Sigma donors

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. 

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. 

The bonding between the transition metal and the group 13 element has been studied extensively by computational methods. Because RM compounds have a valence electroitic structure consisting of a filled orbital of a symmetry and two unfilled orbitals of n symmetry, they are analogous to carbonyl ligands in that they can act as sigma donors and pi acceptors see tv-Acid Ligand). Such an electronic structure... 

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

FIGURE 10-24 Energies of 4 Orbitals in Octahedral Complexes Sigma-Donor and Pi-Donor Ligands. Ao = - Ae.y. Metal s and p... 

Assign the symmetry labels of the d orbitals (labels of the irreducible representations). Repeat the calculations in Part a for a ligand that can act both as a sigma donor and a pi acceptor. 

Attempts to understand chemical reactivity in organometallic systems often involve comparison of some measured observable with pKg or other one parameter criteria of sigma donor strength. Deviations from the sigma donor trends are often interpreted in terms of unusual bonding effects in the organometallic system. In this article, the pitfalls associated with selection of a one parameter criteria of donor strength are discussed. 

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