Strong π‑Backbonding Enables Record Magnetic Exchange Coupling Through Cyanide

The paramagnetic cyano-bridged complex PhB­(tBuIm)3Fe–NC–Mo­(NtBuAr)3 (Ar = 3,5-Me2C6H3) is readily assembled from a new four-coordinate, high-spin (S = 2) iron­(II) monocyanide complex and the three-coordinate molybdenum­(III) complex Mo­(NtBuAr)3. X-ray diffraction and IR spectroscopy reveal that delocalization of unpaired electron density into the cyanide π* orbitals leads to a reduction of the C–N bond order. Direct current (dc) magnetic susceptibility measurements, supported by electronic structure calculations, demonstrate the presence of strong antiferromagnetic exchange between spin centers, with a coupling constant of J = −122(2) cm–1. To our knowledge, this value represents the strongest magnetic exchange coupling ever to be observed through cyanide. These results demonstrate the ability of low-coordinate metal fragments to engender extremely strong magnetic exchange coupling through cyanide by virtue of significant π-backbonding into the cyanide ligand.顺磁性氰基桥联配合物PhB(tBuIm)3Fe–NC–Mo(NtBuAr)3（Ar = 3,5-Me2C6H3）可由新型四配位高自旋（S = 2）单氰合铁(II)配合物与三配位钼(III)配合物Mo(NtBuAr)3便捷合成。X射线衍射与红外光谱表征显示，未成对电子密度离域至氰基的π*轨道会导致C-N键级降低。直流（dc）磁化率测量结合电子结构计算证实，自旋中心间存在强反铁磁交换相互作用，耦合常数J = −122(2) cm–1。据我们所知，该数值是目前通过氰基桥联所观测到的最强磁交换耦合强度。上述结果表明，低配位金属片段可通过向氰基配体进行显著的π反馈键（π-backbonding），实现经由氰基桥联的极强磁交换耦合。