Do Metal-Metal Multiply-Bonded Ligands Have a trans Influence? Structural and Magnetic Comparisons of Heterometallic CrCr•••Co and MoMo•••Co Interactions

Reported here are two new compounds containing either a Cr Cr center dot center dot center dot Co [1, CrCrCo(dpa)(4)Cl-2, dpa = 2,2'-dipyridylamide] or a Mo Mo center dot center dot center dot Co [2, MoMoCo(dpa)(4)Cl-2] framework both having a multiply-bonded unit (Cr Cr in 1, Mo Mo in 2) in close proximity to the Co2+ ion and trans to a Co-Cl bond. Variable temperature magnetic susceptibility measurements reveal 1 to have a temperature-dependent spin equilibrium between a low-spin (S = 1/2) and high-spin (S = 3/2) state, whereas the Co2+ ion in 2 exists solely in its high-spin state. The crystal structures of 1 and 2 were determined. Variable temperature crystallographic data of 1 at 100 K and at room temperature reveal that the spin-transition affects not only the Co-ligand bond lengths but also the terminal Cr-ligand bond lengths, Whereas the Cr center dot center dot center dot Co distance becomes shorter by 0.13 angstrom in the low-spin form, the Co-Cl distance becomes longer by 0.2 angstrom. These observations, along with the crystal structure of 2, suggest that the multiply-bonded M M group has a trans influence on the Co2+ ion. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)