Spin Densities in a Ferromagnetic Bimetallic Chain Compound: Polarized Neutron Diffraction and DFT Calculations

The spin population distribution in the ferromagnetically coupled hetero-bimetallic chain compound
[MnNi(NO2)4(en)2] (en = 1,2-ethanediamine) has been investigated by means of polarized neutron diffraction
experiments, and the results compared with those from theoretical estimates obtained via calculations
based on density functional theory on dinuclear molecular models of the chain. The spin distributions obtained
from experiment and from theory are consistent and reflect a larger spin delocalization from the Ni atom
due to the more covalent character of the Ni−N bonds compared to the Mn−O ones. Also a nearly isotropic
spin distribution is observed for the more ionic d5 Mn2+ ion and a clearly anisotropic distribution for the d8
Ni2+ ion. The use of dinuclear molecular models for the calculation of the exchange coupling constant
between Ni and Mn provide upper and lower limits (+17.6 and −4.2 cm-1) for the experimentally determined
value (+1.3 cm-1), depending on how the missing part of the chain is simulated, but yield essentially the
same spin distribution. The Mn(II)−Ni(II) weak ferromagnetic coupling in the chain is interpreted in a spin
delocalization mechanism as resulting from the weakness of the overlap between the magnetic orbitals
centered on nickel and those centered on manganese which are only weakly delocalized on the ligands.