Rhenium compounds with quadruple bonds

Compounds 4-7 (Table 1) belong to this group. The physicochemical properties of the given binuclear clusters are close to those of the analogous rhenium and molybdenum compounds with the quadruple M-M bonds and are as a rule isostructural with them [1,89,90]. 

The existence of quadruple bonds was first recognized in 1964289 and since then a great many compounds of rhenium, technetium, chromium and molybdenum have been shown to contain them. Both bridged and unbridged compounds are known with a large variety of ligands. The bonding has been described in detail.289... 

The most interesting aspect of these compounds is their structure (Fig. 16.57), which possesses two unusual features. The first is the extremely short Re—Re distance of 224 pm compared with an average Re—Re distance of 275 pm in rhenium metal and 248 pm in Re,Cl. The second unexpected feature is the eclipsed configuration of the chlorine atoms. One might have supposed that since the short Re—Re bond requires that the chlorine atoms lie at distances ( 330 pm) which are less than the sum of their van der Waals radii (—340-360 ppm), the staggered configuration would be preferred (the chlorine atoms would then form a square antiprism rather than a cube). Cotton explained both phenomena by invoking a quadruple bond.147... 

As already mentioned in the Introduction, Cotton [1,2] stated that the two Re atoms are bonded through a quadruple bond in Re2Clg . The short rhenium-rhenium bond between two ReCl4" moieties was described as one a, one degenerate n, and one 8 bond. This work inspired experimental research that brought about the discovery of homo- and hetero-dinuclear compounds with metal-metal quadruple bonds [16]. 

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