Data Collection and Computer Treatment

The crystals were transferred to a CAD4-Enraf Nonius PDP8/M computer-controlled diffractometer. In each case, 25 reflections were used in order to settle the crystals, to check their quality and to refine the cell dimensions. 

The conditions for the data collections are summarized in ref. The intensities of selected reflections [I 2cr(I)] were corrected for Lorentz and polarisation factors but not for absorption. 

Atomic scattering factors corrected for anomalous dispersions were obtained from Cromer and Waber and Cromer and Liberman 

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