Polyhapto ligands

Between the simple metal-main group atom interactions discussed above and clusters discussed below one finds systems in which the main group fragment can be considered a polyhapto ligand. Although the 

In the following, three comparisons of isoelectronic systems are presented. The first consists of a comparison of the isoelectronic ligands ethyne (HCCH) and phosphaethyne (HCP) for different transition metals and in different states of aggregation. The second compares cyclopenta-dienyl and pentadienyl complexes, while the third relates complexes of benzene and hexaphosphabenzene. Although narrow in terms of breadth of compounds, we hope there is sufficient detail in these systems to convey some appreciation of the systematic variation possible with isoelectronic substitution. 

While very few details have been published about the reactions of 34 (88,89), compound 35 is an effective hydrogenation catalyst for carbon-carbon unsaturated bonds, but is inactive toward N2. The coordinated alkyne appears to be an important integral part of the catalyst (90). 

Compound 35 does not form above 30°C when an aromatic solvent is used instead, two alkynes are dimerized to give a titanacyclopentadiene having the structure of l,l-bis(r 5-cyclopentadienyl)-2,3,4,5-tetraphenylzirconole (36) (Fig. 15). It is an important intermediate in alkyne oligomerization reactions on transition metal complexes. 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

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The stereochemical designations employed for organometallic complexes follow an extension of the CIP system2. In all cases, the chiral metal atom has a higher atomic number and thus receives priority. The main feature of this extension of the CIP system concerns treatment of polyhapto ligands such as cyclopentadienyl ... 

As will be shown in detail in the following sections, one or both of these kinds of chirality of the site can be present in the catalytic complexes. However, for the case of catalytic complexes in which two carbon polyhapto ligands are tightly connected through chemical bonds and which we shall call thereafter stereorigid, only the chirality of kind (ii) can change during the polymerization reaction. 

The most successful and widely used synthesis of higher substituted derivatives of M(CO)6 (M = Cr, Mo, W) is via the thermal displacement of mono- or polyhapto ligands from the appropriate carbonyl complex. Thus, from C7HgM(CO)4 (M = Cr, Mo, W C7H8 = norbornadiene, cyclohepta-... 

Many boranes and carboranes can act as very effective polyhapto ligands to form metallaboranes and metallacarboranes. Metallaboranes are borane cages containing one or more metal atoms in the skeletal framework. Metallacarboranes have both metal and carbon atoms in the cage skeleton. In contrast to the metallaboranes, syntheses of metallacarboranes via low- or room-temperature metal insertion into carborane anions in solution are more controllable, usually occurring at a well-defined C2BK open face to yield a single isomer. 

A large number of metallacarboranes and polyhedral metallaboranes of s-, p-, d-, and f-block elements are known. In these compounds, the carboranes and boranes act as polyhapto ligands. The domain of metallaboranes and metallacarboranes has grown enormously and engenders a rich structural chemistry. Some new advances in the chemistry of metallacarboranes of f-block elements are described below. 

It should be noted that there are organometallic compounds involving polyhapto ligands that contain linkages such as Sc— Te that are not found in coordination compounds. In addition, a number of recent reviews on the organometallic chemistry of the lanthanides include reference to scandium compounds. ... 

Scheme 7.2, which describes another case where an Id process may occur, involves substitution on complexes containing a polyhapto ligand, such as a diene or triene. Application of the steady-state approximation with respect to the two intermediates, A and B, gives a rate law of the following form ... 

A specific example of such a reaction, using 1,5-cyclooctadiene (cod) as the polyhapto ligand, is shown in equation 7.20.43... 

Substitution Involving a Substrate with a Polyhapto Ligand... 

There are also complexes containing polyhapto ligands with a more extended TT-system than that in T -allyl or -benzyl groups. Pentadienyl systems are the next higher anionic homolog of an allyl system. Pentadienyl systems are often formed by the addition of protons to, or removal of hydrides from, transition metal polyene complexes. Alternatively, pentadienyl groups have been generated by the addition of... 

Substitution for polyhapto ligands occur in many cases by mechanisms that are similar to those for replacement of monodentate ligands. Thus, substitutions for polyhapto ligands in 18-electron octahedral complexes, like substitutions for CO in the 18-electron octahedral complexes presented in Section 5.4.2.1, often occur by competing dissociative and associative pathways with a two-term rate expression, = k + k ... 

Insertions of Polyhapto Ligands into Metal-Ligand Covalent Bonds... 

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