Quantification of the trans influence in d8 square planar and d6 octahedral complexes: a database study

A systematic search of the Cambridge structural database was undertaken to quantify the trans influence in square planar and octahedral transition metal compounds. For square planar geometry, d⁸ metal centers were studied, while octahedral searches focused on low-spin d⁶ complexes. Two probe ligands (PL) were used to measure the effect of the trans ligand (TL), chloride, and triphenylphosphine (PPh₃). For the TLs O=CX₂, NR₃, pyridine, and Cl^? (X?=?any non-metal, R?=?H or hydrocarbon), the effects on the metal–probe ligand (M–PL) distance were statistically equal and were taken as essentially no trans influence. The other ligands studied showed significant decrease in the mean M–PL bond order, relative to the above ligands: SR₂?=?0.941; S=CX₂?=?0.887; PPh₃?=?0.825; phenyl?=?0.743; CR₃?=?0.719; hydride?=?0.685. Some variation in the trans influence is observed, based on the geometry of the metal center and the PL. In general, electron-donating, σ-bonding ligands lead to a larger trans influence, but π-bonding effects can also be important, particularly when the probe ligand also has π-bonding properties.