  We study the consequences of the approximately trigonal ($D_{3d}$) point
symmetry of the transition metal (M) site in two-dimensional van der Waals
MX$_2$ dihalides and MX$_3$ trihalides. The trigonal symmetry leads to a 2-2-1
orbital splitting of the transition metal $d$ shell, which may be tuned by the
interlayer distance, and changes in the ligand-ligand bond lengths. Orbital
order coupled to various lower symmetry lattice modes may lift the remaining
orbital degeneracies, and we explain how these may support unique electronic
states using ZrI$_2$ and CuCl$_2$ as examples, and offer a brief overview of
possible electronic configurations in this class of materials. By building and
analysing Wannier models adapted to the appropriate symmetry we examine how the
interplay among trigonal symmetry, electronic correlation effects, and $p$-$d$
orbital charge transfer leads to insulating, orbitally polarized magnetic
and/or orbital-selective Mott states. Our work establishes a rigorous framework
to understand, control, and tune the electronic states in low-dimensional
correlated halides. Our analysis shows that trigonal symmetry and its breaking
is a key feature of the 2D halides that needs to be accounted for in search of
novel electronic states in materials ranging from CrI$_3$ to $\alpha$-RuCl$_3$.
