Even-numbered carbon ligands

This is the order based on three features. First, even-numbered carbon ligands react faster than odd-numbered carbon ligands, as exemphhed by the first and second reactions of Scheme 8.2. 

Independent of the chain length, the substituents R and the metal-ligand fragment L M the LUMO in d and d complexes is predominantly localized on the odd carbon atoms. Therefore, nucleophiles are expected to add to the odd carbon atoms in the chain.The HOMO in d and d complexes has contributions mainly from the metal and the carbon atoms in even positions of the chain. As a consequence, electrophiles are expected to add either to the metal or to even-numbered carbon atoms. Deviating from these more general results, the HOMO in [(CO)5Cr=(C)4H2] has been calculated to be more evenly distributed along the chain with major components from the odd-numbered carbon atoms Cl and C3 [10-13]. 

As already stated, DIOP led the way for a number of ligand systems that were built on a carbon framework containing stereogenic centers. Some of these ligands followed closely on the heels of DIOP, such as ChiraPhos (16) where the chelate ring is five-membered [82, 83]. Even one stereogenic center in the backbone, as in ProPhos (17), provides reasonable selectivity [83, 84]. The main problem with these systems is that of slow reactions. 

In complexes vith an even number of carbon atoms in the chain the HOMO is oriented perpendicularly to the plane of the cumulenylidene ligand and the LUMO is an in-plane orbital. Conversely, in complexes with an odd number of carbon atoms the HOMO is an in-plane orbital and the LUMO is perpendicular to the plane [10, 12[. 

The group of monocyclopentadienyl metal carbonyls is the most extensive. In this, in contrast to the pure cyclopentadienyl compounds, it is found that complexes are formed only by transition elements of groups 5 to 8, i.e., metals which can enter into true ferrocene-type bonding. There is a regular relationship for this series transition elements of odd atomic number yield readily volatile compounds of formula CsHsMefCO) , while the even-numbered members form dimers of composition [CsH6Me(CO)ic]2. The actual value of x, the number of ligand carbon monoxide molecules, corresponds to the number of pairs of elec-... 

For polyhydride complexes, photolysis can result in the formation of complexes that still have a single hydride ligand complexed, or, in complexes that lose all of the hydride ligands from the metal in a pairwise fashion. The pathway that is followed largely depends on whether the precursor complex has an odd or an even number of hydride ligands. An example of the former reaction is the photoconversion of ReH3(dppe)2 into ReH(CO)(dppe)2 in the presence of carbon monoxide ... 

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