Relationships between Electron Density and Magnetic Properties in Water-Bridged Dimetal Complexes

The electron densities
in two analogous dimetallic transition metal compounds, namely, [M2(μ-OH2)­(tBuCOO)4(tBuCOOH)2(C5H5N)2] (M = Co­(1), Ni­(2)), were determined from combined X-ray and neutron single-crystal
diffraction at 100 K. Excellent correspondence between the thermal
parameters from X- and N-derived atomic displacement parameters is
found, indicating high-quality X-ray data and a successful separation
of thermal and electronic effects. Topological analysis of electron
densities derived from high-resolution X-ray diffraction, as well
as density functional theory calculations, shows no direct metal–metal
bonding in either compound, while the total energy density at the
bond critical points suggests stronger metal–oxygen interactions
for the Ni system, in correspondence with its shorter bond distances.
The analysis also allows for estimation of the relative strength of
binding of terminal and bridging ligands to the metals, showing that
the bridging water molecule is more strongly bound than terminal carboxylic
acid, but less so than bridging carboxylates. Recently, modeling of
magnetic and spectroscopic data in both of these systems has shown
weak ferromagnetic interactions between the metal atoms. Factors related
to large zero-field splitting effects complicate the magnetic analysis
in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous
magnetic and spectroscopic measurements that there is no appreciable
direct communication between metal centers.

在两种类似的双金属过渡金属化合物[（M2(μ-OH2)­(tBuCOO)4(tBuCOOH)2(C5H5N)2]（M = Co­(1)，Ni­(2)）中，电子密度通过在100 K时结合X射线和中子单晶衍射法测定。X射线与中子衍射得到的原子位移参数的热参数间表现出极佳的一致性，这表明了高质量X射线数据以及热效应与电子效应的成功分离。由高分辨率X射线衍射获得的电子密度拓扑分析，以及密度泛函理论计算显示，这两种化合物中均不存在直接的金属-金属键合，而键合临界点的总能量密度表明Ni系统中金属-氧相互作用更强，与其较短的键长相对应。该分析还允许估计端基配体和桥联配体与金属的相对结合强度，表明桥联水分子比端基羧酸结合得更强，但不如桥联羧酸盐。近期，对这两个体系中磁性和光谱数据的建模表明，金属原子之间存在微弱的铁磁相互作用。与较大的零场分裂效应相关因素使两种化合物中的磁性分析复杂化，尽管在1中这种影响更为显著。当前结果支持先前磁性和光谱测量得出的结论，即金属中心之间不存在显著的直接相互作用。