Neutral and Cationic Complexes of Silicon(IV) Halides with Phosphine Ligands.

The reaction of SiI 4 and PMe 3 in n -hexane produced the yellow salt, [SiI 3 (PMe 3 ) 2 ]I, confirmed from its X-ray structure, containing a trigonal bipyramidal cation with trans -phosphines. This contrasts with the six-coordination found in (the known) trans -[SiX 4 (PMe 3 ) 2 ] (X = Cl, Br) complexes. The diphosphines o -C 6 H 4 (PMe 2 ) 2 and Et 2 P(CH 2 ) 2 PEt 2 form six-coordinate cis -[SiI 4 (diphosphine)], which were also characterized by X-ray crystallography, multinuclear NMR, and IR spectroscopy. Reaction of trans -[SiX 4 (PMe 3 ) 2 ] (X = Cl, Br) with Na[BAr F ] (BAr F = [B{3,5-(CF 3 ) 2 C 6 H 3 } 4 ]) produced five-coordinate [SiX 3 (PMe 3 ) 2 ][BAr F ], but while Me 3 SiO 3 SCF 3 also abstracted chloride from trans -[SiCl 4 (PMe 3 ) 2 ], the reaction products were six-coordinate complexes [SiCl 3 (PMe 3 ) 2 (OTf)] and [SiCl 2 (PMe 3 ) 2 (OTf) 2 ] with the triflate coordinated. X-ray crystal structures were obtained for [SiCl 3 (PMe 3 ) 2 ][BAr F ] and [SiCl 2 (PMe 3 ) 2 (OTf) 2 ]. The charge distribution across the silicon species was also examined by natural bond orbital (NBO) analyses of the computed density functional theory (DFT) wavefunctions. For the [SiX 4 (PMe 3 ) 2 ] and [SiX 3 (PMe 3 ) 2 ] + complexes, the positive charge on Si decreases and the negative charge on X decreases going from X = F to X = I. Upon going from [SiX 4 (PMe 3 ) 2 ] to [SiX 3 (PMe 3 ) 2 ] + , i.e., removal of X - , there is an increase in positive charge on Si and a decrease in negative charge on the X centers (except for the case X = F). The positive charge on P shows a slight decrease.