Transition metal trifluorophosphine complexes are transition metal coordination complexes containing one or more trifluorophosphine (PF3) ligands. These complexes are a subclass of transition metal-phosphine complexes. Although trifluorophosphine complexes lack commercial applications, they have been of enduring fundamental interest. As one distinctive feature, they form many simple homoleptic complexes with zero-valence metals, more so than does carbon monoxide (CO).
Ligand properties Trifluorophosphine is classified as an L ligand in the covalent bond classification method. In the usual electron counting method, it is a two-electron ligand. With respect to HSAB theory, it is soft, reflecting its significant properties as a pi-acceptor ligand. The presence of three electronegative fluorine substituents gives rise to relatively low energy P-F σ* orbitals, which are well positioned to overlap with metal-based orbitals of π-symmetry.
Preparative routes and compounds
Synthetic methodology is complicated by the fact that trifluorophosphine is a highly toxic colorless gas. As the first trifluorophosphine complex to be reported, Ni(PF3)4, an analog of Ni(CO)4, was originally prepared by the reaction of Ni(PCl3)4 with excess PF3, a ligand exchange process. It is a colorless volatile liquid that is stable to water. The same complex can be prepared by combining nickel metal with PF3 at 35 MPa pressure and 100 °C. Cr(PF3)6, the analogue of Cr(CO)6, can be prepared by displacement of benzene from dibenzenechromium:
The analogue of iron pentacarbonyl is , a yellow volatile solid. It was first prepared by reducing ferrous iodide with zinc under a pressure of trifluorophosphine. The approach is analogous to the reductive carbonylation route to metal carbonyls.
In addition to forming analogs of simple metal carbonyls, trifluorophosphine forms a variety of complexes for which the corresponding CO derivatives are unstable or nonexistent. Thus, Pd(PF3)4 is known, but Pd(CO)4 is not.
Reactions Photolysis of under an atmosphere of hydrogen furnishes the dihydride:
This dihydride is a colorless liquid with a boiling point of 87 °C. By contrast, iron tetracarbonyl dihydride () decomposes near its boiling point of -20 °C.
Further reading More modern perspectives and computational analysis: Some early observations:
References