Crystals in Which Some Metal Atoms are More Equal Than Others: Inequalities From Crystal Packing and Their Spectroscopic/Magnetic Consequences

Crystal structures of the heterometallic compounds CrCrFe(dpa)(4)Cl-2 (1), CrCrMn(dpa)(4)Cl-2 (2), and MoMoMn(dpa)(4)Cl-2 (3) (dpa = 2,2'-dipyridylamide) show disorder in the metal atom positions such that the linear M-A M-A center dot center dot center dot M-B array for a given molecule in the crystal is oriented in one of two opposing directions. Despite the fact that the direct coordination sphere of the metals in the two crystallographically independent orientations is identical, subtle differences in some metal -ligand bond distances are observed in 1 and 3 due to differences in the orientation of a solvent molecule of crystallization. The Fe(II) and Mn(II) ions serve as sensitive local spectroscopic probes that have been interrogated by Mossbauer spectroscopy and high-field EPR spectroscopy, respectively. The subtle differences in the two independent Fe and Mn sites in 1 and 3 unexpectedly give rise to unusually large differences in the measured Fe quadrupole splitting (Delta E-O) in 1 and Mn zero-field splitting (D) in 3. Variable-temperature/single-crystal EPR spectroscopy has allowed us to determine that the temperature-dependent D tensors in 3 are oriented along the metal-metal axis and that they show significantly different dynamic behavior with temperature. The differences in Delta E-O and D are reproduced by density functional calculations on truncated models for 1 and 3 that lack the quadruply bonded M-A M-A groups, though the magnitude of the calculated effect is not as large as that observed experimentally. We suggest that the large observed differences in Delta E-Q and D for the individual sites could be due to the influence of the strong diamagnetic anisotropy of the quadruply bonded M M unit.