Explain why $\left[\mathrm{Ni}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\left[{\mathrm{BF}}_4\right]}_2$, is coloured.

Consider the complex, ${\mathrm{K}}_4\left[\mathrm{Fe}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]$. Deduce the condensed electron configuration of the transition metal in this complex.

Consider the complex, ${\mathrm{K}}_4\left[\mathrm{Fe}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]$. State the geometry of the transition metal complex and draw a diagram of the complex.

Consider the complex, ${\mathrm{K}}_4\left[\mathrm{Fe}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]$. Identify the nature of the bonding between the ligand and the transition metal ion in the complex.

In an article written by W.B. Jensen in the Journal of Chemical Education $(85, 9, (2008), \mathrm{p}1182-3)$, it was reported that minute quantities of ${\mathrm{HgF}}_4$, have been detected, using matrix isolation techniques, at $4 \mathrm{K}$ under extreme non-equilibrium conditions. Suggest why, on this basis, in the first instance mercury might now be considered a transition metal. In the publication, however. Jensen challenges this claim. Explore why Jensen's counterargument may have merit, in view of conventional thinking on what IUPAC considers as a transition element.