Cobaltocene reductions of multiply bonded dirhenium complexes: isolation, characterization and reactivity studies of [(.eta.5-C5H5)2Co][Re2(O2CR)4Cl2], [(.eta.5-C5H5)2Co][Re2Cl6(PR3)2] and [(.eta.5-C5H5)2Co][Re2Cl5(PR3)3]