The novel structure of the [Au-11(PMePh(2))(10)](3+) cation: Crystal structures of [Au-11(PMePh2)(10)] [C2B9H12](3)center dot 4thf and [Au-11(PMePh(2))(10)] [C2B9H12](3) (thf=tetrahydrofuran)

Crystal structure analyses of [Au-11(PMePh(2))(10)][C2B9H12](3) . 4thf (thf = tetrahydrofuran) and [Au-11(PMePh(2))(10)][C2B9H12](3) showed the metal framework of the [Au-11(PMePh(2))(10)](3+) cation to approximate to a centred bicapped square antiprism, with idealized D-4d symmetry. Symmetry-related cage distances and angles have similar mean values but different ranges in the two structures, with the latter having greater consistency in the peripheral bond lengths but more distortion in the squares of the antiprism. It is suggested that these differences are directly related to the ligand packing around the metal skeletons. The cations of the two clusters cannot be superimposed in any orientation. It is possible to relate a centred bicapped square antiprism to the previously reported undecagold cage geometry, although they belong to different symmetry point groups. The largest differences between the idealized C-3v and D-4d frameworks centre around three adjacent peripheral sites. The movements required to interconvert the geometries take place about a common mirror plane and appear to be closely related to those of the diamond-square-diamond rearrangement mechanism. Fluxional interconversions of this type provide a possible explanation for the P-31-{H-1} NMR spectra of the Au-11 cluster compounds.