Phosphorus–fluorine compounds. Part XII. Phosphorus-31 and fluorine-19 nuclear magnetic resonance studies on fluorophosphine molybdenum complexes. Evaluation of geminal phosphorus–phosphorus spin-coupling constants, |2JPP′|, in cis-disubstituted octahedral complexes

Abstract

The ¹⁹F and ³¹P n.m.r. spectra of complexes of the type cis-L₂Mo(CO)₄, [where L is a fluorophosphine ligand, e.g. PF₃, CF₃PF₂, CCl₃PF₂, ClCH₂PF₂, Me₂NPF₂, Et₂NPF₂, and (CF₃)₂PF], have been analysed as examples of X_nAA′X_n′ spin-systems, and spin-coupling constants |¹J_PF|, |³J_PF′|, and |²J_PP′| have been accurately evaluated. For the [RPF₂]₂Mo(CO)₄ series of compounds, |²J_PP′| decreases in the order PF₃ > CF₃PF₂∼ CCl₃PF₂ > ClCH₂PF₂ > Me₂NPF₂∼ Et₂NPF₂, which is the order of decreasing electron-withdrawing power of R. Factors which affect the magnitude of |²J_PP′| are discussed. The n.m.r. spectra of the dialkylaminodifluorophosphine complexes provide very strong evidence for co-ordination to the metal via phosphorus rather than nitrogen.