STUDY OF THE RELATIVE PI-ACCEPTOR ABILITY OF UNSATURATED LIGANDS C(=C)(N)PH-2 (N=0, 1, 2 AND 4) BASED ON CYCLIC VOLTAMMETRY OF COMPLEXES TRANS-[RHCL(L)((PPR3)-PR-I)(2)] (L=C(=C)(N)PH-2, C2H4 AND CO)(1,2)

The set of carbene (I), vinylidene (II), allenylidene (III), pentatetr aenylidene (IV), ethylene (V) and carbon monoxide (VI) rhodium(I) comp lexes of the general formula trans-[RhCl(L) ((PPr3)-Pr-i)(2)] (L=:C(=C )(n)Ph-2, C2H4 and CO) was studied by cyclic voltammetry. As the study revealed, I-VI undergo one-electron oxidation in the potential range 0.49-1.36 V (versus NHE). The comparison of corresponding E-1/2(Rh2+/) values, as well as the values of the anodic peak potential E-pa(Rh2/+), allowed us to order the ligands by increasing pi-acceptor ability . The order :CPh2 <:C=C=CPh2 congruent to:C=C=C=C=CPh2 < C2H4 <:C=CPh2 < CO is in good correlation with observed trends in metal-carbon bond distances in various complexes trans- [MCl{=C(=C)(n)R1R2}((PPr3)-Pr-i )(2)] (M = Rh(I) and Ir(I); n = 0, 1, 2 and 4). It was also attempted to assign the electrochemical ligand parameters E-L(L) of the four fra gments :C(=C),Ph, (n = 0, 1, 2 and 4) using Lever's electrochemical pa rametrisation of metal complex redox potentials. (C) 1998 Elsevier Sci ence S.A. All rights reserved.