The coordination number of the [Fe H2O 6]2+ complex ion is 6, as there are six water molecules H2O surrounding the central iron Fe atom. The geometry of this complex ion is octahedral, as the six ligands are arranged symmetrically around the central metal atom, with each ligand occupying a vertex of an octahedron. UV-Vis spectroscopy can be used to study the electronic transitions in the complex, which can provide information about the geometry and coordination number. In the case of [Fe H2O 6]2+, the UV-Vis spectrum would show a characteristic absorption band due to the d-d transition of the central Fe II ion in an octahedral environment.