Empirical and numerical (Gaussian) absorption corrections, determined by face indexing and integration were applied to the data for (I), (II), (III) and (IV). The structures were solved by applying the intrinsic phasing in SHELXT and refined by full-matrix least-squares techniques against F 2 (SHELXTL) (REF X2,X3). Anisotropic thermal factors were applied for all atoms except for the hydrogen atoms. H atoms were placed in idealized positions and refined using a riding model. The structural and refinement parameters can be found in Table and all the figures were generated using ORTEP. Numbers in parentheses are the errors in the least significant digits.