Metal-sigma interactions

For octahedral complexes, ligands can interact with metals in a sigma fashion, donating electrons directly to metal orbitals, or in a pi fashion, with ligand-metal orbital interactions occurring in two regions off to the side. Examples of such interactions are shown in Figure 10.3. 

As in Chapter 5, we will first consider group orbitals on ligands based on Oh symmetry, and then consider how these group orbitals can interact with orbitals of matching symmetry on the central atom, in this case a transition metal. We will consider sigma interactions first. The character table for Oh symmetry is provided in Table 10.4. 

These are octahedral ions with only sigma interactions. The ammonia ligands have no 77 orbitals available for significant bonding with the metal ion. The donor orbital of NH3 is mostly nitrogen orbital in composition, and the other p orbitals are used in bonding to the hydrogens. ... 

Using the angular overlap model, determine the relative energies of d orbitals in a metal complex of formula ML4 having tetrahedral geometry. Assume that the ligands are capable of sigma interactions only. How does this result for A compare with the value... 

Halide ions donate electron density to a metal via orbitals, a sigma interaction the ions also have p and p orbitals that can interact with metal orbitals and donate additional electron density by pi interactions. We will use [MXs]", where X is a hahde ion or other ligand that is simultaneously a a and a tt donor. 

Fig. 11(c) shows the sigma-type interaction between a filled O2 tt-orbital and the empty d sp hybrid orbital on the iron atom. And Fig. 11(b) shows the 7T-type interaction between a filled c -orbital on the metal e.g.dyf) and one of the empty tt dioxygen orbitals. 

As shown in (4.72a) and (4.72b), such interactions give rise to formal sigma bonds (omb or oma) that are expected to be oppositely polarized, with atomic charges of opposite sign on the metal atom depending on whether donation is to (4.72a) or... 

Figure 4.34 shows the form of the interacting metal and ligand NHOs and the final own NBO for H4W(NH3), n = 1. (Corresponding plots for n = 2 and 3 are very similar.) The complementary lock-and-key overlap of the donor and acceptor hybrids is apparent in Fig. 4.34(a). The localized ammine— metal (nL nM ) interaction depicted in Fig. 4.34(a) is representative of sigma donation in a large number of metal-ligand complexes. 

From this example one can judge that insertions of metal atoms into sigma bonds may be precluded by unfavorable spin or steric/promofion factors, despite the formal possibility of attractive interactions such as depicted in Fig. 4.62. Long-range s2 steric/promofion barriers may be expected to block direct insertion of metal atoms... 

A common motif in organometallic chemistry is the agostic interaction, which can act to stabilize low-coordination low-e-count complexes. The requirement is an alkyl group with a / - or a y-C—H bond attached to the metal within reach of (i.e., cis to) an empty coordination site. An attractive interaction occurs with the C—H bond acting as a 2e donor into the low-lying metal valence orbital that occupies that site. In the case of a / -C—H bond, hydride transfer may occur with little activation, resulting in an M—H sigma bond and complex with an alkene as discussed above. 

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