Metal p orbitals

Theoretical explanation of the trans-effect (and /rans-influence) has centred on two theories, one based on cr-bonding the other on 7r-bonding. The tr-bonding argument considers two frans-ligands sharing a metal p orbital (Figure 3.83). 

The three-fold degenerate set of p orbitals are labelled tiu t for three-fold, u for odd under inversion through the centre of symmetry). As shown in Fig. 6-6, each metal p orbital matches symmetry with ligand group orbitals comprising just two... 

Fig. 14. The overlap of (ligand)p orbital with a vacant CIS (metal)p orbital without rearrangement.

However, significant stabilization is also contributed by nN—>-szn donor-acceptor interactions, each with estimated second-order interaction energy 49.4 kcal mol-1, as depicted in Fig. 4.52. Each ammine ligand thereby donates about 0.061 e to the zinc cation, primarily to the vacant 4s orbital which acquires about 0.371 e total occupancy. As before, the high formal hypervalency at the metal center is achieved within the limits of the duodectet rule, i.e., without significant involvement of extravalent metal p orbitals. 

Three equivalent 4x4 blocks corresponding to rTu. One block will produce four energy levels with MOs formed from a metal p, orbital, a combination of ligand s-orbitals (y. a)) a combination of ligand cr-type p-orbitals (y. a)) and a combination of... 

The three LGOs of r1u symmetry that will overlap with the metal p orbitals are constructed in a similar manner, as shown in Fig. 11.19. Since the metal crbkals cannot participate in a-overlap, they are considered nonbonding molecular orbitals in complexes where there is no possibility for it bonding. In cases where there are ligand orbtals of appropriate symmetry available, the orbitals will be involved m n bonds. 

Fig, 13.4 Competition of trans ligand (T) and leaving group (X) for a metal p orbital in a square planar complex. 

Figure 2-25. The interaction of metal d or metal p orbitals with the ji -levels of a ligand such as carbon monoxide or an imine.

We normally think of protons as acidic rather than electrophilic but an acid is just a special kind of electrophile. In the same way, Lewis acids such as BF3 or AICI3 are electrophiles too. They have empty orbitals that are usually metallic p orbitals. We saw above how BF3 reacted with Me3N. In that reaction BF3 was the electrophile and Me3N the nucleophile. Lewis acids such as AICI3 react violently with water and the first step in this process is nucleophilic attackby water on the empty p orbital of the aluminium atom. Eventually alumina (AI2O3) is formed. 

The model also predicts that the position of the lithium atom should vary with solvation number. If a trisolvated lithium atom were formed, the empty metal p orbital no longer would be available, and the bonding site again would probably be that predicted by the electrostatic model (Figure 29). 

Concerning the question whether the metal p orbitals should be considered as valence orbitals or polarization function, the results in Table 13.18 show that the contribution of the tiu term (metal p orbitals) is always larger than the ajg term (metal s orbital). Thus, the p orbitals are as important as the s orbitals for the bonding in these compounds and thus, must be considered as valence functions. 

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