A zwitterionic phosphanyl-substituted phosphonio-benzo[c]phospholide as bridging ligand towards dinuclear transition metal carbonyls

The zwitterionic 1-triphenylphosphonio-3-diphenylphosphanyl-benzo[c]phospholide (3) reacts with M2(CO)10 (M=Mn, Re) to form the dinuclear complexes [{M(CO)4}2(μ-3)] (7, 8) in which the metalmetal bond is retained and both metals are bound via the two phosphorus lone-pairs. In contrast, reaction of 3 with Cp2Mo2(CO)4 (MoMo) proceeds with complete cleavage of the metalmetal bond to yield as final product a complex [{MoCp(CO)2}2(μ-3)] (10) in which both phosphorus lone-pairs of 3 co-ordinate to one metal atom, and the second metal atom is attached in a η2(π)-co-ordination mode to the benzophospholide π-electron system. All complexes 7, 8, and 10 were characterised by analytic and spectroscopic data and X-ray diffraction studies. The formation of 10 is preceded by that of an intermediate which could likewise be isolated and whose constitution was assigned on the basis of spectroscopic studies as an isomer of 10 which still contains a metalmetal single bond. The behaviour of 3 marks an interesting contrast to that of 2-phosphanyl-phosphinines which react under similar conditions exclusively under co-ordination via the phosphorus lone-pairs, and fail to induce any breakage of metalmetal bonds. The origin for the observed deviations is discussed in terms of the different π-acceptor properties of the two types of ligands.