title: Do penta- and decaphospha analogues of lithocene anion and beryllocene exist? Analysis of stability, structure, and bonding by hybrid density functional study.
abstract: Stability in penta- and decaphospha analogues of lithocene anion and beryllocene is investigated by complete structural optimization at the B3LYP/6-31G level. Natural bond orbital analysis is carried out to examine the bonding between the metal and the ligands. The heterolytic dissociation energies of 667 and 608 kcal/mol predicted by B3LYP/6-311+G//B3LYP/6-31G calculations for CpBeP(5) and (P(5))(2)Be are comparable with the observed value of 635 +/- 15 kcal/mol in ferrocene. The high stability in CpBeP(5) and (P(5))(2)Be shows that these species are isolable under appropriate conditions. Lithocene anion and its phospha analogues possess lower stability toward dissociation into ionic fragments. A novel observation of the present study is that CpBeP(5) and (P(5))(2)Be have lowest energies when the two planar ligands are arranged perpendicular to each other such that one of the ligands, cyclo-P(5), is eta(1)-coordinated while the second ligand is eta(5)-coordinated to Be. The resulting structure having C(s)() point group (denoted as C(s)()(p)) is predicted to be 22 and 28 kcal/mol lower than the staggered sandwich geometry in CpBeP(5) and (P(5))(2)Be, respectively, at the B3LYP/6-311+G//B3LYP/6-31G level. In the analogous lithocene anions [CpLiP(5)](-) and [(P(5))(2)Li](-) also the C(s)()(p) structures are found to be the lowest energy structures, though their relative stabilities are small. We also characterized the geometry with both ligands eta(1)-coordinated to the metal in a linear arrangement having the D(2)(h)() point group in the decaphospha analogues [(P(5))(2)Li](-) and (P(5))(2)Be. This structure is found to be higher in energy than the C(s)()(p) structure. The D(2)(h)() structure could not be located as a potential minimum in the biscyclopentadienyl complexes and their pentaphospha analogues. Both the C(s)()(p) and D(2)(h)() structures are characterized for the first time in metallocenes. The D(2)(h)() structure seems to be a unique feature in the decaphospha metallocenes under consideration. Covalent bond formation between beryllium and phosphorus atom P(1) of eta(1)-(cyclo-P(5)) is more pronounced (bond orders 0.43-0.49) than that between Be and C(1) of eta(1)-Cp (bond orders 0.24-0.27). Though both eta(1)-coordinated cyclo-P(5) and Cp exhibit C(2)(v)() point groups, bond alternation is less pronounced in the former. The Wiberg P-P bond orders in the eta(1)-(cyclo-P(5)) of CpBeP(5) and (P(5))(2)Be having C(s)()(p) structures are in the range 1.29-1.47. These ring bond orders indicate that the P(5) ring retains aromaticity to a large extent in the eta(1)-mode of bonding with Be. Second-order perturbational energy analysis of the Fock matrix in the natural bond orbital basis reveals that there is a significant stabilizing interaction of approximately 123 kcal/mol between the lone pair orbital of P(1) and the 2s orbital of Be in the C(s)()(p) structures.
authors.affiliation: Department of Physical Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India. ejpmalar@vsnl.net
authors.forename: E J Padma
authors.initials: EJ
authors.lastname: Malar