Crystal structure of μ -2,2' -bipy rimidine-hexaca rbonyl-bis(triph enyl- phosphine)dirhen ium(I) bis(trifluorome thanesulfonate) acetonitrile tetrasolvate, (Re2(C8H6N4)(C0 )6(Ci8H 15 P)2)(F3 CS03)2· 4CH 3 CN