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

A total of 11 crystal structures, in six different space groups of the compounds M₂(hpp)₄Cl₂ (hpp=the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-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 4·4CH₂Cl₂ and the structure has been refined in space group I4/m instead of space group I4/mmm as reported for 2·4CH₂Cl₂ and 5·4CH₂Cl₂. Compound 6 crystallizes in tetragonal space group P4/n with the molecule residing on a crystallographic C₄ symmetry position. Idealized D₄ 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.