Developing complex metallic heterocomplexes requires knowledge of
synthesis methods based on oxidation-reduction reactions, complexation to boiling
point or basification of mixtures of salts of chromium, aluminum, iron, zirconium or
titanium, the main tanning metals for natural leather. The purpose of these syntheses is
developing heterocomplexes with higher stability to alkali than that of metal salts from
which the natural leather tanning properties derive. Chromium-aluminum salts are the
only heterocomplexed salts that are currently commercially available, with restricted
use to certain types of applications related to maintaining a lighter colour of leather and
improved polishing ability. Knowing the possible structure of complex metallic
heterocomplexes obtained is a complicated endeavor due to the polydispersity of
tanning solutions and the difficulty of separating the tanning heterocomplex in crystal
form. IR analyses enabled the formulation of hypotheses on the structure of chromiumzirconium
and aluminum-titanium heterocomplexes, while X-ray diffraction of chromezirconium-
cerium crystal enabled the identification of the role of sulfate ion of ligand
tridentate and not bidentate, as it was previously thought, the identification of
oximetallic O-Cr-O bond length of 10Å, identical to the distance between the collagen
macromolecules forming the three-macromolecule helix. The close connection between
the synthesis method and the stability of tanning metallic heterocomplexes is
highlighted by examples of research in the field. The topicality of tanning metallic
heterocomplexes is emphasized by their superior properties compared to organic
tanning variants, both in terms of economy and technology, and the greater possibility
of combining metals or organic ligands to develop structures with different collagen
crosslinking properties.
Keywords: Synthesis methods, Chromium-aluminum tanning agents, Tanning
heterocomplexes, Structure of heterocomplexes, Properties of heterocomplexes.
