Modes of bonding

The Irihalides of A1 form a large number of addition compounds or complexes and these have been extensively studied because of their importance in understanding the nature of Friedel-Crafts catalysis. The adducts vary enormously in stability from weak interactions to very stable complexes, and they also vary widely in their mode of bonding, structure and... 

With such structural diversity it is perhaps not surprising that no certain method has been devised for theoretically predicting the mode of bonding to be expected in specific cases, although... 

Such Hgands may also coordinate to the surface of nanoparticles and strongly influence their chemistry, it is therefore important to gain information on their presence and mode of bonding. 

The bond strength to enamel (2-6 to 9-9 MPa) is greater than that to dentine (1-5 to 4-5 MPa) (Wilson McLean, 1988). Bond strength develops rapidly and is complete within 15 minutes according to van Zeghbroeck (1989). The cement must penetrate the acquired pellicle (a thin mucous deposit adherent to all surfaces of the tooth) and also bond to debris of calciferous tooth and the smear layer present after drilling. Whatever the exact mode of bonding to tooth stmcture, the adhesion is permanent. The principles and mechanism of adhesion have already been discussed in Section 5.2. 

XH NMR also provides evidence that the predominant mode of. bond cleavage is at the bond opposite to the backbone carbon in the polymer. If the bond adjacent to the backbone carbon had been broken this... 

The mode of bond cleavage is extremely sensitive to alkyl substitution/73 75 ... 

This review is intended as an account of the coordination chemistry of both dimethyl sulfoxide and the higher sulfoxides, with particular reference to the mode of bonding and the extent to which this affects the chemistry of transition-metal sulfoxide complexes. No attempt has been made to provide an exhaustive listing of all known sulfoxide complexes, many of which contain coordinated sulfoxide moieties only coincidentally to their other important functions. 

In all the above examples, the carbonyl group donates two electrons to the metal cluster unit. Four-electron donation by the carbonyl group has recently been observed although this appears to be a much less frequent mode of bonding in cluster systems, it has often been invoked to explain the properties of absorbed carbon monoxide on a metal surface. 

The various modes of bonding that have been observed for alkenes to the trinuclear osmium clusters are shown in Fig. 7 [see (88)]. The simple 77-bonded structure (a) is relatively unstable and readily converts to (c) the vinyl intermediate (b) is obtained by interaction of alkene with H2Os3(CO)10 and also readily converts to (c) on warming. Direct reaction of ethylene with Os3(CO)12 produces (c), which is considered to be formed via the sequence (a) — (b) — (c) and (d). Both isomers (c) and (d) are observed and involve metal-hydrogen and metal-carbon bond formation at the expense of carbon-hydrogen bonds. In the reaction of Os3(CO)12 with C2H4, the complex 112088(00)902112, (c), is formed in preference to (d). Acyclic internal olefins also react with the carbonyl, with isomerization, to yield a structure related to (c). Structure (c) is... 

The variety of these products and the complexity of the structural forms observed emphasize the difficulties in elucidating the structures of these compounds by methods other than X-ray analysis. The bonding of the alkyne fragment in these complexes involves major changes in the carbon-carbon distance, as well as deviation of the molecule from linearity. Table VIII (57,100,113,118-128) includes some of the C- C distances for various modes of bonding. 

Os3(CO)i2, nine products were obtained. Three of these were identified as the substituted adducts Os3(CO)12(Ph3P)i2- Gc = 1, 2, or 3), but for others, the fission of C-H and C-P bonds had occurred to yield a variety of compounds in which there were radical variations in the structure and mode of bonding of the triphenylphosphine group. Figure 29 shows details of the structures of six of these complexes. As indicated above, significant ligand rearrangements have taken place. 

Table 2 The reaction off Pf.Pf.Pf) with iron carbonyl compounds Reactant Product Mode of bonding...

The question of the mode of bonding of the cyclopentadienyl rings in complexes of the type U(C5Hs)sX was settled by the structural determination of tris(ben-zylcyclopentadienide)uranium(IV) chloride (69). The structure as viewed down... 

The allyl group is able to form both a- and jr-bonded complexes with the actinides. The 71 complexes will be considered here because of the similarities of the homoal-lyls with the lanthanide and actinide homoalkyls. The limiting modes of bonding in metal allyl complexes and the ratio of PMR intensities from magnetically equi valent protons are illustrated in Fig. 14. 

The tetraallylic complexes of thorium and uranium can be isolated as red sohds and are stable to 0° and — 20°C, respectively, whereas the uranium homoalkyls defy isolation at temperatures as low as — 80°C. The greater stability of the thorium complex has been attributed partially to the great instability of Th(III) as compared with U(III) (96). The tc mode of bonding and the resultant increase in coordination number to eight of the allyl complexes significantly increases their thermal stability relative to the uranium homoalkyls. 

Determination of the mode of bonding of the allylic intermediate formed would provide direct evidence that an M-O-C species produces acrolein. This is difficult because formation of the allylic intermediate is rate-determining and its subsequent reaction is fast. Thus, the surface concentration of the intermediate is small and not amenable to standard spectroscopic observation. 

Cross sections for desorption of adsorbed species are very sensitive to the mode of bonding. In general, weakly bound adsorbates have a much higher cross section than adsorbates which are much more strongly bound. 

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