Why is Re-Re bond formation/cleavage in [Re(bpy)(CO)3]2 different from that in [Re(CO)5]2?: Experimental and theoretical studies on the dimers and fragments

The Re(NN)(CO)(3)(THF) (NN = bpy = 2,2'-bipyridine or dmb = 4,4'-dimethyl-2,2'-bipyridine) radical, produced by homolysis of [Re(NN)(CO)(3)](2) in THF solution by visible irradiation, dimerizes with rate constants k(d) = 20 +/- 3 and 11 +/- 4 M-1 s(-1) for NN = dmb and bpy, respectively. The dimerization processes are strikingly slow compared to those of typical metal radicals including Re(CO)(5) (k(d) approximate to 10(9) M-1 s(-1)). In order to explain such slow reactions, we have performed B3LYP hybrid DFT and fully ab initio RHF and MP2 calculations on several conformations of [Re(bpy)(CO)(3)](2) (cis, trans, skewed cis, skewed trans) and [Re(CO)(5)](2) (staggered) and on their constituent monomer radicals and anions. The calculations show that the most stable geometry of [Re(bpy)(CO)(3)](2) is skewed cis, and the experimental infrared spectrum and photochemical properties of the [Re(bpy)(CO)(3)](2) dimer are best described by the calculated properties of the skewed cis conformer in which there is no low-lying unoccupied orbital that is predominantly sigma(MM)* in character. The Re(bpy)(CO)(3)(THF) ligand radical is more stable than the 5-coordinate 17-electron metal radical, Re(bpy)(CO)(3), Suggesting that the extremely slow dimerization rate most likely arises from the solvent blocking the binding site (i.e., the estimated fraction of the five-coordinate monomer is 1.6 x 10(-2)). Theoretical results are consistent with our experimental results that the dimerization process proceeds via the Re centered radical, which is involved in a pre-equilibrium favoring the ligand-centered radical. Furthermore, time-dependent DFT calculations on [Re(bpy)(CO)(3)](2) and [Re(bpy)(CO)(3)](-) identify the origin of UV-vis absorption in THF.