Allyl ligand molecular orbitals

The arrangement of the molecular orbitals of the allyl species will be useful when discussing the bonding of this ligand in metal complexes (Chapters 16 and 21). 

Curtis and Eisenstein355 have made a molecular orbital analysis of the regioselectivity of the addition of nucleophiles to 77-allyl complexes and on the conformation of the 773-allyl ligand in [MoX(CO)2L2(773-allyl)] type complexes. A detailed study of the chirality retention in rearrangements of complexes of the type [MX(CO)2(dppe)(rj3-C3H5)] has been made.356 Studies of the photoelectron spectra,357 electrochemical properties,358 infrared spectroelectrochemistry,359 and fast atom bombardment mass spec-... 

A series of DFT calculations see Molecular Orbital Theory) on Rh() -C3H5)3 indicate that the ground-state structure has no symmetry. Calculated ionization energies agree well with values obtained from photoelectron spectra. The calculated potential-energy surface indicates the presence of three transition states, one of which involves an n] -allyl ligand between the several minima that are found, and variable-temperature NMR measurements appear consistent with there being three distinct fluxional processes see Stability Constants their Determination) ... 

Allyllithium. The highest occupied allyllithium molecular orbital, shown in 25a, is composed of the allyl highest occupied molecular orbital, (HOMO) and a vacant p orbital on lithium. This interaction is favored when lithium adopts the bridging position indicated by NMR studies (39). (The available allyllithium X-ray structure is that of a polymer (40), but it may be possible with suitable lithium ligands to obtain the monomer.) The alternative ion pair formation (25b) would also place the lithium cation equidistant between the two negatively charged carbon centers at C(l) and C(3). 

Table 8 gives the data on the electron structure of valence shells of atoms of the corresponding transition metals [86]. Taking into account the fact that the total number of electrons provided by three allyl (crotyl) ligands to the molecular orbitals of the complex is nine, it may be concluded that the complexes of Cr (chromium), Nb (niobium) and Ti (titanium) should be paramagnetic and those of Rh (rhodium) should be diamagnetic. In this case the total... 

The TT-molecular orbital of the organic ligand can be delocalized over more than two carbon atoms, of course. There are well-characterized compounds in which the 17-orbital is associated with three, four, five, six, and seven carbon atoms. Some examples of structures with allyl ligands are illustrated in Figure S.2. 

There are two filled molecular orbitals on the allyl ligand that act as good fT-donors. Thus, the allyl ligand is a four-electron donor in the donor pair electron counting formalism. 

In order to determine which of the metal orbitals may interact with the molecular orbitals of the allyl ligand it is necessary to make several assumptions. Brown (7) and Orgel 88) used the approximation that it is permissible to classify the metal electrons under the local symmetry of the attached ligand. Hence, since only orbitals of the same symmetry may combine, the possible bonding and nonbonding orbitals may be separated. These are shown in Table III. 

Figure S.la shows that of the three molecular orbitals of the allyl fragment, 1 1 can interact with a suitable metal da orbital, and 2 with an M(d ) orbital on the metal ifrj is not a frontier orbital and so probably of lesser importance. As the number of nodes increases, the MOs of the free ligand become less stable (Fig. 5.1b). Two peculiarities of the structures of t -allyl complexes can be understood on this picture. First, the plane of the allyl is canted at an angle 6 with respect to the coordination polyhedron around the metal, as shown in Fig. S.lc 9 is usually 3° -10°. The reason is that the interaction between 2 and the dxy orbital on the metal is improved if the allyl group moves in this way, as can be seen in Fig. S.lc. The structures also show that the terminal CH2 groiqrs of the allyl are twisted about the C—C vector so as to rotate the anti hydrog, Ha, away from the metal, and the syn hydrogens, Hj, toward the metal as shown by the arrows in Fig. 5.W. This allows the p orbital on these carbons to point more directly...

Figure 3-7 shows the molecular orbitals of the ethylene, 7i-allyl, and a n-cyclopentadienyl ligands. The corresponding atomic orbitals of the central metal atom that can interact are given to the right of each 7c-ligand. 

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