EXCHANGE OF AXIAL AND EQUATORIAL CARBONYL GROUPS IN PENTACOORDINATE METAL-CARBONYLS IN THE SOLID-STATE - THE VARIABLE TEMPERATURE MAGIC ANGLE SPINNING C-13 NMR-SPECTROSCOPY OF FE(CO)5, [PH3PNPPH3][HFE(CO)4], AND [NET4][HFE(CO)4]

The pentacoordinate anion, [HFe(CO)4]-, undergoes rapid axial-equatorial exchange of carbonyl groups in the solid state as demonstrated by variable temperature magic angle spinning carbon-13 NMR spectroscopy. The exchange occurs in crystalline samples with either Ph3PNPPh3+ (PPN+) or NEt4+ as the counterion. From the variation in line width as a function of temperature in the fast-exchange region the activation energy for axial-equatorial exchange in [HFe(CO)4] [NEt4] is estimated to be 7.0 ± 0.7 kcal mol-1 11. A comparison of coalescence temperatures suggests that the activation barrier is higher in the PPN+ salt. The crystal structure of the PPN+ is known and shows a pentacoordinate iron atom with a geometry that can be described as intermediate between a face-capped tetrahedron and a trigonal bipyramid. Although the structure for the NEt4+ salt is not known it is likely that the anion has essentially the same geometry determined in the PPN+ salt. A hydride tunnelling mechanism is considered to be most likely for the exchange of carbonyl ligands. In contrast the related molecule, Fe(CO)5, shows no evidence for rapid axial-equatorial exchange of carbonyl groups in the solid state. Previous breadline NMR results estimated the rate of axial-equatorial exchange in solid Fe(CO)5 to be 2.4 ⨯ 104 s-1 at-60 °C. However, magic angle spinning NMR results suggests that the rate of axial-equatorial exchange in solid Fe(CO)5 cannot be greater than 102 s-1. © 1990, American Chemical Society. All rights reserved.