Fluorine penta-coordinated

Penta-coordinated fluorophosphoranes have been extensively considered by Schmutzlera965,12andl4) and discussed in detail in ref. 1966, 1, p. 281 seq. The special interest was in the existence of two well-defined types of fluorine substituents, as well as for the hexa-coordinated (PVI)e compounds. Fluorine exchange generally occurs by pseudorotationf but steric hindrance, substituent conjugation and low temperatures may stabilize fluorine atoms in their respective position. As a rule (see Section... 

High conformational mobility of the molecules is a specific feature of penta-coordinate species of transition and main group elements including silicon 95-397) Thus, at room temperature all fluorine atoms of SiFJ are equivalent on the NMR time scale which points to inter- or intramolecular exchange processes. Since in NMR spectra the Si satellites are observed (148 Hz) the magnetic... 

At lower temperatures, the F signals of XFMeSi(o-CgH )CH2NMe2 shift down-field for X = H, 0-t-Bu and NEt whereas they shift upfield for X = Cl. This has been attributed to axial F in the former cases and equatorial in the latter case. However, for X = OCOCgHj a mixture of two penta-coordinate conformers with axial and equatorial fluorine arrangements was obtained below the coalescence temperature... 

The interpretation of observations on penta- or hexa- coordinated compounds is not so clear as CF3 groups may occupy axial positions, even when competing with fluorine substituents as in (44).<1968,41)... 

Careful purification of solutions of fluorosilane and fluorosilicate anions and a decrease in concentration and temperature result in a fine structure of F NMR spectra due to a non-equivalent arrangement of ligands (Table 16). For example, in the F NMR spectra of octahedral complexes of the type RSiFj" a doublet (equatorial atoms) and a quintuplet are observed whose mutual arrangement depends on the nature of the ligand In fluorine derivatives of penta- or hexa-coordinate silicon with slow exchange, the Hgip values (if they can be determined) are greater for the fluorine atoms with chemical shifts in low field. 

---