The fluxionality and crystal structures of [Fe-3(CO)(10)L-2] (L-2 = dppm, (Ph2P)(2)NH, {(EtO)(2)P}(2)O) and [Fe-3(CO)(9)(dppm){P(OMe)(3)}]

The crystal structures of [Fe-3(CO)(10)L-2] (L-2 = dppm, (Ph2P)(2)NH, {(EtO )(2)P}(2)O) and [Fe-3(CO)(9)(dppm){P(OMe)(3)}] (dppm = (Ph2P)(2)CH2) have b een determined. [Fe-3(CO)(10)L-2] (L-2 = dppm, (Ph2P)(2)NH, {(EtO)(2)P}(2)O ) adopt a stereochemistry in the crystal with the bidentate ligand across a n unbridged Fe-Fe edge, resulting in one phosphorus ligand being unusually in the axial position. In contrast, [Fe-3(CO)(9)(dppm){P(OMe)(3)}] adopts a stereochemistry in the crystal with the bidentate ligand across the bridge d Fe-Fe edge. The structures of the [Fe-3(CO)(10)L-2] (L-2 = dppm, (Ph2P)(2 )NH, {(EtO)(2)P}(2)O) support the Mann concerted bridge - opening bridge - closing mechanism of carbonyl fluxionality rather than the Johnson libratio n mechanism. The fluxionality of the compounds in the C-13 NMR spectra is e xplained by a combination of the concerted bridge - opening bridge - closin g, merry-go-round, and trigonal twist mechanisms, and DeltaG(double dagger) values were determined.