Do Metal–Metal Multiply‐Bonded “Ligands” Have a <i>trans</i> Influence? Structural and Magnetic Comparisons of Heterometallic CrCr···Co and MoMo···Co Interactions

Abstract

Abstract Reported here are two new compounds containing either a CrCr ··· Co [ 1 , CrCrCo(dpa) 4 Cl 2 , dpa = 2,2′‐dipyridylamide] or a MoMo ··· 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 Co 2+ 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 Co 2+ 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 Co 2+ ion.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)