tc 2 X 8 3− , as the decrease in formal bond order from 4 to 3.5 normally tends to result in an increased bond length. Geometry optimizations using the functional BH&HLYP and basis sets SDD/aug-CC-PVDZ were able to reproduce this irregular change in Tc–Tc bond length. Natural bond orbital (NBO) analyses demonstrated that an increase in both σ and π bond strength, likely induced by the change in charge on the Tc atoms, resulted in the shortening in the Tc–Tc bond length. This study also suggested that DFT calculations on compounds with metal–metal bonds are sensitive to the choice of functionals and basis sets. |
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