Even ligand

More than twenty years ago, Nesmeyanov s group showed that chlorine can be substituted by a variety of nucleophiles in FeCp(r 6-PhCl)+ [83, 84]. Indeed the chlorine substituent in the chlorobenzene (even) ligand is 1000 times more reactive than when it is located on the cyclopentadienyl (odd) ligand [85]. The FeCp+ is a good withdrawing group which is equivalent to two nitro groups in terms of activation. The reactions proceed under ambient conditions with primary or secondary amines and have been extended to other substituted chloroarene complexes [86, 87] Eq. (22), Table 2. 

Charge-transfer excitations from odd ligand levels to the even metal ys and y>3 levels clearly represent formally Laporte-allowed u - -g transitions, and consequently should be intense. Ligand to metal transitions involving even ligand orbitals are of course also possible, but would be parity forbidden and are therefore rather seldom observed. For many of the ions here treated though the data are derived from reflectance measurements and the intensity criterion is of limited value because of the increase in the scattering coefficient which usually occurs above about 25 kK. [c.f. (7)]. 

The central point in this consideration is the Ln-OH moiety, the preferred formation of which is considered to be a dilemma in organolanthanide chemistry. Organolanthanide compounds containing Ln-X a-bonds such as alkyls, amides and alkoxides readily hydrolyze when exposed to moist air, with the formation of the hydroxides. Lanthanide complexes with Ln-C linkages are considered to be oversensitive compounds [89]. Even ligands with lower pKa values than water, as exemplified by substituted phenol ligands, tend to hydrolyze in organic solvents because the insoluble hydroxides formed act as... 

DGM and others have attempted to explain the basis for these rules. Rule 1 may be explained on the basis of the reactions being under charge control. Consider the orbitals of the n ligand once attached to the metal (Figure 8-6). For even ligands the HOMO is doubly occupied.46 If the metal is quite electron deficient,... 

DIOP, [8] achieved an enantiomeric excess of only 22% ee (Table 1) for the preparation of 5-Me. [9] Even ligands such as BINAP [8], or Chiraphos [8] have so far only shown a high selectivity for few substrates. [10] As a result, ligands were developed which can reach over to the exo side of the allylic complexes 2, for example the ferrocene 6 designed... 

Rule 1 takes precedence over rule 2 whenever they conflict. Polyenes or even ligands are simply ones having an even electron count on the covalent... 

In 8.10, we see that we can treat the alkene and the allyl parts of the bicy-clooctadienyl as independent entities the even alkene part is attacked. CO is an even ligand, but it among the least reactive of these, as shown in 8.11 and 8.13. The examples also illustrate what might be called the zeroth rule of nucleophilic addition a nucleophile usually adds once to a monocation, twice to a dication and so on. 8.12 and 8.13 also show the operation of rule... 

Even ligands bringing one or several electron pairs to the metal (e.g., ethylene). 

For instance, in the cationic molybdenum complex below containing the following ligands benzene (even, closed), butadiene (even, open) and allyl (odd, open), the nucleophilic attack will occur in exo on the terminal butadiene carbon. The first rule indicates that an even ligand (benzene or butadiene) should be preferably attacked the second rule says that the open butadiene ligand is attacked preferably to the closed benzene one, and the third rule states that a terminal butadiene carbon is preferred ... 

In the following rhodium complex, the fiTSt Tule does not apply in the absence of even ligand, the second rule gives preference to the cyclohexadienyl ligand, and the third rule to one of its terminal carbon atoms ... 

Bis-arene metal sandwich complexes are known for all the transition metals, and their structure and chemistry resembles that of metallocenes. However, the even ligands are less strongly bonded to metals than the odd Cp ligand, and their chemistry suffers from easy decomplexation and is less rich than that of metallocenes. 

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