Orbital‐Like Motion of Hydride Ligands around Low‐Coordinate Metal Centers

Abstract

Hydrogen atoms in the coordination sphere of a transition metal are highly mobile ligands. Here, a new type of dynamic process involving hydrides has been characterized by computational means. This dynamic event consists of an orbital‐like motion of hydride ligands around low‐coordinate metal centers containing N‐heterocyclic carbenes. The hydride movement around the carbene–metal–carbene axis is the lowest energy mode connecting energy equivalent isomers. This understanding provides crucial information for the interpretation of NMR spectra.

Complexes of High-Valent Rhenium Supported by the PCP Pincer

“…T 1 inversion recovery experiments at low temperature allow for the observation of hydride bonding modes and have even led to the reformulation of the heptahydride, (PPh 3 ) 2 ReH 7 , as the pentahydride, dihydrogen species: (PPh 3 ) 2 ReH 5 (H 2 ) . At low temperatures, polyhydrides can “freeze out”, breaking their symmetry and allowing for the resolution of multiple hydride resonances by 1 H NMR spectroscopy . Variable-temperature T 1 measurements conducted on the triplet hydride resonance of (PCP iPr )­ReH 6 (−6.2 ppm, 1 H NMR, {500 MHz, THF- d 8 }) showed that even at −75 °C the symmetry of the hexahydride is maintained, providing a single triplet resonance and a T 1 minimum measurement of 225 ms.…”

“…33 At low temperatures, polyhydrides can "freeze out", breaking their symmetry and allowing for the resolution of multiple hydride resonances by 1 H NMR spectroscopy. 34 Variable-temperature T 1 measurements conducted on the triplet hydride resonance of (PCP iPr )ReH 6 (−6.2 ppm, 1 H NMR, {500 MHz, THF-d 8 }) showed that even at −75 °C the symmetry of the hexahydride is maintained, providing a single triplet resonance and a T 1 minimum measurement of 225 ms. Although this measurement is most consistent with a classical hexahydride formulation, some ambiguity remains, as fast hydride exchange between classical and nonclassical hydrogen positions could account for these observations, even at low temperature.…”