The [Formula: see text] cation, where chiraphos is 2,3-bis(diphenylphosphino)butane, has been synthesized and fully characterized by spectroscopy and single crystal data on the chloride salt (1). Crystals are monoclinic, a = 23.106(3), b = 12.4987(8), c = 21.455(3) Å, β = 128.539(5)°, Z = 4, space group C2. The structure was solved by conventional heavy atom methods and was refined by full-matrix least-squares procedures to R = 0.046 and Rw = 0.032 for 4509 reflections with I > σ(I). The coordination about Rh is square planar distorted toward tetrahedral within the five-membered chelate rings, similar to that in [Formula: see text], where dpe is 1,2-bis(diphenylphosphino)ethane, but the distortion toward tetrahedral is significantly greater in 1; the methyl substituents further restrict rotational freedom about the P—Ph bonds, which leads to some blocking of the axial ligand sites by the four H atoms of the chelate-ring carbon atoms. In solution, 1 is unreactive towards O2, CO, and H2, but oxidatively adds HX (X = Cl, Br) to give trans-[Formula: see text] species.