THE STRUCTURAL VARIATIONS OF MONOMERIC ALKALINE-EARTH MX2 COMPOUNDS (M = CA, SR, BA, X = LI, BEH, BH2, CH3, NH2, OH, F) - AN ABINITIO PSEUDOPOTENTIAL STUDY

The geometries of a series of monomeric alkaline earth MX2 compounds (M = Ca, Sr, Ba; X = Li, BeH, BH2, CH3, NH2, OH, F) have been calculated at the Hartree-Fock level, using quasirelativistic pseudopotentials for Ca, Sr, and Ba. The energies of fully optimized structures are compared with those of linear X-M-X geometries. Most barium compounds studied (except BaLi2) are bent with angles between 115 and 130-degrees and linearization energies up to ca. 8 kcal/mol (for Ba(CH3)2). The degree of bending for M = Sr is smaller but is still significant (except for X = Li, BeH). However, most of the Ca compounds may be termed quasilinear, i.e., they either are linear or nearly so and bend easily. The XMX bond angles for M = Sr, Ba do not decrease monotonously along the series X = Li, BeH, BH2, CH3, NH2, OH, F but show a minimum for X = CH3! Natural atomic orbital population analyses indicate the larger angles with O, N, and F to be due to pi-type interactions of lone pairs with empty metal d-orbitals. These p(pi) --> d(pi) interactions tend to favor linear structures. For the diamides (X = NH2), significant pi-bonding contributions lead to a preference for C2-upsilon structures with the hydrogen atoms in the N-M-N plane over C(s) or out-of-plane C2-upsilon geometries. The barriers to rotation around the M-N bonds are significant. For X = BH2, C(s) and out-of-plane C2-upsilon arrangements are slightly more favorable than an in-plane C2-upsilon geometry. The dimethyl compounds generally exhibit almost free MCH3 rotation.