Trans-spanning ligands are bidentate ligands that can span opposite sites of a complex with square-planar geometry. A wide variety of ligands that chelate in the cis fashion already exist, but very few can link opposite vertices on a coordination polyhedron. Early attempts to generate trans-spanning bidentate ligands relied on polymethylene chains to link the donor functionalities, but such ligands often lead to coordination polymers.

History

A diphosphane linked with pentamethylene was claimed to span across a square planar complex. This early attempt was followed by ligands with more rigid backbones. "TRANSPHOS" was the first trans-spanning diphosphane ligand that usually coordinates to palladium(II) and platinum(II) in a trans manner. TRANSPHOS features benzo[c]phenanthrene substituted by diphenylphosphinomethyl (Ph₂PCH₂) groups at the 1 and 11 positions. The polycyclic framework suffers sterically clashing hydrogen centers.

Xantphos, SPANphos, TRANSDIP and related ligands

Xantphos

Xantphos

Xantphos is an organophosphorus compound derived from the heterocycle xanthene. It is used as a bidentate ligand and is noteworthy for having a particularly wide bite angle. Such ligands are useful in the hydroformylation of alkenes. Illustrative of its wide bite angle, it forms both cis and...

 is a trans-spanning ligand, without the steric problems associated with TRANSPHOS. SPANphos

SPANphos

SPANphos is an organophosphorus compound used as a ligand in organometallic and coordination chemistry. This compound is a rare example of a trans-spanning ligand and rigidly links mutually trans coordination sites. By virtue of its chiral backbone that forms a chiral cavity over the face of a...

 is comparable to XANTPHOS but more reliably trans-spanning. TRANSDIP, based on a α-cyclodextrin

Cyclodextrin

Cyclodextrins are a family of compounds made up of sugar molecules bound together in a ring ....

, is the first ligand to give exclusively trans-spanned complexes, even with d⁸ metal ion halides.