Reaction of cis-bis(acetato)hexakis(acetonitrile)dimolybdenum(2+) tetrafluoroborate with dmpe and S,S-dppb: preparation, crystal structure, and spectroscopic properties of trans-[Mo2(MeCO2)2(dmpe)2][BF4]2 (dmpe = bis(dimethylphosphino)ethane, S,S-dppb = (2S,3S)-bis(diphenylphosphino)butane)