The Structures of Pentamethyltantalum and ‐Antimony: One Square Pyramid and One Trigonal Bipyramid

Unexpected structures for pentacoordinate d0 metal complexes were revealed in these theoretical and experimental studies. Ab initio calculations on [TaH5] and [Ta(CH3)5] (1) show that square-pyramidal coordination is preferred over trigonal-bipyramidal in both cases. This was also confirmed for the pentamethyl complex by gas-phase electron diffraction experiments. In contrast, the main group element compound [Sb(CH3)5] (2) has a trigonal-bipyramidal geometry which is, however, separated from the square-pyramidal form by only a small energy barrier.