A THEORETICAL-STUDY OF THE LINEAR VERSUS BENT GEOMETRY FOR SEVERAL MX2 MOLECULES - MGF2, CAH2, CAF2, CEO2 AND YBCL2

We have completed Hartree-Fock-Slater theoretical studies on the title molecules. The minimum energy configuration corresponds to a linear geometry for MgF2, whereas the remainder of the molecules are bent with a bond angle in the range 120-132°. These results are in agreement with experimental evidence where it exists. The bent geometries are more stable than the linear by only 1-3 kcal mol-1. The simplest rationalization for the (anomalous) bent geometries is that the central metal (n - 1)d orbitals are more important to the bonding than are the np orbitals. In valence bond language we can summarize this as sd (bent) vs sp (linear) hybridization. The 4&{itf} orbitals of cerium and ytterbium also do not contribute enough for the molecules to become linear as a result of {itf} orbital involvement. This analysis is borne out by close scrutiny of the eigenvalues and changes in Mulliken atomic populations for the linear and bent structures. There is no need to invoke a "reverse" polarization argument to rationalize the geometry of the bent molecules, as has been done in the classical multipole expansion model. We show that this model is very sensitive to the polarizability value chosen for the metal. Furthermore, in the multipole expansion the dipole-dipole term is of equal importance to the charge-dipole term, although in qualitative discussions only the latter is emphasized. © 1990.