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 long hydrocarbon chains to link the donor functionalities, but such ligands often lead to

coordination polymer A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, ...

History

A diphosphane linked with

pentamethylene Cyclopentane (also called C pentane) is a highly flammable alicyclic hydrocarbon with chemical formula C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occu ...

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 henanthrene 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 is an organophosphorus compound derived from the heterocycle xanthene. It is used as a bidentate diphosphine ligand and is noteworthy for having a particularly wide bite angle (108°). Such ligands are useful in the hydroformylation ...

is a trans-spanning ligand, without the steric problems associated with TRANSPHOS. SPANphos is comparable to XANTPHOS but more reliably trans-spanning. TRANSDIP, based on a α-

cyclodextrin Cyclodextrins are a family of cyclic oligosaccharides, consisting of a macrocyclic ring of glucose subunits joined by α-1,4 glycosidic bonds. Cyclodextrins are produced from starch by enzymatic conversion. They are used in food, pharmaceutica ...

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

References

{{reflist Coordination chemistry