Triazenido complexes

The inner coordination spheres of these two complexes are presented in Figure 1. While the Ru-H distance is well defined in the amidinato, it is not in the triazenido complex. Yet reference to Table I indicates that the triazenido structure is better if one uses the unreliable criterion that the lower the R index, the better the structure. In this instance, the low R index in the triazenido complex results from an elaboration of the usual group refinement model (3), allowing for anisotropic motion of the group atoms. This elaboration introduces a large number of additional variables and provides us with an opportunity to lower the R index More importantly, we established that the other features of the structure were virtually unaffected by this elaboration. We conclude that with problems of this type, such an elaboration probably is not justified by the expense involved. Why can t the hydride position be located accurately in the... 

In the triazenido complex, there is a slight trans labilizing effect of the hydride over the carbonyl, as reflected in the trans Ru-N distances. This effect is much more pronounced in the amidinato complex because of the absence of disorder and perhaps because the four-membered ring has opened up so that atom N(2) is more nearly trans to the hydride ligand. 

These two structures illustrate a fundamental and disturbing point about diffraction experiments. It is not until the late stages of refinement, after considerable time and money has been spent on the experiment, that one sometimes discovers his inability to define accurately a salient feature of the structure—in this instance the hydride position in the triazenido complex. There is no way from the formula, space group, or films to have predicted this nor are there any usefully consistent methods that enable one to predict, especially in common low-symmetry space groups, when disorder will occur. 

A very convenient general synthesis for an extensive range of metal triazenido-complexes has been described (see p. 412). 

A very convenient general synthesis for an extensive range of metal triazenido-complexes has been described.154 The complexes, which contain unidentate or chelated 1,3-diaryltriazene, were isolated as highly coloured, air-stable crystalline products. The new complexes cis- and trans-[M(ArNNNAr)2(PPh3)2] (M = Pd or Pt), [Pt(ArNNAr)2 Ph2P(CH2) PPh2 ] (n = 2- ), [PtCl(ArNNNAr)(PR3)2] (R = Et or Ph) were characterized from i.r. and n.m.r. spectra. 

Unlike all the other complexes described in this subsection the bis di(/)-tolyl)triazenido complex (137) contains six-coordinate rhodium. It can be prepared from the reaction between the triazene and [Rh(N0)Cl(N02)(PPh3)2] (equation 312). The single methyl resonance at t7.80 in its proton NMR spectrum is the reason for assigning it the symmetric structure.231... 

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