Imido complexes bonding modes

The bonding mode is dependent on the nature of the metal centre as well as the steric or electronic properties of the imido substituent. For example, the M(CO)5 unit in the (AO-trigonal complex [W(CO)5 S(NMe)2 ] undergoes a 1,3-shift between the two nitrogen donors, whereas the tert-butyl forms a N,N -chelated [W(CO)4 S(NtBu)2 ] complex, as is also observed with main-group metal halides, e.g., [SnCl4 E(NtBu)2 ] (E=S, Se).153... 

The nudeophile is activated by the formation of a titanium(IV)-imido complex 19. The next step is a [2 + 2] cydoaddition with one of the jt-bonds of the allene, depending on the regioselectivity leading to either 20 or 22. Compound 20 then delivers 21 by twofold stepwise proto-demetallation and the latter enamine tau-tomerizes to the imine 24 (Scheme 15.3). Compound 22, on the other hand, should provide allylamines 23, but as we shall see, there are no examples of that mode of reaction known so far. 

The different modes of activation for the different organoactinides are indeed very unusual. For both organoactinide-imido complexes, a selective metathesis with the alkyne 7i-bond is operative (yielding the hydroamination products), whereas for the thorium complex a competing protonolysis reaction also takes... 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

A 2000 study from Buffon [69] examined the reaction of Schrock-type, alkoxy-Mo-alkylidenes with the surface OH groups of silica, silica-alumina, and niobium oxide. The specific mode of attachment of the Mo-complex was found to be highly dependent upon the acid-base properties of the support. For silica, it appears to be a Lewis acid-base interaction between the Mo center and surface silanols, whereas in the case of silica-alumina, the attachment appears to occur via the addition of a surface OH group across the Mo-imido bond. In the case of niobium oxide, it is possible that the OH adds across the Mo-alkylidene, deactivating the complex, as the resulting material was completely inactive for metathesis. The activities of both immobilized catalysts were less than the parent homogeneous... 

As stated by Gibson, the most striking feature in the structure of the Bis(imido) chromium(VI) precursor is the manner in which the two benzyl ligands are bonded to the chromium atom through and r coordination modes. Figure 3.30 shows the two cationic complexes prepared from the precursor complex to form ethylene polymerization catalysts. 

---