Coplanar refers to objects lying in the same plane (torsional angle 0 degree or 180 degree).
Coplanar ligands means you can draw a single plane that contains all the ligands.
A general simple rule is that the molecule will not be planar if there is an SP3 hybridized carbon (or nitrogen) atom or two SP2 hybridized atoms of carbon/nitrogen which are separated by an even number of double bonds and no single bonds. Otherwise, its structure allows it to be planar.
