Further studies on the reactivity of complexes of the type cis-Re2(μ-O2CR)2X2(μ-dppm)2. The structural characterization of cis-Re2(μ-O2CCH3)2(NCBH3)2(μ-dppm)2 and [Re2Cl3(μ-dppm)2(1,2-C6H4(CN)2)2]PF6