A transition metal phosphido complex is a coordination complex containing a phosphido ligand. With two lone pairs on phosphorus, the phosphido anion is comparable to an amido anion, except that the M-P distances are longer and the phosphorus atom is more sterically accessible. For these reasons, phosphido is often a bridging ligand.
Synthesis
Phosphido ligands are often installed by salt metathesis reactions. Sources of R2P+ and R2P− are provided by phosphorus halides and alkali metal phosphides respectively. Illustrative of the use of R2PCl-like reagents is the synthesis of a diiron diphosphide: The alternative salt metathesis route involves the reaction of alkali metal diorganophosphides with metal halides. A typical phosphide reagent is lithium diphenylphosphide. Alkali metal phosphides sometimes reduce the metal center. Another way to generate the transition-metal phosphido complexes are by direct activation of P-H bonds and is mostly seen in the late-transition-metal complexes. For example, the reaction of Vaska's complex analogs with the parent phosphine generate the following transition-metal phosphido complex.
Structure
Complexes of the phosphide ligand can be classified into one of three classes:
those where the phosphide is a terminal ligand and phosphorus is pyramidal,
those where the phosphide is a terminal ligand and phosphorus is planar,
those where the phosphide is a bridging ligand and phosphorus is tetrahedral.
Terminal phosphido ligands
In most complexes with terminal phosphido ligands, phosphorus is pyramidal, as expected with a stereochemically active lone pair of electrons. The M-P bond length in the pyramidal phosphide complex is longer than the M-P bond length in corresponding transition metal phosphine complexes. The pyramidal phosphido complex. In the complex, the Os-PHPh bond is 0.11 Å longer than the Os-PPh3 and the Os-P-C angle is 113o. The elongated Os-PHPh bond is often attributed to the electronic repulsion of the lone pair and nonbonding electrons on Os. Also, in another ruthenium complex, the Ru-PPh bond is 0.17 Å longer than Ru-PHPh in the related phosphine ligand version of the complex, +. Additionally electronic repulsion of the P-centered lone pair and metal-based electrons enhance the nucleophilicity of the phosphide ligand. This high basicity and high nucleophilicity leads to the dimerization reaction. As implied in the resonance structures A and B, some terminal phosphido ligands engage in M-P multiple bonding. In the resonance structure A, the lone pair from the p-orbital on phosphorus donates to the vacant orbital on the metal to form a π-bond. Because of the π-bonding interaction in resonance structure A, it is planar at phosphorus and M-P bond-length is shorter and M-P-R bond-angle is larger. Planar phosphido complexes usually have shorter M-P bonds and larger M-P-R angles. In the tungsten complex, the W-PHPh bond is 0.26 Å shorter than W-PEt3 bond in the same complex, and the W-P-C angle is 140°. Another example is a ruthenium complex. In those complex, the Ru-PCy2 bond is 0.11 Å shorter than Ru-PPh3 bond and the Ru-P-C angle is 127°. While the planar and pyramidal phosphides can be distinguished clearly, in one case, a pyramidal phosphide can be converted to planar phosphide by one-electron oxidation. The inversion of configuration at pyramidal terminal phosphides has been observed by 31P NMR spectroscopy.
Bridging phosphido ligands
In most of its complexes, the phosphido ligand is a bridging ligand. No lone pairs remain on phosphorus. These complexes have the formula2. One example is 2.
Applications
Metal phosphido complexes are however intermediates the catalytic hydrophosphinations. Some late metal hydrophosphination catalysts rely on oxidative addition of a P-H bond. For example, a Pt catalyst that undergoes oxidative addition of a secondary phosphine to form the corresponding Pt phosphido complex, which react with electrophilic alkenes such as acrylonitrile. This P-C bond forming step proceeds through an outer-sphere, Michael-type addition. cAlkene insertion into the metal-hydrogen bond is also invoked in some hydrophosphinations. Metal phosphide have been used in the synthesis of P-stereogenic phosphines by exploiting the high nucleophilicity in the pyramidal phosphide complex.
