STERIC STABILIZATION OF 20-ELECTRON SEMI-SANDWICH COMPLEXES - CRYSTAL-STRUCTURE OF [(C5H5)CO(P(OME)2O)3NI(C5PH5)]

The nickel complexes [(C5Ph5)Ni(CO)Br] and [(C5Ph5)Ni(CO)(ClO4)] react with the sodium salts Na[(C5H5)Co{P(R)2O}3] in dichloromethane to yield the compounds [(C5H5)Co{P(R)2O}3Ni(C5Ph5)] (R = OMe (2a), OEt (2b), O(i)Pr (2c)). The crystals of 2a are triclinic, space group P1BAR with a 1226.4(2), b 2014.7(4), c 2047.3(4) pm, alpha-61.09(3), beta-81.33(3), gamma-89.56(3)degrees, Z = 4, and R = 0.042 for 9353 observed reflections. Apart from the cyclopentadienyl ring the molecules approximate to three-fold symmetry. Three OP(OMe)2-units bridge the two metals with all three phosphoryl oxygen atom-nickel distances being equal. The 20-electron compounds 2a-c are stable at room temperature and paramagnetic in solution and in the solid state. The magnetic moment of 2a and 2b in the solid state has been determined, it corresponds to two unpaired electrons. The cyclovoltammograms of 2a-c demonstrate that they can be oxidised quasi-reversibly at almost-equal-to +0.26 V (vs SCE). The cations formed could not be isolated; they decompose with elimination of the stable pentaphenylcyclopentadienyl radical and formation of the nickel(II) complex [{(C5H5)Co[P(R)2]3}2Ni]. The cyclic voltammogram of [{(C5H5)Co[P(OEt)2O]3}2Ni] shows a reversible nickel(II/III) wave at +1.2 V (vs SCE).