Nb-X bonds

The broken bonds (boldface=dissociated atom) BDEs (boldface = recommended data reference in parentheses) Methods (reference in parentheses)  

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The X-ray structure of 83 was in accord with the prediction of Scheme 4 in that the Si-Cl bond lies in the niobocene bisecting plane trans to the hydride and is elongated compared with classical complexes of the type [M(SiR2Cl)L ]. There was, however, no good reference system that would allow for the comparison of the Nb-Si bond lengths. The metal-silyl bond lengths are strongly affected by Bent s... 

Fig. 10. Variation of the Nb-Si bond length in 87 with X. (Reproduced from Ref 174, with permission...

The data for these compounds can be found in Tables 27 and 28 173). The frequencies for TI3MX4, where M is V, Nb or Ta and X is S or Se, can readily be taken from Table 22. In every case the chalcogen-metal vibrations are markedly decreased. The compounds such as CU3MX4 where M is V, Nb or Ta and X is S or Se form a three dimensional crosslinked network due to strong Cu—X bonding. The coppermetaUates crystallize in the sulvanite form. 

The controlled hydrolysis of MC15 or [Ta(OR)5]2 (R = Et, SiMe3) led to polymeric oxo alkoxides. On the other hand the hydrolysis of [NbCl2(OR)3]2 in the presence of bipyridyl led to [NbOCl2(OR)(bipy)] (R = Et, Pr1) adducts. 13 >364>412 An X-ray structure for R = Et showed a distorted coordination octahedron, with the two chlorine atoms trans to each other (Nb=Or 1.71(3) A). The Nb—OR bond is only 0.16 A longer than the Nb=0 oxo bond, and was presumed to have considerable double bond character, which is also consistent with the very open NbOC bond angle of 149°. 

As an illustration of the phenomena involved, consider the photocycloaddition of fumaronitrile to 5-X adamantanone, 6 (Scheme 2 X = F, Cl, Br, OH, Ph, or r-Bu) [94], In isotropic solvents, different quantities of adducts to the two carbonyl faces are formed. When 6 is complexed by /J-CD, the intrinsically more reactive face of the carbonyl group becomes more hindered toward attack by fumaronitrile than the less reactive one. As a result of this attractive interaction (NB, hydrogen bonding between the carbonyl oxygen and a hydroxyl on the fl-CD torus), the distribution of photoadducts is reversed. In this example, fl-CD serves the function of a reaction cavity with active walls (i.e., a template). 

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