Unusual Intramolecular Motion of ReH 9 2- in K 2 ReH 9 Crystal: Circle Dance and Three-Arm Turnstile Mechanisms Revealed by Computational Studies.

The nonahydridorhenate dianion ReH 9 2- is a unique rhenium polyhydride complex due to its remarkably high coordination number; however, its detailed polytopal rearrangement process in either solution or crystal is so far unclear. In this work, our quantum chemical calculations have identified two previously unreported fluxional mechanisms for the ReH 9 2- dianion in the K 2 ReH 9 crystal: three-arm turnstile rotation and circle dance mechanism. These two polytopal rearrangements in the crystal offer an alternative interpretation to the pulse and wide-line NMR spectra (Farrar et al. J. Chem. Phys . 1969 , 51, 3595). The previously postulated hindered rotation of the whole ReH 9 2- dianion in K 2 ReH 9 (White et al. J. Chem. Soc., Faraday Trans. 2 1972 , 68, 1414) turns out to be a combination of the above-mentioned two elementary fluxional processes. In addition, our calculations have confirmed the Muetterties' D 3 h ⇌ C 4 v rearrangement as the intramolecular motion for the ReH 9 2- dianion in solution.