Synthesis, Geometry, and Bonding Nature of Heptacoordinate Compounds of Silicon and Germanium Featuring Three Phosphine Donors

Structural studies were performed
on heptacoordinate compounds
of silicon {(o-Ph2P)­C6H4}3SiX (X = F (1), Cl (3), H (5)) and germanium {(o-Ph2P)­C6H4}3GeX (X = F (2), Cl (4), H (6), Me (7)) compounds featuring three phosphine donors. We found that 5, 6, and 7 have approximately a C3 symmetry similar to Corriu’s compounds
(heptacoordinate silane {(o-Me2NCH2)­C6H4}3SiX (X = F (8), H) and germane {(o-Me2NCH2)­C6H4}3GeX (X = H, F) with
three nitrogen donors coordinating to the central Si/Ge trans to the Cipso atoms).
In contrast, the Si compounds 1 and 3 and
the Ge compounds 2 and 4 have novel heptacoordinate
geometries; the incorporation of such electronegative substituents
as F and Cl results in the change of one phosphine donor from the
position trans to the Cipso atom to that trans to the X
atom. Compounds 1–4 retain this unprecedented
geometry in solution but show dynamic behavior. The structural modification
observed upon changing the substituent at Si and Ge is rationalized
by electrostatic and charge transfer interactions.