The Rhenium Tris(dithiolene) Electron Transfer Series: Calibrating Covalency

Four members of the rhenium tris­(dithiolene) electron transfer series have been prepared, [Re­(S2C2R2)3]z {R = Ph, z = 1+ (1), 0 (2), 1– (3); R = CN, z = 2– (4)}, with the anions in 3 and 4 structurally characterized. The intraligand C–S and C–C bond lengths for 3 vs 2 are indicative of ligand reduction concomitant with an overall distorted trigonal prismatic geometry (Θ = 26.3° cf. 3.8° in 2). The distorted octahedral ReS6 polyhedron in 4 (Θ = 38.3°) indicates reduction of the metal to a Re­(IV) d3 central ion. This series has been probed by sulfur K-edge X-ray absorption spectroscopy (XAS), and the electronic structures are unambiguously defined as follows: [ReV(L34–)]1+ (S = 0) for the monocation in 1; [ReV(L35–•)]0 (S = 1/2) for neutral 2; [ReV(L36–)]1– (S = 0) for the monoanion in 3; and [ReIV(L36–)]2– (S = 1/2) for the dianion in 4. The sulfur 3p character in the frontier orbitalsthe covalencyis estimated by two different approaches. Method A utilizes the radial dipole integral (Is) derived from the S 1s → 4p transition, whereas method B, involves time-dependent density functional theoretical (TD-DFT) calculation of the pre-edge transitions and calibrated to the intensity in [Re­(pdt)3] (pdt2– = 1,2-diphenyl-1,2-dithiolate). The two estimates are contrasted for the rhenium series and extended to the [V­(pdt)3]0/1–, and [Mo­(mdt)3]0/1–/2– (mdt2– = 1,2-dimethyl-1,2-dithiolate) series, ultimately providing a refined description of the contested electronic structure of neutral molybdenum (and tungsten) tris­(dithiolenes) compounds.