  Two-dimensional materials (2DM) have attracted much interest due to their
distinct optical, electronic, and catalytic properties. These properties can be
by tuned a range of methods including substitutional doping or, as recently
demonstrated, by surface functionalization with single atoms, increasing even
further 2DM portfolio. Here we theoretically and experimentally describe the
coordination reaction between MoS$_2$ monolayers with 3d transition metals
(TMs), exploring the nature and the trend of MoS$_2$-TMs interaction. Density
Functional Theory calculations, X-Ray Photoelectron Spectroscopy (XPS), and
Photoluminescence (PL) point to the formation of MoS$_2$-TM coordination
complexes, where the adsorption energy trend for 3d TM resembles the
crystal-field (CF) stabilization energy for weak-field complexes. Pearson's
theory for hard-soft acid-base and Ligand-field theory were applied to discuss
the periodic trends on 3d TM coordination on the MoS$_2$ surface. We found that
softer acids with higher ligand field stabilization energy, such as Ni$^{2+}$,
tend to form bonds with more covalent character with MoS$_2$, which can be
considered a soft base. On the other hand, harder acids, such as Cr$^{3+}$,
tend to form bonds with more ionic character. Additionally, we studied the
trends in charge transfer and doping observed in the XPS and PL results, where
metals such as Ni led to an n-type of doping, while Cu functionalization
results in p-type doping. Therefore, the formation of coordination complexes on
TMD's surface is demonstrated to be a promising and effective way to control
and to understand the nature of the single-atom functionalization of TMD.
