Influence of stereochemistry on the formation and reactivity of phosphorus-bridged dinuclear metal carbonyls. Part 1. Reactions of Di-µ-dimethylphosphidobis(tetracarbonylmolybdenum) with phosphines and the molecular structure of one of the products

Abstract

Di-µ-dimethylphosphidobis(tetracarbonylmolybdenum) usually reacts with mono- and bi-dentate phosphines to give products in which two carbonyl groups have been replaced by phosphine. The yields depend upon the solvent used, and with chelating diphosphines the structure of the product depends on the length of the hydrocarbon chain between the phosphorus atoms. The molecular structure of [Et₃P(OC)₃Mo{P(CH₃)₂}₂Mo(CO)₃PEt₃] has been determined by X-ray methods, and the three-dimensional data refined to give a final discrepancy of R= 0·115 between observed and calculated values of the 1900 independent structure factors. The molecule is centrosymmetric, and two metal atoms are linked by a planar di-µ-phosphido-bridge. The environment of each metal atom is approximately octahedral, and there is in addition a metal–metal bond 3·09 Å long. It is concluded that the interaction between the metal atoms is direct, through d_xy orbitals, and not via the phosphorus atoms. The possible role of π-bonding in this molecule is discussed. The difficulty experienced in obtaining a monosubstituted product from the reaction between [(OC)₄Mo{P(CH₃)₂}₂Mo(CO)₄] and PEt₃ is attributed to a labilising influence transmitted across the diphosphidobridge, but there is insufficient evidence to show whether or not this influence is directive in determining the orientation of substitution, or whether or not it is transmitted via the metal–metal bond.