Dedicated to Professor Joachim Bargon (Universit‰t Bonn) on the occasion of his 65 th birthday.Abstract: The synthesis of a novel class of sterically demanding diphosphonites 1 ± 8, based on rigid backbones, is described. The starting materials are all commercially available and the methodology allows for a modular approach. All ligands have been fully characterized, including an X-ray crystal structure for compound 1, 4,5-bis{di[(2-tert-butyl)phenyl]phosphonito}-9,9-dimethylxanthene. The coordination of these diphosphonite ligands towards Ni(II) and Ni(0) precursors is investigated, both by NMR spectroscopy as well as X-ray crystallography and compared with the behaviour of diphosphine ligands such as Xantphos. The molecular structure for complex 9, trans-[NiBr 2 (1)] is described in detail. The nickel-catalyzed isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile is studied, a relevant step in the industrially important hydrocyanation of butadiene (the DuPont adiponitrile process). Good activities and selectivities to the desired 3-pentenenitrile are obtained in this reversible C À C bond activation reaction.