Do Penta- and Decaphospha Analogues of Lithocene Anion and Beryllocene Exist? Analysis of Stability, Structure, and Bonding by Hybrid Density Functional Study

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 CpBeP5 and (P5)2Be are comparable with the observed value of
635 ± 15 kcal/mol in ferrocene. The high stability in CpBeP5 and (P5)2Be 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 CpBeP5 and (P5)2Be have lowest energies
when the two planar ligands are arranged perpendicular to each other such that one of the ligands, cyclo-P5, is
η1-coordinated while the second ligand is η5-coordinated to Be. The resulting structure having Cs point group
(denoted as Cs(p)) is predicted to be 22 and 28 kcal/mol lower than the staggered sandwich geometry in CpBeP5
and (P5)2Be, respectively, at the B3LYP/6-311+G**//B3LYP/6-31G* level. In the analogous lithocene anions [CpLiP5]-
and [(P5)2Li]- also the Cs(p) structures are found to be the lowest energy structures, though their relative stabilities
are small. We also characterized the geometry with both ligands η1-coordinated to the metal in a linear arrangement
having the D2h point group in the decaphospha analogues [(P5)2Li]- and (P5)2Be. This structure is found to be
higher in energy than the Cs(p) structure. The D2h structure could not be located as a potential minimum in the
biscyclopentadienyl complexes and their pentaphospha analogues. Both the Cs(p) and D2h structures are characterized
for the first time in metallocenes. The D2h structure seems to be a unique feature in the decaphospha metallocenes
under consideration. Covalent bond formation between beryllium and phosphorus atom P1 of η1-(cyclo-P5) is more
pronounced (bond orders 0.43−0.49) than that between Be and C1 of η1-Cp (bond orders 0.24−0.27). Though both
η1-coordinated cyclo-P5 and Cp exhibit C2v point groups, bond alternation is less pronounced in the former. The
Wiberg P−P bond orders in the η1-(cyclo-P5) of CpBeP5 and (P5)2Be having Cs(p) structures are in the range
1.29−1.47. These ring bond orders indicate that the P5 ring retains aromaticity to a large extent in the η1-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 ∼123 kcal/mol between the lone pair orbital of P1 and
the 2s orbital of Be in the Cs(p) structures.