Stereochemical Non-rigidity of Clusters in Ligand Shells

Stereochemical non-rigidity has been observed in all the various types of clusters described in the following sections and has been reported for the ligand shell, the metal core, or both. Furthermore, this phenomenon is not limited to clusters in solution but, with some obvious differences, can also be present in the solid state. This behavior has mostly been observed from multinuclear NMR 

A different case is represented by X-ray diffraction studies. Although the frequencies of the X rays employed in these studies imply a time-scale of ca. 10 s, it must be realized that this value has no meaning in a diffraction experiment because the measurement of a single reflection may take up to several minutes. As a result, several types of molecular motions may take place during this time and can include small and large amplitude oscillations of the metal core with respect to the ligand shell, or vice versa. The relevant point then becomes how many of these occur during a measurement. [72] 

Figare 3-2. Proposed mechanism for the carbonyl intra-exchange pro-In thp 

In contrast, intermolecular carbonyl exchange processes seem to play only a secondary role and just a few examples, which take place through either dissociative or associative mechanisms, have been reported. [70] 

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