Question

WEB Platinum(II) forms many complexes, among them those with the following ligands. Give the formula and charge of each complex. (a) two ammonia molecules and one oxalate ion \(\left(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\right)\) (b) two ammonia molecules, one thiocyanate ion \(\left(\mathrm{SCN}^{-}\right)\), and one bromide ion (c) one ethylenediamine molecule and two nitrite ions

Short answer

Complex (a): Two ammonia molecules and one oxalate ion Formula: WEB Pt(NH3)2(C2O4) Charge: 0 Complex (b): Two ammonia molecules, one thiocyanate ion, and one bromide ion Formula: WEB Pt(NH3)2(SCN)(Br) Charge: 0 Complex (c): One ethylenediamine molecule and two nitrite ions Formula: WEB Pt(en)(NO2)2 Charge: 0

Step-by-step solution

Complex (a) - Two ammonia molecules and one oxalate ion

To find the formula and charge of the complex, we combine the platinum with the ligands: 1. WEB Platinum(II) has the formula WEB Pt with a charge of +2. 2. Two ammonia molecules have the formula \(\mathrm{NH}_{3}\) and a charge of 0. 3. One oxalate ion has the formula \(\mathrm{C}_{2}\mathrm{O}_{4}\) and a charge of -2. We combine these ligands and balance the charges: \(\mathrm{WEB Pt}^{2+} + 2\mathrm{NH}_{3} + \mathrm{C}_{2}\mathrm{O}_{4}^{2-} \rightarrow \mathrm{WEB Pt(NH}_{3}\mathrm{)_{2}\mathrm{(C}_{2}\mathrm{O}_{4})}\) The charge on the complex will be the sum of the charges: \(+2 + 0 + (-2) = 0\) Thus, the formula and charge of complex (a) is \(\mathrm{WEB Pt(NH}_{3}\mathrm{)_{2}\mathrm{(C}_{2}\mathrm{O}_{4})}\) and 0.

Complex (b) - Two ammonia molecules, one thiocyanate ion, and one bromide ion.

To find the formula and charge of the complex, we combine the platinum with the ligands: 1. WEB Platinum(II) has the formula WEB Pt with a charge of +2. 2. Two ammonia molecules have the formula \(\mathrm{NH}_{3}\) and a charge of 0. 3. One thiocyanate ion has the formula \(\mathrm{SCN}\) and a charge of -1. 4. One bromide ion has the formula \(\mathrm{Br}\) and a charge of -1. We combine these ligands and balance the charges: \(\mathrm{WEB Pt}^{2+} + 2\mathrm{NH}_{3} + \mathrm{SCN}^{-} + \mathrm{Br}^{-} \rightarrow \mathrm{WEB Pt(NH}_{3}\mathrm{)_{2}\mathrm{(SCN)(Br)}\) The charge on the complex will be the sum of the charges: \(+2 + 0 + (-1) + (-1) = 0\) Thus, the formula and charge of complex (b) is \(\mathrm{WEB Pt(NH}_{3}\mathrm{)_{2}\mathrm{(SCN)(Br)}\) and 0.

Complex (c) - One ethylenediamine molecule and two nitrite ions

To find the formula and charge of the complex, we combine the platinum with the ligands: 1. WEB Platinum(II) has the formula WEB Pt with a charge of +2. 2. One ethylenediamine molecule has the formula \(\mathrm{en}\) (en is given for ethylenediamine) and a charge of 0. 3. Two nitrite ions have the formula \(\mathrm{NO}_{2}\) and a charge of -1 each. We combine these ligands and balance the charges: \(\mathrm{WEB Pt}^{2+} + \mathrm{en} + 2\mathrm{NO}_{2}^{-} \rightarrow \mathrm{WEB Pt(en)(NO}_{2}\mathrm{)_{2}}\) The charge on the complex will be the sum of the charges: \(+2 + 0 + 2(-1) = 0\) Thus, the formula and charge of complex (c) is \(\mathrm{WEB Pt(en)(NO}_{2}\mathrm{)_{2}}\) and 0.

Key concepts

Coordination Chemistry

Coordination chemistry is a branch of inorganic chemistry that deals with compounds typically consisting of a central metal atom or ion (usually referred to as the coordination center) and a surrounding array of bound molecules or anions known as ligands. Ligands are ions or neutral molecules that donate a pair of electrons to the metal, forming coordinate covalent bonds.

These complex compounds exhibit a fascinating array of behaviors and properties, driven by the nature of the metal-ligand interactions. The number of ligands bound to the central metal atom/ion is called the coordination number, which can vary from two to large numbers in some complexes. The geometrical arrangement of the ligands is governed by the coordination number and is crucial for the properties of the complex.

In the textbook exercise, the complexes formed by platinum(II) with different ligands demonstrate the versatility of coordination chemistry. The central platinum(II) ion acts as the coordination center with a fixed oxidation state of +2, while varying ligands like ammonia, oxalate, thiocyanate, bromide, and ethylenediamine coordinate to it. In each case, the ligands determine the final charge and the geometry of the complex.

Ligand Field Theory

Ligand field theory is a theoretical model that describes the electron distribution around the central atom in a coordination compound. It accounts for the bond formation between the central metal ion and the ligands through the splitting of the metal's d orbitals in the presence of an electrostatic field created by the ligands. This theory is especially useful for understanding the electronic structures, color, and magnetic properties of coordination complexes.

According to ligand field theory, when the ligands approach the central metal ion, the degenerate (equally energized) d orbitals split into two sets of orbitals with different energy levels. The specific splitting pattern depends on the geometry of the complex (e.g., octahedral, tetrahedral, square planar).

For instance, in a platinum(II) complex, the square planar geometry typically leads to a significant splitting, with a pair of d orbitals having much higher energy than the others. This affects the observable properties of the complex such as absorption of light and magnetism, which are central to numerous applications in material science and medicine.

Chemical Formula and Charge

The chemical formula of a coordination complex conveys the composition of the complex, specifying the metal center, the attached ligands, and their stoichiometry. It is crucial not only for understanding the structure of the complex but also for predicting its reactivity and physical properties. When writing the formula, the central atom is first listed followed by the ligands within the coordination sphere, all enclosed in square brackets.

The charge of a coordination complex is the net charge resulting from the sum of the oxidation state of the metal ion and the charges of the individual ligands. It is important when considering the reactivity of the complex, as it affects the compound's ability to form additional bonds or interact with other charged species. For example, in the given exercises, the platinum(II) complexes with ammonia, oxalate, thiocyanate, bromide, and ethylenediamine are all neutral, having a net charge of zero.

To ensure the charge balance, the total positive charge contributed by the metal ion must be balanced by the total negative charge from the ligands and any counterions, if present. Misinterpreting the charge can lead to incorrect predictions about the compound's solubility, color, and potential chemical reactions.