The three-center-four-electron (3c-4e) bond nature revisited. An atoms-in-molecules theory (AIM) and ELF study

Theoretical calculations (B3LYP/6-311 + + G**) were performed on a series of formally hypervalent compounds showing linear three-center geometries. The bonding nature was analyzed by the electron density, rho (r), and electron-localization function (ELF) topologies, including calculations of the AIM charges and NMR chemical shifts (GIAO method). In addition. a quantitative analysis was also performed of the localization and delocalization indexes, obtained from the electron-pair density in conjunction with the definition of an atom in a molecule. Furthermore, the populations and fluctuations in the ELF basins were also evaluated. The compounds studied presented linear (15), T-shaped (6-9), and bipyramidal structures (10-15). Our results support the 3c-4e model for the linear (1-5) structures, but reveal for the T-shaped (6-9) structures only a small contribution from this model. In addition, there is no evidence to support the 3c-4e bond scheme for the bipyramidal compounds (10-15).