THE NOVEL STRUCTURE OF THE [AU-11(PMEPH(2))(10)](3- CRYSTAL-STRUCTURES OF [AU-11(PMEPH2)(10)] [C2B9H12](3)CENTER-DOT-4THF AND [AU-11(PMEPH(2))(10)] [C2B9H12](3) (THF=TETRAHYDROFURAN)() CATION )

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 approxi mate to a centred bicapped square antiprism, with idealized D-4d symme try. Symmetry-related cage distances and angles have similar mean valu es 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 differenc es are directly related to the ligand packing around the metal skeleto ns. The cations of the two clusters cannot be superimposed in any orie ntation. It is possible to relate a centred bicapped square antiprism to the previously reported undecagold cage geometry, although they bel ong to different symmetry point groups. The largest differences betwee n the idealized C-3v and D-4d frameworks centre around three adjacent peripheral sites. The movements required to interconvert the geometrie s take place about a common mirror plane and appear to be closely rela ted to those of the diamond-square-diamond rearrangement mechanism. Fl uxional interconversions of this type provide a possible explanation f or the P-31-{H-1} NMR spectra of the Au-11 cluster compounds.