The highly electropositive nature of calcium, strontium, and barium is associated with metal�ligand (Ae�L) interactions that are strongly polar. Lacking metal valence electrons to influence the orientation of ligands, AeL2 compounds would be expected to have linear geometries, but it has been long known, for example, that Ae dihalides are bent in the gas phase, that the unsolvated dialkyl compound Ca[C(SiMe3)3]2 is nonlinear, and that almost all Cp�2Ae (and related organolanthanide metallocenes) possess non-parallel cyclopentadienyl rings. Reconciliation of an electrostatically-based bonding scheme for AeL2 compounds with their nonlinear structures has been attempted by proposing that: 1) reverse (core) polarization of the metal cation by the ligands is operative; 2) that the (n - 1)d orbitals of the cation participate in bonding; 3) and that in appropriate complexes, attractive van der Waals (dispersion) interactions between the ligands strongly influence the structures. The latter explanation has risen to prominence in recent years (e.g., Organometallics 2012,  32, 1265), especially as computational disentanglement of these (not mutually exclusive) possibilities has become more feasible with the development of density functionals that capture the effects of dispersion interactions. In some cases these take the form of heavily parameterized functionals (e.g., Truhlar's Minnesota M0X functionals), whereas in other cases the dispersion interaction is added as a correction to an existing functional (as in the �-DX� developments by Grimme). We chose to emphasize the latter functionals in investigating the importance of dispersion effects in heavy Group 2 complexes, as it is possible to optimize a structure both with and without dispersion corrections, while maintaining the same underlying functional form. The effects of dispersion corrections on the structures of a range of heavy organoalkaline-earth complexes with cyclic and acyclic ligands will be presented.

Structure and bonding in heavy group 2 organometallics: Does dispersion hold the key?

247th National Meeting (2014)

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