_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The main Groups IV and V
Dimethyltin fluoride (m.p. 360°C) crystallizes in an infinite two dimensional
lattice resembling that of SnF 4 • The tin atoms are octahedrally coordinated
(Fig. 4.2). While Me 2 SnCl 2 is unassociated in the vapour, in the solid association
occurs through highly unsymmetrical chlorine bridges. The bromide and iodide
are probably similar. In continuation ofthe trend to greater association, the lead
fluorides R,PbF and R 2 PbF 2 are in volatile, sparingly soluble solids which ha ve
high melting points or which decompose without mdting. These properties are
consistent with a high degree of ionic character. Ph,PbCl. m.p. 205oC and the
similar bromide have solid state structures similar to that of Me,SnF, but in
contrast to the molecular Ph,SnCl. Ph 2 SnCl 2 forms double chlorine-bridged chain
polymers in which lead is octahedrally coordinated ( cf. R 2 SnF 2 but contrast
Ph 2 SnCl 2 ).
4.3.2 Lewis acidity
Intermolecular association in covalent halides occurs through Lewis acid-base
interactions. This leads to the expectation that the halides R,MX 4 _" would also
form complexes with donor molecules or ions and that this Lewis acidity would
in crease
Si - Ge < Sn < Pb; 1 < Br < Cl < F; R,MX < R 2 MX 2 < RMX,
While Me,SiF does not complex with neutral Lewis bases, it reacts with
tetramethylammonium fluoride
Satm +
(Me 4 N) 2 Me,SiF~-
Fluoride ions also add to R 2 SiF 2 and to RSiF 3 •
RSiF 5
2 -
The 19 F n.m.r. spectrum of Ph 2 SiF; in a non-polar solvent shows only one
resonance at room temperature, but below - 60° this splits into two groups of
resonances of relative intensity 2: 1 (Fax: F eq). Rapid intramolecular scrambling
between axial and equatorial positions is a common phenomenon in trigonal
bipyramidal species (p. 130). This normally occurs through a square pyramidal
transition state (Berry pseudorotation). The most stable trigonal bipyramidal
structure is often the one in which the most electronegative substituents are in
the axial positions.
104