Znorg. Chem. 1987, 26, 160-765
760
Contribution from the Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003
Steric Hindrance in Pentacoordinated Fluorosilicates. Synthesis and Molecular Structure of the Diphenyl- 1-naphthyldifluorosilicate Anion and the Phenylmethyltrifluorosilicate Anion'$2 John J. H a r l a ~ ~Jeffrey d , ~ S. P a ~ n e Roberta ,~ 0. Day, and Robert R. Holmes* Received November 4 , 1986 New pentacoordinated anionic silicate derivatives, as salts of phenylmethyltrifluorosilicate,[PhMeSiF,] [N(n-Bu),] (1) and [PhMeSiF,] [NEt,], and the tris(dimethylamino)sulfonium salt of diphenyl- 1-naphthyldifluorosilicate, [Ph,( 1-Np)SiF,] [S(NMe2)3] ( 2 ) , were synthesized. The X-ray crystal structures of 1 and 2 were obtained. The molecular structures are trigonal bipyramidal with fluorine atoms occupying axial positions. The structural data on these and other acyclic five-coordinated anionic silicates are shown to form a series increasing in steric properties in the order SiF5-, PhSiF;, PhMeSiFC, Ph2SiF3-, (I-Np)Ph,SiF2-, t-BuPhSiFF, and Xy12SiF< (Xyl = xylyl). The steric effects are reasoned to partly produce the observed increases in axial bond lengths, which are comparable to increases found for equatorial bond lengths. The steric effects are superimposed on changes in electronic effects, Le., VSEPR terms increasing in the same direction. Bond angle changes over the series show an increase similar to that of the bond length data. As a measure proportional to the steric effect, it is found that the sum of the axial and equatorial angular displacements from the ideal values of 180 and 120°, respectively, increases from 3.1' for SiF< to a maximum of 11.8' for the dixylyl anion, Xyl,SiF
