Six- and five-co-ordinate ruthenium(II) complexes with 1,2-bis(dicyclohexylphosphino)ethane. X-Ray crystal structure of the d6 five-co-ordinate [RuCl{(C6H11)2PCH2CH2P(C6H11)2}2]PF6

Six- and five-co-ordinate complexes of ruthenium(II) with the bulky dipyhosphine ligand 1,2-bis(dicyclohexylphosphino)ethane (dcpe) have been prepared. In solution, octahedral trans-[RuX2(dcpe)2](X = Cl or Br) and trans-[RuH(Cl)(dcpe)2] can dissociate giving the unsaturated species [RuX(dcpe)2]+ and [RuH(dcpe)2]+, respectively. Phosphorus-31 n.m.r. studies show that the five-co-ordinate d6 cations, which are stereochemically non-rigid at room tempreature, possess a trigonal-bipyramidal geometry with the halide or hydride in an equatorial position. The X-ray crystal structure of [RuCl(dcpe)2]PF6 has been determined: the crystals are monoclinic, space group P21/c, with a= 14.614(3), b= 14.625(3), c= 26.196(3)A, β= 100.41(1)°, and Z= 4; final R factor 0.052 for 3 219 observed reflections. The ruthenium cation has an approximate trigonalbipyramidal geometry, with the bidentate dcpe ligands bridging axial and equatorial postions. This is the first five-co-ordinate complex of ruthenium(II) structurally characterized as trigonal bipyramidal. According to their co-ordinatively unsaturated nature, the complexes [RuX(dcpe)2]+ react with L(L = CO or MeCN) to form the six-co-ordinate [RuX(L)(dcpe)2]+, whose stereochemistry and dynamic behavious were studied by 31P n.m.r. spectroscopy. The influence of the steric properties of the diphosphine on the formation of the [RuX(dcpe)2]+ species is discussed, and a dominant role for steric effects is suggested.