Cundari Th.R. -- Computational Organometallic Chemistry-0824704789

TABLE 6 Optimized Distances of Cp–E for LiCp, LiCp*,

MgCp, [MgCp*] , AlCp, and AlCp* at Various

˚

Theoretical Levels (distance in A, angle in degrees)

The effects caused by methyl substitution (Cp → Cp*) on the main structural features of metallocenes are shown in Table 6. Comparison of calculated Cp–E distance between Cp-containing half-sandwich metallocenes and Cp*-con- taining half-sandwich metallocenes suggests that methyl substitutions on the Cp rings has little affect on Cp–E distances. This fact is also supported by previous calculations of Cp2Ca, in which optimized geometrical parameters are in good

agreement with the experimental data on Cp*Ca (28).

2

One of the interesting features for main group metallocenes is electron distribution between main group elements and Cp rings. We have evaluated atomic charges and bond populations on the basis of some useful partitioning schemes for the total electron density distributions, such as Mulliken population analysis, NPA, CHelpG, and MKS, and results are shown in Tables 7 and 8. Calculated atomic charges of different elements, such as Li, Mg, and Al, shows

TABLE 7 Calculated Atomic Charges of Some Half-

Sandwich and Full-Sandwich Metallocenes at the

B3LYP/6-31G*//B3LYP/6-31G*

TABLE 8 Calculated Mulliken Bond

Populations and Wiberg Bond Indices of

C(Cp)–E for Some Half-Sandwich and Full-

Sandwich Metallocenes at the B3LYP/6-31G*//

B3LYP/6-31G*

positive charges, as expected. The values of electrostatic charges are between those of Mulliken and NPA charges. Mulliken charges overestimate the polarity of main group elements, while NPA and electrostatic charges give a reasonable interpretation of the electron distribution of main group elements. Mulliken bond populations for metallocenes of Li and Mg show that there is a certain covalent interaction between C(Cp) and E, which is somewhat controversial given that these metallocenes have a large amount of ionic bonding character between C(Cp) and E. On the other hand, the Wiberg bond indices (WBI), which are included in the NBO analysis, shows a reasonable description of C(Cp)–E bonding. Thus, the C(Cp)–Al bonding is predicted to be stronger than C(Cp)–Li and C(Cp)–Mg bonding, which explains why Group 13 metallocenes have larger amount of covalent bond character between carbon and the central atom. When a Cp ligand is replaced by Cp*, the Mulliken bond populations and WBI are changed very little, which implies that methyl substitution on the Cp ring should not significantly influence the nature of bonding of metallocenes.

6. CONCLUSIONS

While metallocenes are often thought to be entirely in the domain of transition metal chemistry, we have shown that main group chemists have a legitimate claim as well. The lack of d orbital participation in the metal–ligand bonding may result in less thermodynamic stability but does not preclude the possibility of novel structural motifs or future ‘‘real-world’’ applications. Computational

chemistry is expected to play a very active role in the development of this field, calculating properties of known compounds and predicting properties of unknown compounds. Even at the present stage of computer software and hardware, it can be expected that the calculation of main group metallocene potential energy surfaces will give the experimentalist clues for the successful synthesis of new compounds. We hope that this chapter provides an introduction and motivation to continued explorations in this field. Happy hunting!

7. RECENT DEVELOPMENTS

A series of Group 14 metallocenes with substituted Cp rings [C5Me4(SiMe2But)] was recently reported (87). Single-crystal X-ray structural analysis of each metallocene (E Ge, Sn, Pb) showed that the mixed alkyland silyl-substituted Cp rings are parallel for all three metallocenes (in contrast to most Group 14 with unsubstituted Cp rings). Theoretical calculations at the DFT level indicated that the preference of parallel Cp rings over bent ones is due to the SiR3 substituent

on the Cp rings, which lowers the a* orbital (stereochemically active lone pair)

1g

below the e1u orbital.

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