Phosphinimide ligands

Phosphinimide ligands, also known as phosphorane iminato ligands, are any of a class of organic compounds of the general formula NPR₃⁻. The R groups represent organic substituents or, in rare cases, halides or NR₂ groups. NPR₃⁻ is isoelectronic with phosphine oxides (OPR₃) and siloxides ([OSiR₃]⁻), but far more basic.^[1][2] By varying the R groups on P, a variety of ligands with different electronic and steric properties can be produced, and due to the high oxidation state of phosphorus, these ligands have good thermal stability.^[3] Many transition metal phosphinimide complexes have been well-developed as have main group phosphinimide complexes.^[1]

In main group phosphinimide complexes, only terminal and μ²-N-bridging bonding modes are observed.^[2] The terminally bound bent ligands are primarily commonly have M-N-P bond angles ranging from 120-150°.^[2] Both the M-N and N-P bond lengths are appropriate for double bonds. This bonding can best be described by a covalent single bond with an overlaying share of polar bonding. The μ²-N-bridging mode arises when the free electron pair at nitrogen gives rise to dimerization.^[2] These dimeric complexes yield different M-N bond lengths depending on the ligands present in the rest of the ligand sphere of M.^[2] When the complex contains two or four identical ligands, nearly equal M-N distances are observed, whereas, when different or odd-numbered identical ligands are in the complex, the M-N distances are all of significantly different length.^[2]