Metallaborane chemistry. Part VI. Molecular and crystal structures of the closo-carbaplatinaoctaboranes 6,8-dimethyl-1,1-bis(trimethylphosphine)- and 1,1-bis(trimethylphosphine)-6,8-dicarba-1-platinaoctaborane

The molecular structure of the title compounds have been determined by single-crystal X-ray diffraction. The C-methylated metallacarbaborane exhibits two crystalline modifications, an A2/a (α) and a P2₁/c (β) form. The latter crystallises in a cell of dimensions a = 11.892(4), b = 9.280(4), and c = 18.724(8) Å, β = 106.17(3)°, and its structure was determined by the heavy-atom method and refined to R 0.044 for 2 487 independent observed reflections. Both compounds exhibit C₂ molecular symmetry and have close-polyhedral cages with geometries approximating to those of tricapped trigonal prisms, whose low connectivity 'cap' positions are occupied by the three heteroatoms. Some experimental data are given for (I)-α and (II): the gross features of their molecular geometries were determined but their structures were not refined.