page_title: Transition metal phosphido complexes
section_title: Terminal phosphido ligands
hierarchical_section_title: Transition metal phosphido complexes / Structure / Terminal phosphido ligands
caption_attribution_description: English: Two resonance structure of phosphido complex
context_page_description: 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.
context_section_description: 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-P(Me)Ph bond is 0.17 Å longer than Ru-PH(Me)Ph in the related phosphine ligand version of the complex, [(dmpe)₂Ru(H)PH(Me)Ph]⁺. 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.
caption: Resonance structures for complexes of planar phosphido ligands.
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