Studies of chelation IV. Steric influences on the chelation of ditertiary phosphine complexes of group VI metal carbonyls

Abstract

The preparation of the complexes [M(CO) n (dcpe)] [M  Cr, Mo, W; n  4, 5; dcpe is ((cyclo-C 6H 11) 2PCH 2) 2] is reported. Attempts to prepare [M(CO) 2(dcpe) 2] by many different methods gave only cis-[M(CO) 4(dcpe)] and [M(CO) 5(dcpe)]. Heating cis-[M(CO) 4(dcpe)] with (Me 2PCH 2) 2(dmpe) gives cis-[M(CO) 2(dmpe) 2] only. These observations are explained in terms of unfavourable intramolecular non-bonded interactions between substituents at phosphorus. The rate of chelation of [M(CO) 5(dcpe)] to give cis-[M(CO) 4(dcpe)] has been measured at various temperatures in the range 360–420 K. The activation parameters indicate the dominance of a dissociative process leading to the observed steric acceleration in the chelation step. The rate of chelation is correlated satisfactorily with the ligand cone angle; the operation of an apparent saturation effect is noted.