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) (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) (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) (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) is representative. W(6)X(8)X'(6) and Re(6)S(8)Cl(6) possess disparate electronic structures owing to the greater covalency of the metal-sulfur bond and hence of the Re(6)S(8) core. Low-lying virtual orbitals are raised in energy in Re(6)S(8)Cl(6) 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) 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) luminescence (E(em) increasing in the order W(6)Cl(14) < W(6)Br(14) < W(6)I(14)), 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) clusters. The inapplicability of isoelectronic analogies between clusters of Group 6 and Group 7 is emphasized.