Investigations on organoantimony compounds : VII. The stereochemistry of (acetylacetonato)organoantimony(V) compounds, RnSbCl(4-n)Acac (n = 1-4)

The complete series of (acetylacetonato)organoantimony(V) complexes of the type RnSbCl(4−n)Acac (R = Me or Ph and n = 1–4) has been synthesized. The influence of progressive substitution of chlorine atoms by organic groups on the spectroscopic parameters, the stability, and the stereochemistry of these compounds has been investigated. Particular attention has been given to a comparison of the merits of the electric field model of Smith and Wilkins and the inductive model of Kawasaki et al., for explaining the effect of substitution at antimony on the spectra characteristics of the Acac ligand. The PMR data display an excellent linear correlation with the inductive effect of the substituents at antimony and thus strongly support the inductive model. For phenyl-substituted complexes aromatic ring current effects must be taken into account to explain the observed PMR chemical shift data. IR and PMR data indicate that throughout the series RnSbCl(4−n)Acac progressive substitution of electron-withdrawing chlorine substituents by organic groups R leads to a weakening of the Sb---Acac interaction. For Ph3SbClAcac and Me4SbAcac this is further reflected in the observed non-rigidity of these complexes in solution. The stereochemistry of the various compounds has been unambiguously deduced from spectroscopic and molecular dipole moment data. The diphenyl derivative Ph2SbCl2Acac is noteworthy in that it provides an example of stereoisomerism in a hexacoordinate organometallic complex. In the isomer which shows the Acac proton PMR signals at higher field, the two phenyl groups are in trans-position, whereas in the “low-field isomer” the phenyl groups occupy cis- and the two chlorine atoms trans-positions