Compounds with Bond Order Three

As mentioned above, species with a metal-metal triple bond may be achieved by both addition or substraction of electrons in species with an electron configuration (T n S. In both bond order three electron configurations and 

As observed in Table 1.8 most examples of compounds with electron configuration arise from rhenium chemistry. The most important group of 

Compounds of type M2L6 (M = Mo or W L = N2, OR) with structures as those illustrated in Fig. 1.40 possess a triple metal-metal bond with a 

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The boranes are electron-deficient compounds (Section 3.8) we cannot write valid Lewis structures for them, because too few electrons are available. For instance, there are 8 atoms in diborane, so we need at least 7 bonds however, there are only 12 valence electrons, and so we can form at most 6 electron-pair bonds. In molecular orbital theory, these electron pairs are regarded as delocalized over the entire molecule, and their bonding power is shared by several atoms. In diborane, for instance, a single electron pair is delocalized over a B—H—B unit. It binds all three atoms together with bond order of 4 for each of the B—H bridging bonds. The molecule has two such bridging three-center bonds (9). 

A comprehensive list of P-gp modulators or inhibitors, classified according to their chemical structures, has been published recently [87]. This shows that the structures of inhibitors are almost as heterogeneous as those of the substrates. A small but representative selection of inhibitors is shown in Fig. 20.12 and Table 20.1. In an attempt to clarify the different mechanisms of P-gp modulation or inhibition, the H-bonding concept discussed above is applied. To this end, the modulators or inhibitors in Table 20.1 were ordered according to their H-bond acceptor potential and divided in three groups comprising compounds with (i) a low EUh (<2 i.e., not transported) (ii) an intermediate EUh (— 3—6) and (iii) a high H-bond acceptor potential ( EUh > 10 i.e., transported slowly). 

The I-I bond distances are varying from 2.79 A, in case of complexes with weak I-S interaction, to 3.08 A, as a result of a strong 1-S interaction. The corresponding I-I bond is subseqnently elongated with respect to the corresponding distance in free I-I in the solid state (2.717 A at 110 K). Bigoli et al. [10] has classified iodine adducts of sulfur donors into three classes, depending on 1-1 bond order (n). When n > 0.6 and d(I-I) < 2.85 A the adduct is type A and when n < 0.4 and d(l-l) > 3.01 A it is type C. Componnds with intermediate values were classified as type B. Thus, compounds (18) and (20) are classified to A type, compounds (19), (21) and (22) to B type, whereas componnd (17) is of C type. 

The reaction of [WCl4(MeS)2] with Et2SiH gives compound (22) from which the anion (23) can be prepared.285 The structure of (23) possesses the W(jU-H)(/i-SMe2)2W bridge with a W—W distance of 2.410 A, consistent with a W—W bond of order three. 

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