Structural differences in nickel(II) and palladium(II) complexes containing cis-1,2-bis(diphenylphosphino)ethene or 1,2-bis(diphenylphosphino)ethane

Article abstract of DOI:10.1016/S0277-5387(97)00009-0

Monometallic Ni^II and Pd^II complexes of cis-1,2-bis(diphenylphosphino)ethene (cis-dppen) or 1,2-bis(diphenylphosphino)ethane (dppe) have been prepared and characterized by X-ray diffraction methods, ³¹P ¹H NMR spectroscopy, FAB mass spectrometry, elemental analyses and melting points. The X-ray structure of [NiCl₂(cis-dppen)] (1) is given for the first time. In 1 the mean deviation of the nickel, the phosphorus and the chlorine atoms from a least-squares plane through these atoms is only 0.025 A. The ethene bridge is nearly coplanar with this plane, which can be explained by a π-bonding interaction. It is shown that the small deviation from coplanarity are caused by a crystal packing effect. The reaction of Ni(CN)₂ with cis-dppen leads to the novel compound [Ni(CN)₂(cis-dppen)] (2). The removal of chloride in 1 by AgBF₄ followed by the reaction with 1,10-phenanthroline (phen) leads to the novel octahedral Ni^II complex [Ni(cis-dppen)(phen)₂](BF₄)₂ (3). In an analogous manner 2,2′-bipyridine (bipy) produces [Ni(cis-dppen)(bipy)₂] (BF₄)₂ (4). The reaction of trans-1,2-bis(diphenylphosphino)ethene (trans-dppen) with PdCl₂ and subsequent treatment with LiI leads to the unexpected product [PdI₂(dppe)] (5). Compound 5 is also fully characterized by an X-ray structure analysis for the first time. The palladium, phosphorus and iodine atoms show a mean deviation of only 0.017 A from a least-squares plane through these atoms. However, the PdP₂C₂ chelate ring has a twist conformation, where one carbon atom of the aliphatic chain of dppe lies - 0.420 A to one side, and the second carbon atom 0.318 A to the other side of the coordination plane. In 5 the P-Pd-P chelate angle of 85.59(5)° is significantly smaller than the corresponding P-Ni-P angle of 87.79(4)° in 1. Furthermore, the X-ray structure of the recently reported complex [Pd(cis-dppen)₂](BPh₄)₂ (6) is given. The palladium atom is located on a centre of symmetry leading to the complete planarity of the coordination plane due to crystallographic constraints. However, the two carbon atoms of the ethene bridge of cis-dppen lie 0.547 and 0.532 A, respectively, to one side of the coordination plane. Compound 6 significantly shows the smallest P-M-P chelate angle of 84.78(3)° compared with the corresponding values in 1 and 5. A comparison of the X-ray structures of 1, 5 and 6 with several known X-ray structures of Ni^II and Pd^II complexes containing cis-dppen or dppe is given. Differences in the chelate angles and coplanar parts of these structures are discussed with respect to the observed reactivities of compounds 1-6.