Anthony KERMAGORET - THESES ET MEMOIRES DE L'UDS

Chapitre III
Figure 4. Visible spectra (400-800 nm) of 1a/b in MeCN/CH2Cl2 (5:2) recorded in the 293-193 K
range and compared to that of solid 1a.
A strong absorption centered at 517 nm dominates the visible spectrum recorded at 293 K but
two weak bands centered at 615 and 700 nm were also detected. Upon decreasing the temperature, the
main absorption gradually disappears, while the two minor ones remain almost unchanged. The
absorption pattern of the 193 K spectrum is similar to that of the solid state form 1a. It can therefore
be envisaged that 1a is present in solution and favored at low temperature, being thermodynamically
stabilized with respect to 1b. The equilibrium was fully restored when the system was allowed to
reach 293 K.
The presence of two distinct species in equilibrium in solution is consistent with the
paramagnetism detected by NMR (Evans method). The values of 2.8 and 2.3 B for 1 and 2 in
CD2Cl2, respectively, are not consistent with either a square-planar or an octahedral coordination. The
paramagnetic NMR data recorded for 1-4 could thus result from an equilibrium between a
mononuclear diamagnetic complex and a polynuclear, probably tetranuclear, paramagnetic one. An
equilibrium between square planar and octahedral Ni complexes in solution is not uncommon and
previous examples involve solvent molecules entering the coordination sphere, [9] and hemilability. [10]
Other examples include tetrahedral-square planar isomerisms. [11] However, the equilibrium between
mononuclear and polynuclear Ni complexes remains rare. Recently, an irreversible nuclearity
decrease from 6 to 4 was observed, induced by coordinating solvents. [12] The tetranuclear form is
more stabilized in the case of PNth than of PNox, as observed by (i) the Evans method and (ii) the solid
state isomerization pressure.
In conclusion, the situations encountered in this work reveal many unprecedented features and
8