The actinides \(\mathrm{Pa}, \mathrm{U},\) and \(\mathrm{Np}\) form a series of complex ions, such as the anion in the compound \(\mathrm{Na}_{3}\left[\mathrm{UF}_{8}\right],\) in which the central metal ion has an unusual geometry and oxidation state. In the crystal structure, the complex ion can be pictured as resulting from interpenetration of simple cubic arrays of uranium and fluoride ions. (a) What is the coordination number of the metal ion in the complex ion? (b) What is the oxidation state of uranium in the compound? (c) Sketch the complex ion.

The coordination number is a key concept in coordination chemistry. It indicates the number of ligand atoms that are directly bonded to a central metal ion. This tells us about the spatial arrangement and stoichiometry of the complex. The compound \(\text{Na}_{3}[\text{UF}_{8}]\) features an example of a coordination number. Here, the uranium ion (\text{U}) is surrounded by eight fluoride ions (\text{F}^{-}). This means the coordination number of the uranium in this complex is 8. Counting the number of ligands can provide insight into the shape and geometry of the complex.

The oxidation state of a metal ion in a complex determines its electronic configuration and reactivity. To figure out the oxidation state of uranium in \(\text{Na}_{3}[\text{UF}_{8}]\), we first need to consider the oxidation states of the other ions. Fluoride ions (\text{F}^{-}) typically have an oxidation state of -1. In the \([\text{UF}_{8}]^{3-}\) ion, there are eight fluoride ions, making the total negative charge from the ligands -8. Let the oxidation state of uranium be \(x\). We set up the equation: \[ x + 8(-1) = -3 \] Solving this, \ x - 8 = -3, \ so \ x = +5. Therefore, uranium has an oxidation state of +5 in this complex.

Ligands are ions or molecules that combine with a central metal ion to form a complex. They donate pairs of electrons to the metal ion, forming coordinate covalent bonds. In our example of \(\text{Na}_{3}[\text{UF}_{8}]\), fluoride ions (\text{F}^{-}) act as ligands to uranium. These ligands are monodentate, meaning they use one donor atom to bind to the central metal ion. Ligands can significantly affect the stability, solubility, and reactivity of the coordination complex. In \([\text{UF}_{8}]^{3-}\), eight \text{F}^{-} ions surround the uranium, encapsulating it in a cubic geometry. Understanding ligands and their interactions with metal ions is crucial for predicting and manipulating the behavior of coordination complexes.