A Density Functional Study of the Highly Adaptable Molecular Structure of Mo(V) and W(V) Dithiolene Complexes:  From Three-Dimensional Antiferromagnet to Spin Ladder

Cp2M(dithiolene) (M = MoIV, WIV) d2 complexes exhibit a folding (θ) of the MS2C2 metallacycle along the S−S axis upon oxidation to the d1 paramagnetic cation. The evolution of the peculiar unfolded structure (θ = 0°) of Cp2Mo(dmit)•+ (dmit2-, 2-thioxo-1,3-dithiole-4,5-dithiolate) in its AsF6 - salt toward the introduction of heavy atoms (W, Se) is examined here by substituting dsit2- (dsit2-, 2-thioxo-1,3-dithiole-4,5-diselenolate) for dmit2- and/or W for Mo in those AsF6 - salts. While [Cp2Mo(dsit)•+][AsF6 -] (orthorhombic, Cmcm, a = 9.071(2), b = 20.868(4), c = 10.243(2) Å, V = 1938.9(7) Å3, Z = 4) and [Cp2W(dmit)•+][AsF6 -] (orthorhombic, Cmcm, a = 9.0295(15), b = 20.568(2), c = 10.2641(12) Å, V = 1906.2(4) Å3, Z = 4) crystallize with an unfolded geometry and are isostructural with [Cp2Mo(dmit)•+][AsF6 -], the introduction of both W and dsit2- in Cp2W(dsit) stabilizes a fully different structure for [Cp2W(dsit)•+][AsF6 -] (monoclinic, P21/c, a = 6.8442, b = 15.9923(13), c = 17.6594(16) Å, β = 90.934(11)°, V = 1932.6(3) Å3, Z = 4) with the Cp2W(dsit)•+ cation in a folded geometry (θ = 30.1(1)°). DFT calculations show that the variety of folding angles and hence of molecular structures found in those open-shell d1 Cp2M(dithiolene)•+ complexes is attributable to a very low energy cost (