Ki is a kinetic constant for binding an inhibitor to a receptor. It is independent of enzyme concentration and associated with the binding and unbinding reaction of receptor and ligand molecules.
In this 2D representation of the bypyridine ligand, N stands for nitrogen:
By Su-no-G – selfmade with ChemDraw, public domain, https://commons.wikimedia.org/w/index.php?curid=1371770
The bypyridine ligand in 3D would be as follows:
Created by Jynto (talk) – own work of this vector using Discovery Studio Visualizer, CC0, https://commons.wikimedia.org/w/index.php?curid=15603670