Stibine five-coordinate

A very large number of phosphine, phosphite, stibine, and arsine complexes of type four-coordinate [NiX2(ER3)2] and five-coordinate [NiX2(ER3)3] (X = halide) have been reported, although this area is dominated by complexes of tertiary phosphines. The complexes [NiX2(ER3)2] can be either tetrahedral (67) or square-planar (68), depending upon the electronic and steric effect of the coordinated phosphine, arsine, and stibine ligands (see Section 2). ... 

P) to 2.42 (As) to 2.58 A (Sb). The relatively easy formation of five-coordinate stibine-containing complexes may be a consequence of these changes. 

In the reactions of palladium(II), the solvent path seems to be more important than with either platinum(II) or gold(III), and conversely the reactions involve less bond making at the transition state. A most interesting reaction in rhodium(I) chemistry from the point of view of the solvent, was recently studied by Cattalini, Ugo and Orio. They found the rate of replacement of stibine (Sb(/ -tolyl)3) by amines from [RhCl(l,5-cyclooctadiene)(SbR3)] in a range of solvents, to be virtually independent of the nature of the solvent, independent of the concentration of the amine, but dependent on the nature of the amine. These results have been interpreted in terms of a relatively inert five-coordinate intermediate. 

---