Studies of the Structures and Bonding of Gold-Bridged Dirhenium Carbonyl Cluster Complexes

The compounds Re2­(CO)8­(μ-AuPPh3)2, 1, a dimer of Re­(CO)4­(μ-AuPPh3), and ax,ax-Re2­(CO)8­(PPh3)2 were obtained from UV–vis
radiation-induced decarbonylation of the compound Re­(CO)5­[Au­(PPh3)]. Compound 1 contains two rhenium atoms bridged by two AuPPh3 groups.
The complex has 32 valence electrons and is formally unsaturated by
the amount of two electrons. The Re–Re bond distance in 1 is unusually short (Re–Re = 2.9070(3) Å), as
found by a single-crystal structural analysis. The nature of the metal–metal
bonding in 1 was investigated by DFT computational analyses,
which have provided evidence not only for σ-bonding but also
significant complementary π-bonding directly between the two
rhenium atoms. The electronic structure of Re2­(CO)8­(μ-H)2, 2, was similarly
analyzed and is compared with that of 1. Compound 1 is intensely colored due to low-energy, metal-based electronic
transitions between the HOMO and HOMO-2 and the LUMO. Compound 1 reacts with I2 to yield Re2­(CO)8­(μ-AuPPh3)­(μ-I), 3, and the known compound Re2­(CO)8­(μ-I)2, 4, by substitution of
the bridging AuPPh3 groups with bridging iodide ligands.
Compound 3 is electronically saturated, 34 valence electrons,
and contains a formal Re–Re single bond: Re–Re = 3.2067(5)
Å. Compound 3 was also in a high yield (83%) from
the reaction of Re2­(CO)8­(μ-H)­(μ-CHCHC4H9) with Au­(PPh3)­I. The Re–Re
bonding in compounds 3, 4, and Re2­(CO)10 was also analyzed computationally, and this
bonding was compared with their bonding in 1 and 2.