Pt(II)
and Pd(II) complexes (1 and 2,
respectively) with thiomorpholine-4-carbonitrile (TM-CN), an N-substituted
thiomorpholine derivative, were synthesized from tetrachlorido precursors
in water. Structural analysis has shown that 1 represents
the first monomeric metal complex with this ligand type with an axial
M–S bond with respect to the TM-CN ring chair conformation,
while in 2 a typical equatorial M–S bond position
with respect to the ring chair conformation was observed. A detailed
DFT investigation revealed that axial conformers are more stable for
molecular forms of both metals, while intermolecular interactions
in the crystals stabilize the axial conformer for Pt(II) and the equatorial
conformer for Pd(II). The magnitude of this stabilization in the case
of 2 is large enough to change the most stable axial
conformer in the molecular form to the equatorial conformer in the
crystal. Further investigation of the strength of individual intermolecular
interactions revealed significant differences of some interactions
between the two structures. The likely cause of the difference in
the crystal structures of experimentally obtained complexes is the
fact that 1 and 2 exhibit different dominant
interactions: C–H/M and C–H/S are more dominant in 1 and C–H/Cl interactions are more dominant in 2. In addition, DFT calculations have shown that while the
axial position of the Pt–S bond with respect to the ring chair
conformation results in a significantly shorter C–H/Pt interaction
distance than that in the hypothetical equatorial conformer, there
is very little difference in C–H/Pd interaction distances in
conformers with axial and equatorial positions of Pd–S bond
with respect to the ring chair conformation.