From isolated dimers to an ordered antiferromagnetic ground state in cation radical salts of Cp2Mo(dmit) with small anions (Br-, BF4-)

The structural and magnetic properties of two 1:1 salts of the open-shell d (1) organometallic radical cation Cp2Mo(dmit)(+.) (dmit(2-) for 2-thioxo -1 ,3-dithiole-4,5-dithiolate) with BF4- and Br- are described and analyzed. T he two structures differ not only from each other but also from those repor ted for the three isostructural PF6-, AsF6-, SbF6- salts with an unfolded M oS2C2 metallacycle, In [Cp2Mo(dmit)][BF4], the MoS2C2 metallacycle is folde d by 23.21(5)degrees along the S-S hinge and the cations are associated int o inversion-centered dimers weakly interacting with each others as reflecte d by the magnetic susceptibility measurements. In [Cp2Mo(dmit)][Br], the Mo S2C2 metallacycle is folded by 30.45(4)degrees and the Br- anion is involve d in C-H (...) Br hydrogen bonds. The cations interact antiferromagneticall y, giving rise to a three-dimensional set of HOMO-HOMO intermolecular inter actions and confirmed by the stabilization of an antiferromagnetic ground s tate below T-Neel = 4.5 K. (C) 2001 Academic Press.