Resolving conformational ambiguities in M2(hpp)4Cl2 paddlewheel compounds:: M = Mo, W, Re, Ru, Os, Ir, Pd, Pt

A total of I I crystal structures, in six different space groups of the compounds M-2(hpp)(4)Cl-2 (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[l 2-a]pyrimidine) with M = Mo (1), W (2), Re (3), Ru (4), Os (5), Ir (6), Pd (7), and Pt (8) have been studied with a view to seeing that ambiguities caused by disorder and difficulties in distinguishing between similar space groups are correctly resolved. The importance of this goes beyond mere crystallographic pedantry because the existence (or not) of internal torsion, the correct angle when torsion does exist, and the accuracy of bond lengths depend on correct refinement. Compounds 1-4 are isomorphous in terms of crystal space group symmetry (I4/m) and unit cell dimensions, but the molecular structure of 4 is different from the others. The internal M2N8 cores in 1, 2 and 3 are eclipsed whereas the Ru2N8 core in 4 is twisted. The twisted configuration in 4 has also lead to merohedral twinning for the dichloromethane solvate 4.4CH(2)Cl(2) and the structure has been refined in space group I4/m instead of space group I4/mmm as reported for 2-4CH(2)Cl(2) and 5.4CH(2)Cl(2). Compound 6 crystallizes in tetragonal space group P4/n with the molecule residing on a crystallographic C-4 symmetry position. Idealized D-4 symmetry is found for 7, which crystallizes in tetragonal space group P4/nnc. It has been found that 8 can be better refined in space group I4/m instead of the published space group I422. (C) 2003 Elsevier Science B.V. All rights reserved.