Polyhapto ligands complexes

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-... 

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 ... 

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 ... 

Stanley, K. Baird, M. C. Demonstration of controlled asymmetric induction in organoiron chemistry. Suggestions concerning the specification of chirality in pseudotetrahedral metal complexes containing polyhapto ligands. J. Am. Chem. Soc. 1975, 97, 6598-6599. 

The nucleophilic attack on a polyhapto ligand occurs with a decrease of the hapticity by one unit in the reaction product. In addition, three rules established by Davies, Mingos and Green appear, when the reaction is under charge control, to determine the regioselectivity of the attack, if several hydrocarbon ligands are present in the same complex or if several possible sites are available for the nucleophilic attack ... 

To most chemists the term n complex is connected with compounds in which it ligands are polyhapto bonded to a transition metal. Indeed, this class of compounds is highly important and has experienced fantastic development during the last three decades. On the other hand, polyhapto bonding of n liagnds is not restricted to transition elements. Main-group elements can also serve as central atoms, as documented by many relatively recent examples. 

Equation (65) is an example of a process that may be readily promoted by a cluster but not necessarily with a mononuclear complex. Scheme 8 shows how the multiple-coordination capabilities of a trinuclear cluster could reduce the N—C bond and eventually cleave it. To date, examples of the NCO ligand bridging more than two metals in either a mono- or polyhapto fashion are unknown. The reverse process represents a unique method for forming carbon-nitrogen bonds. Although the above chemis-... 

Stable thiocarbonyl complexes containing the elusive CS ligand are also now well established and known coordination modes, which include terminal, bridging and polyhapto, are as shown at the top of the next column. ... 

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