Transition Elements: Question 7

Worked solution

Part (a): Oxidation state of platinum and overall neutrality

Ammonia, $\text{NH}_3$, is a neutral ligand, so the two $\text{NH}_3$ ligands contribute $2\times0=0$ to the overall charge. Chloride, $\text{Cl}^-$, carries a $1-$ charge, so the two $\text{Cl}^-$ ligands contribute $2\times(-1)=-2$.

For the complex $[\text{Pt(NH}_3)_2\text{Cl}_2]$ to be electrically neutral overall: $(\text{oxidation state of Pt}) + 0 + (-2) = 0 \implies \text{oxidation state of Pt} = +2$

So platinum is in the $+2$ oxidation state, consistent with the two neutral $\text{NH}_3$ and two singly-charged $\text{Cl}^-$ ligands exactly balancing it to give a neutral overall complex.

Part (b): Coordination number and shape

Platinum forms four coordinate bonds in total, two to the nitrogen atoms of the $\text{NH}_3$ ligands and two to the $\text{Cl}^-$ ligands, so the coordination number is 4. A coordination number of 4 can give either a tetrahedral or a square-planar arrangement; platinum(II) [and nickel(II)] complexes of this type adopt the square-planar shape.

Part (c): Cis and trans arrangements, and why tetrahedral geometry cannot show them

In the square-planar complex, the four ligands lie at the corners of a square around platinum.

• In the cis isomer, the two $\text{Cl}^-$ ligands occupy adjacent corners of the square (at $90°$ to each other), and the two $\text{NH}_3$ ligands occupy the other two adjacent corners.
• In the trans isomer, the two $\text{Cl}^-$ ligands occupy opposite corners (at $180°$ to each other, diagonally across the square), as do the two $\text{NH}_3$ ligands.

This distinction cannot arise in a tetrahedral $\text{MA}_2\text{B}_2$ complex because every vertex of a tetrahedron is equidistant from, and adjacent to, every other vertex (there is no pair of vertices that is “opposite” in the way two square-planar corners can be. Any tetrahedral arrangement of two $\text{A}$ and two $\text{B}$ ligands can always be rotated to coincide with any other such arrangement, so only one structure exists) cis-trans isomerism therefore requires a geometry (such as square planar or octahedral) in which distinct adjacent and opposite positions exist.

Part (d): Type of isomerism and biological significance

The cis/trans relationship between the two forms of $[\text{Pt(NH}_3)_2\text{Cl}_2]$ is an example of cis-trans (geometric) stereoisomerism. The two isomers have the same molecular formula and the same bonds, but a different spatial arrangement of ligands around the central platinum atom; they are not mirror images of each other, so this is not optical isomerism.

Although the two isomers are chemically very similar in formula, their differing three-dimensional shapes mean they interact differently with biological molecules. Cisplatin’s anticancer activity depends on binding at two adjacent sites (after loss of the two $\text{Cl}^-$ ligands) on a target biomolecule such as DNA; because biological binding sites are highly shape-specific, only the cis arrangement, with its two available adjacent binding positions, fits and reacts effectively, while the trans isomer’s ligands point in opposite directions and so cannot bind in the same way, giving it far weaker biological activity.

Final answers

• (a) Platinum is $+2$: $(+2) + 0 + 2(-1) = 0$, so the complex is neutral.
• (b) Coordination number $=4$; shape $=$ square planar.
• (c) Cis: the two $\text{Cl}^-$ ligands are adjacent ($90°$ apart); trans: they are opposite ($180°$ apart). Tetrahedral geometry has no distinct “opposite” vertices, so this isomerism cannot occur there.
• (d) Cis-trans (geometric) stereoisomerism; the two isomers have different shapes, and shape-specific biological binding means only the cis form is therapeutically active.