Divergent Electronic Structures of Isoelectronic Metalloclusters: Tungsten(II) Halides and Rhenium(III) Chalcogenide Halides

Same but different : DFT calculations on hexanuclear tungsten(II) halide clusters [W 6 X 8 X′ 6 ] 2− (X, X′=Cl, Br, I) indicate a breakdown in the isoelectronic analogy between themselves and the isostructural rhenium(III) chalcogenide clusters [Re 6 S 8 X 6 ] 4− (see figure).The hexanuclear tungsten(II) halide clusters and the sulfido‐halide clusters of rhenium(III) are subsets of a broad system of 24‐electron metal–metal bonded assemblies that share a common structure. Tungsten(II) halide clusters and rhenium(III) sulfide clusters luminesce from triplet excited states upon ultraviolet or visible excitation; emission from both cluster series has been extensively characterized elsewhere. Reported here are density‐functional theory studies of the nine permutations of [W 6 X 8 X′ 6 ] 2− (X, X′=Cl, Br, I). Ground‐state properties including geometries, harmonic vibrational frequencies, and orbital energy‐level diagrams, have been calculated. Comparison is made to the sulfide clusters of rhenium(III), of which [Re 6 S 8 Cl 6 ] 4− is representative. [W 6 X 8 X′ 6 ] 2− and [Re 6 S 8 Cl 6 ] 4− possess disparate electronic structures owing to the greater covalency of the metal–sulfur bond and hence of the [Re 6 S 8 ] 2+ core. Low‐lying virtual orbitals are raised in energy in [Re 6 S 8 Cl 6 ] 4− with the result that the LUMO+7 (or LUMO+8 in some cases) of tungsten(II) halide clusters is the LUMO of [Re 6 S 8 Cl 6 ] 4− species. An inversion of the HOMO and HOMO−1 between the two cluster series also occurs. Time‐dependent density‐functional calculations using asymptotically correct functionals do not recapture the experimentally observed periodic trend in [W 6 X 14 ] 2− luminescence ( E em increasing in the order [W 6 Cl 14 ] 2− < [W 6 Br 14 ] 2− < [W 6 I 14 ] 2− ), predicting instead that emission energies decrease with incorporation of the heavier halides. This circumstance is either a gross failure of the time‐dependent formalism of DFT or it indicates extensive multistate emission in [W 6 X 8 X′ 6 ] 2− clusters. The inapplicability of isoelectronic analogies between clusters of Group 6 and Group 7 is emphasized.