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. 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. Many transition metal phosphinimide complexes have been well-developed as have main group phosphinimide complexes.

In main group phosphinimide complexes, only terminal and μ²-N-bridging bonding modes are observed. The terminally bound bent ligands are primarily commonly have M-N-P bond angles ranging from 120-150°. 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. These dimeric complexes yield different M-N bond lengths depending on the ligands present in the rest of the ligand sphere of M. 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.