Sigma-Donor Interactions

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.  

The energy changes for the hgand orbitals are the same as those above for each interaction. The totals, however, are taken across a row of Table 10.10, inclnding each of the d orbitals. 

Ligands in positions 1 and 6 interact strongly with d 2 and are lowered by e . Ligands in these positions do not interact with the other d orbitals. 

Ligands in positions 2, 3,4, and 5 are lowered by e by interaction with and by by interaction with /, for a total stabilization of e for each donor orbital. Overall, each hgand orbital is lowered by e.  

TABLE 10.10 Angular Overlap Parameters Sigma Interactions 

The angular overlap approach is best described by example. We will consider first the most common geometry for coordination complexes, octahedral. 

Metal d Orbitals orbital The interaction is strongest with ligands in positions 1 and 6, along the z axis. Each interacts with the orbital to raise its energy by e . The ligands in positions 2, 3, 4, and 5 interact more weakly with the df orbital, each raising the energy of the 

FIGURE 10-20 Energies of d Orbitals in Octahedral Complexes Sigma-Donor Ligands. A, = 

Metal s and p orbitals also contribute to the bonding molecular orbitals, 

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FIGURE 10.4 Sigma-Donor Interactions with Metal d Orbitals. 

FIGURE 10.5 Sigma-Donor Interactions with Metal s, p, and d Orbitals. As in Chapter 5, the symmetry labels of the atomic orbitals are capitalized, and the labels of the molecular orbitals are in lowercase. The six filled ligand donor orbitals contribute 12 electrons to the lowest six molecular orbitals in this diagram. The metal valence electrons occupy the and,... 

As a simple example of non-d coordination, let us consider the hexaammine-zinc(II) cation [Zn(NH3)6]2+, whose optimized structure is shown in Fig. 4.51. Each ammine ligand serves as a formal two-electron sigma donor, and the total electron count atZn therefore corresponds to a 22e system, again violating the 18-electron rule. Each ammine ligand is bound to the Zn2+ cation by about 60.7 kcal mol-1, which is in part attributable to classical electrostatic interactions of ion-dipole type. 

Figure 4.10. (a) Sigma-type donor interaction between the bond and the LUMO, cr, of a polarized a bond (shown as a p orbital). (donor interaction between the a bond and the LUMO (shown as a p orbital), which may be the n orbital of an adjacent Z-type substituent, or cr of a polarized cr bond. 

Another control experiment was run to further confirm the significance of the cation-pi interaction in these bibracchial lariat ether model complexes. In this case, a diaza-18-crown-6 derivative was prepared in which a 2-phenylethyl pi-donor sidearm was attached to one nitrogen and a 2-methoxyethyl sigma donor was attached to the other <2002CC1808>. The structure is illustrated as 14, above. The solid-state structure of the 14 KI complex showed the typical apical solvation of the ring bound cation. In this case, however, one apex was solvated in the pi-fashion (benzene) and the other by the oxygen sigma donor. 

FIGURE 10.7 Sigma-Donor and TT-Acceptor Interactions in an Octahedral Complex. ... 

Li QZ, Guo X, Yang X, Li WZ, Cheng JB, Li HB (2014) A sigma-hole interaction with radical species as electron donors does single-electron tetrel bonding exist Phys Chem Chem Phys 16(23) 11617-11625. doi 10.1039/c4cp01209g... 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

Sigma- and pi-type donor acceptor interactions Further details of the leading hp —hM donor-acceptor interactions are gathered in Table 2.5 and Figure 2.21. For each such interaction the table shows the hybrid form of the donor (hp)23 and acceptor (hM) orbitals, the occupancy of the acceptor, and the second-order estimate (cf. Eq. (1.35) or (2.7)) of the donor-acceptor stabilization energy. Let us now discuss some of the trends displayed in Table 2.5. 

Figure 2.21 Leading sigma- (left) and pi-type (right) donor-acceptor interactions of representative TM monofluorides (see Table 2.5).

Thus, fluorine is the strongest sigma-withdrawing element known, but is a mild donor (pi base) in pi-type interactions. 

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