This paper is devoted to the determination of the spin
density in the ferrimagnetic ground state of the
bimetallic chain compound
MnCu(pba)(H2O)3·2H2O,
with pba = 1,3-propylenebis(oxamato). The crystal
structure,
previously determined at room temperature through X-ray diffraction,
was redetermined at 10 K through unpolarized
neutron diffraction (orthorhombic system, space group Pnma,
a = 12.727(11) Å, b = 21.352(19)
Å, c = 5.153(3)
Å, Z = 4). The experimental spin density has been
deduced from polarized neutron diffraction data recorded at
10
K under 50 kOe. Positive spin densities were observed on the
manganese side and negative spin densities on the
copper side. The delocalization of the spin density from the metal
centers toward the oxamato bridging ligand was
found to be more pronounced on the copper side than on the manganese
side, so that the nodal surface (of zero spin
density) is closer to manganese than to copper. The experimental
spin distribution has been compared to the theoretical
distributions deduced from density functional theory (DFT)
calculations, using both DGauss and DMol programs.
The experimental results for the title chain compound have also
been compared to the spin distribution for the
binuclear compound
[Mn(Me6-[14]ane-N4)Cu(oxpn)](CF3SO3)2
with Me6-[14]ane-N4 =
(±)-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, and oxpn =
N,N‘-bis(3-aminopropyl)oxamido,
recently reported. The most
striking difference between pair and chain compounds concerns the
positive P
+ and negative
P
- spin populations
carried by the manganese and copper sides, respectively. For the
pair compound P
+ was found as 4.67(8)
μB, and
P
- as −0.67(8) μB while
for the chain compound these values are 5.05(7) μB
and −1.05(10) μB, respectively.
The
spin distribution for the ferrimagnetic chain compound is very close to
a Neel state (P
+ = 5 μB and
P
- = −1 μB).