Multiply-bonded species

We can generalize from these examples to the description of a multiply bonded species according to valence-bond theory ... 

As in main-group chemistry, hypovalent hydrides of the transition series have pronounced tendencies to form bridging tau bonds. In addition to H4TaTaH4, the multiply bonded species HTaTaH features two symmetrically bridging hydrides. Despite the complexity introduced by such tau bonds, the Ta-Ta interaction clearly has high bond order, as simple Lewis-like structures prescribe. 

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

The tandem radical cyclization of tetrayne (97) and its derivatives has been performed to generate the polycyclic pyran (98) via a biradical intermediate.238 The cycloaddition reaction of a biradical species (or diyl) and a multiply bonded species (the diylophile)239 has been observed with unique allene diylophiles.240 The short-lived biradical fonned by the irradiation of the diazene (99) is trapped by an allene diester to form a second biradical species (100). Intramolecular cyclization occurs such that all steric interactions are minimized and so enforces stereocontrol in the formation of the cycloadduct (101) see Scheme 14. A paper reports the rearrangement of 2-vinyhnethylenecyclopropane (102) to 3-methylcyclopentene (103) via the triplet biradical (104), which has been characterized for the first tune by IR spectroscopy.241... 

The reactivity of SiN multiply bonded species has in part been discussed in Section IV. B, dedicated to synthesis, because the existence of reactive and short-lived species is often proven by trapping experiments. This part of the review therefore focusses on more comprehensive reactivity studies of Si=N systems. 

G. GeN, SiP and SiAs Multiply Bonded Species and Related Compounds... 

This chapter outlines the recent progress in the chemistry of multiply bonded species of Ge, Sn, and Pb. As can be concluded from this chapter and related recent reviews which deal with the chemistry of this family of compounds which are stable at room temperature, the field of low-coordinated species of heavier group 14 elements has matured considerably in recent years. 

The mechanism of bond-shift isomerization of alkanes is still a subject of controversy. Several suggested mechanisms involved an intermediacy of multiply bonded species [Anderson-Avery (63), Muller-Gault (64), and Garin-Gault (65) mechanisms] (Fig. 2). More recent data (51, 52, 66-68) seem to support the 1,2 bond-shift isomerization involving monoadsorbed alkyl species (69). Figure 2 shows these mechanistic ideas. 

A second type of metathesis reaction involves metals multiply bonded to nonmetals (equation 68). In one sense, this corresponds to a special case of dinuclear reductive elimination of neutral molecules in which multiply bonded species are involved. This reaction can occur in both directions. Multiply bonded metal-metal complexes can be cleaved by multiply bonded main group species. 

For metal-metal bonded complexes that contain multiple bonds, addition of hgands can reduce the bond order but does not have to destroy the metal metal interaction entirely. The reactions shown below are written in the forward direction, which highhghts addition reactions leading to a loss in multiple-bond character. It should be kept in mind that these are often in equilibria, and hgand elimination in the reverse direction can be a source of the multiply bonded species. 

By analogy to other multiply bonded species such as 02, dissociation of multiply bonded species can be defined by equation 4. 

In this section we will review the computational studies on reactions of silylenes, mainly addition and insertion reactions. Some reactions, in particular the following isomeriz-ations of silylenes to the corresponding multiply bonded species, were discussed above (a) to silaethylene in Section V.A.l.a.v, (b) to substituted silenes in Section V.A.l.b.iv, (c) to disilenes in Sections V.A.2.d and f, (d) to silanimines in Section Y.A.3, (e) to silanephos-phimines in Section V.A.4, (f) to silanones in Section V.A.5, (g) to silanethiones in Section V.A.6, (h) to silynes in Section V.B.l, (i) to disilynes in Section V.B.2, (j) to aromatic compounds in Section VI.A and to antiaromatic compounds in Section VI.D. 

Most of the simple silylenes of the type RR Si were discussed in this section and at the beginning of Section VII. A.3, where citations were given to isomerization of such silylenes to multiply bonded species. In Table 36 and Figure 56 we list all the silylenes not mentioned in Section VILA, and refer the reader to other sections in which these silylenes are discussed. For silylenes which were not mentioned in earlier sections, references to the original literature are given. 

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