Quantifying the electronic cis effect of phosphine, arsine and stibine ligands by use of rhodium(I) Vaska-type complexes

Abstract

The cis effects of phosphine, arsine and stibine ligands have been evaluated by measuring the IR stretching frequency in dichloromethane of the carbonyl ligand in a series of Rh(I) Vaska-type complexes, trans-[RhCl(CO)(L) 2]. These data were correlated with those obtained by Tolman for the electronic trans influences in the [Ni(L)(CO) 3] complexes. The electronic contribution, χ Fc, of ferrocenyl was determined as 0.8 from these plots by evaluating PPh 2Fc as ligand. In order to accommodate arsine and stibine ligands an additional correction term, to compensate for differences in the donor atom, was added to Tolman’s equation for calculation of the Tolman electronic parameter of phosphine ligands. In the resulting equation: ν(CO Ni)=2056.1+∑ i=1 3 χ i + C L values for C L of C P=0, C As=−1.5 and C Sb=−3.1 are suggested for phosphine, arsine and stibine ligands, respectively. The crystal and molecular structures of trans-[RhCl(CO)(PPh 2Fc) 2] · 2C 6H 6, trans-[RhCl(CO){P(NMe 2) 3} 2] and trans-[RhCl(CO)(AsPh 3) 2] are reported. The Tolman cone angles for PPh 2Fc and P(NMe 2) 3 were determined as 169° and 166°, while the effective cone angles for PPh 2Fc, P(NMe 2) 3 and AsPh 3 were determined as 171°, 168° and 147°, respectively.