Do Metal–Metal Multiply‐Bonded “Ligands” Have a trans Influence? Structural and Magnetic Comparisons of Heterometallic CrCr···Co and MoMo···Co Interactions

Abstract

AbstractReported here are two new compounds containing either a CrCr···Co [1, CrCrCo(dpa)4Cl2, dpa = 2,2′‐dipyridylamide] or a MoMo···Co [2, MoMoCo(dpa)4Cl2] 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···Co distance becomes shorter by 0.13 Å in the low‐spin form, the Co–Cldistance becomes longer by 0.2 Å. 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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)