Carbonyl halides of the Group VI transition metals. XXIV. 1,2-Bis(diphenylarsino)ethane complexes of the molybdenum(II) halocarbonyls

Abstract

Complexes of the general formulae Mo(CO)₃(dae)X₂ and [Mo(CO)₂(dae)_1.5X₂]₂ [X = Cl, Br, I; dae = 1,2-bis(diphenylarsino)ethane] have been prepared and characterized. The chloro- and bromo-tricarbonyl species exist in two isomeric forms. All of the dae-bridged dimeric complexes can be cleaved by carbon monoxide, to reform the corresponding tricarbonyl, and by excess dae to give soluble dicarbonyl compounds which are thought to be of the type Mo(CO)₂(dae)₂X₂. Unfortunately, the equilibria                [Mo(CO)₂(dae)_1.5X₂]₂+dae ↔ 2Mo(CO)₂(dae)₂X₂ are very labile, and all attempts to isolate the bis(dae) complexes in a pure form resulted in the precipitation of the sparingly soluble dimeric species.     Comparison of the complexes derived from the halocarbonyls with dae and other ditertiary phosphines and arsines shows that, as the halogen is varied, the changes in the relative stabilities of the complexes obtained within each ligand series are all explicable on the basis of a self-consistent steric argument, but comparison of the complexes of dae and its diphosphine analogue, dpe, suggests that electronic effects become significant on changing from arsenic to phosphorus donor atoms.