The past few years have seen an enormous growth in the applications of molecular mechanics to organometallic systems. Although there is no fundamental reason why molecular mechanics should not be applied in this area, there are two significant problems which arise:
. To define the atom connectivity . To define, unambiguously, the type of bonding between the atoms.
Examples are the assignment of the hapticity of p-bonded systems coordinated to metal centers, the recognition of bonds such as metal-metal bonds, and the modeling of p-bonding, trans-, and other strong electronic effects. These aspects are also of importancealbeit to a lesser extentin the modeling of organic and classical coordination compounds. Therefore, it is not surprising that the application of molecular mechanics to the field of organometallic chemistry has developed slowly, and that QM modeling wasand in many cases still isthe method of choice for computing the structures and properties of this class of compound [29, 438].
In order to model organometallic systems it is necessary to be able to define the type of bonding, knowing only the metal center and the type of ligand. It is difficult to imagine how a method that is based exclusively on empirical force-field calculations can be used to predict the hapticity of p-ligands such as cyclopentadienyl or allyl systems and the coordination mode of carbonyl groups (Figure 14.1). The additional information needed may be available through electron counting, and therefore be relatively easy to implement in modeling systems, but a more thorough understanding of the electronic properties may also be required. The fact that many systems are fluxional indicates that the equilibrium between various bonding modes is often a subtle balance between electronic and steric factors.
Thus, it is not surprising that, with few exceptions, force-field calculations of organometallic systems start with a predefined bonding scheme. This is not unreasonable since the type of bonding can usually be determined from spectroscopic results, and it is often more or less constant within a class of similar compounds. Force-field calculations can then be used to obtain a more detailed picture of the structural and dynamic properties of a molecule with a given connectivity. In spite of these restrictions to the modeling of organometallics, the Molecular Modeling of Inorganic Compounds. Third Edition.