Density Functional Investigation of Metal−Metal Interactions in d⁴d⁴ Face-Shared [M₂Cl₉]^3- (M = Mn, Tc, Re) Systems

Abstract

The molecular and electronic structures of the d(4)d(4) face-shared [M(2)Cl(9)](3)(-) (M = Mn, Tc, Re) dimers have been calculated by density functional methods in order to investigate metal-metal bonding in this series. The electronic structures of these systems have been analyzed using potential energy curves for the broken-symmetry and other spin states arising from the various d(4)d(4) coupling modes, and closed energy cycles have been utilized to identify and quantify the parameters which are most important in determining the preference for electron localization or delocalization and for high-spin or low-spin configurations. In [Tc(2)Cl(9)](3)(-) and [Re(2)Cl(9)](3)(-), the global minimum has been found to be a spin-triplet state arising from the coupling of metal centers with low-spin configurations, and characterized by delocalization of the metal-based electrons in a double (sigma and delta(pi)) bond with a metal-metal separation of 2.57 A. In contrast, high-spin configurations and electron localization are favored in [Mn(2)Cl(9)](3)(-), the global minimum for this species being the ferromagnetic S = 4 state with a rather long metal-metal separation of 3.43 A. These results are consistent with metal-metal overlap and ligand-field effects prevailing over spin polarization effects in the Tc and Re systems, but with the opposite trend being observed in the Mn complex. The ground states and metal-metal bonding observed for the d(4)d(4) systems in this study parallel those previously found for the analogous d(2)d(2) complexes of V, Nb, and Ta, and can be rationalized on the basis that the d(4)d(4) dimer configuration is the hole equivalent of the d(2)d(2) configuration.

Periodic trends in metal–metal bonding in edge-shared [M2Cl10]4− systems

“…Nevertheless, we have shown in our previous work that density 0277 functional theory in combination with the broken-symmetry approach of Noodleman and coworkers [5,6] can accurately describe the entire range of metal-metal interactions, from weak antiferromagnetic coupling through to multiple metal-metal bonds, as well as encompassing both high-spin and low-spin metal configurations. The success of this procedure can be exemplified by noting that our broken-symmetry density functional calculations have accurately reproduced experimentally observed properties in a large number of [M 2 X 9 ] zÀ dimers with electronic configurations ranging from d 1 d 1 through to d 5 d 5 [7][8][9][10][11][12][13][14] as well as mixed-valence [15,16]. We have also used this methodology to successfully investigate metal-metal interactions in a variety of other dimer systems, including edge-shared [M 2 X 10 ] zÀ and phosphinebridged [M 2 X 6 (P-P) 4 ] zÀ systems [17,18], indicating that this theoretical approach is broadly applicable across many dinuclear transition metal complexes.…”

Large Positive Zero-Field Splitting in the Cluster Magnet Ba3CeRu2O9

“…When there are eight valence electrons per Ru dimer, this metal–metal bonding scenario leads to an S tot = 1 electronic ground state with no orbital degree of freedom and a S tot = 0 first excited-state multiplet, as illustrated in Figure a and b, respectively. An axial zero-field splitting D > 0 can lift the 3-fold degeneracy of the S tot = 1 manifold, leading to a S tot = 1, S tot z = 0 singlet with a spin gap D to an excited doublet S tot = 1, S tot z = ±1, and in Ba 3 CeRu 2 O 9 this splitting can arise from the combined effects of spin–orbit coupling, the trigonal elongation of the Ru 2 O 9 complex, and the octahedral off-centering of the Ru ions.…”

“…Intriguingly, Ba 3 MRu 2 O 9 (M = Ce, Pr, Tb) has eight valence electrons per Ru dimer and even shorter intradimer Ru–Ru distances, which likely leads to metal–metal bonding as well. Large ZFS may play an important role in these materials, as an S tot = 1 electronic ground-state multiplet is predicted in a molecular orbital scenario . Because Ba 3 CeRu 2 O 9 consists of well-separated Ru dimers due to nonmagnetic Ce 4+ occupying the M site and no evidence for long-range magnetic order was found previously down to 2 K, it should be considered a SMM candidate.…”

Periodic trends in metal–metal interactions in face-shared [M2Cl9]z−systems

“…The energetic difference between the S = 0 and S = S max states, which corresponds to the difference between the metal-metal bonding and spin polarization terms represented by the quantity DE spe − DE mbe in Table 3 11 The global minima for 3d-3d dimers normally correspond to states characterized by weak coupling and localization of the metal-based electrons, whereas those for 4d-4d and 5d-5d dimers tend to occur at relatively short metalmetal distances where electron delocalization and metal-metal bond formation are favoured.…”