Staggered configurations

Polynuclear Carbonyls. Several stmctures consist of dinuclear metal carbonyls as shown in stmctures (4)—(6). The metal atoms in Mn2(CO) Q, as also for technetium and rhenium, are held together by a metal—metal bond and the compound contains 10 terminal CO ligands, five coordinated to each atom. The CO ligands of Mn2(00) 0 adopt a staggered configuration as illustrated in stmcture... 

In the solid state sulfamic acid forms a strongly H-bonded network which is best described in terms of zwitterion units +H3NSO3" rather than the more obvious formulation as aminosulfuric acid, H2NS02(0H). The zwitterion has the staggered configuration shown in Fig. 15.50a and the S-N distance is notably longer than in the sulfamate ion or sulfamide. 

Figure 19.29 The structure of [Cr( ) -C6H6)(CO)3] showing the three CO groups in staggered configuration with respect to the benzene ring the Cr-O distance is 295 pm and the plane of the 3 O atoms is parallel to the plane of the ring.

In solution the compounds exhibit solvent-dependent colours in dilute solution in non-polar solvents, planar monomers are present but in more concentrated solutions oligomerization occurs. In the solid state a dimeric structure has been identified (X-ray, Figure 2.30) with R = Ph there is a staggered configuration (Rh-Rh 3.193 A) but with other isocyanides (R = 4-FC6H4) the configuration is eclipsed. 

The methylisocyanide complex has a dimeric structure with a direct metal-metal bond (2.531 A) and only terminal isocyanides, in a staggered configuration (Figure 3.22). 

It is hard to make a prediction about hydrazine, although the staggered configuration with the unshared electron pairs in the trans position might be expected from the foregoing considerations. The configuration has not yet been determined for the gas molecule the ar-... 

Perhaps this can best be emphasized by taking an example. Let us consider ethane in its staggered configuration. The C—C line defines a C3 axis, but certainly not a C6 axis. Yet there is an Sbi as the diagram shows. Observe that II and III are equivalent to each other but that neither is equivalent to I that is, neither a nor C is by itself a symmetry operation. But the combination of both, in either order, which we call S6, is a symmetry operation since it produces IV, which is equivalent to I. 

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... 

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