Transition elements such as iron, copper, platinum, silver, and gold have played a central role in t — Inorganic Chemistry Chemistry Question
Isomerism of Coordination Compounds of Metals
Transition elements such as iron, copper, platinum, silver, and gold have played a central role in the development of human society. At the end of the 19th century Alfred Werner developed the field of coordination chemistry, and ideas from that field were important in the overall development of modern chemistry. These elements and their compounds are now used in countless ways, and their importance in biology is widely recognized.
A. Isomerism
Coordination compounds exhibit several forms of isomerism.
• Stereoisomers are isomers that possess identical constitution, but which differ in the arrangement of their atoms in space. Stereoisomers include optical isomers (enantiomers) and geometric isomers (diastereoisomers).
• Structural or constitutional isomers have the same empirical formula but differ in their atom-to-atom connections.
How many stereoisomers are expected for each of the following four-coordinate, square planar platinum(II) compounds? Draw the structure of each.
i) (PPh3)2PtCl2 (Ph = phenyl);
ii) [Pt(NH3)(pyridine)(NO2)(NH2OH)]+ (The Russian chemist Chernyaev first synthesized the diastereoisomers of this compound in 1926.) (Here both NO2 – and NH2OH are N-bonded to the platinum(II) ion);
iii) Pt(en)Cl2 (where en = ethylenediamine, H2NCH2CH2NH2).
Model Answer
i)
ii)
iii) This has only a single isomer. The ethylenediamine ligand cannot bridge across the molecule to place the N atoms in a trans position.
Draw each stereoisomer of the following octahedral, six-coordinate cobalt(III) and chromium(III) complexes.
i) Co(py)3Cl3 (where py = pyridine);
ii) [Cr(ox)2(H2O)2] – (where ox = oxalate ion, [O2C–CO2] 2–);
iii) [Co(en)(NH3)2Cl2] +.
Model Answer
i)
ii) Oxalate is depicted as O–O.
iii) Ethylenediamine is depicted as H2N–NH2.
B. Chemotherapy Agents
There has been a concerted effort to find transition metal complexes that can act as drugs in the treatment of cancers. A particularly important recent example is a Ru(III) complex, the anion of which has the formula [Ru(DMSO)(imidazole)Cl4] –. Complexes of DMSO, dimethylsulfoxide [(CH3)2SO], are interesting in part because the DMSO ligand can bind to the metal ion either through the O atom or the S atom.
What is the total number of stereoisomers and structural isomers possible for [Ru(DMSO)(imidazole)Cl4] –?
Model Answer
Four isomers are possible. The imidazole and DMSO can be arranged cis or trans (see above). In each case, the DMSO can be S-bonded or O-bonded.
C. OLEDs and an Aluminum Coordination Compound
In an organic light-emitting diode (OLED), a film of an organic compound emits light in response to a current. OLEDs are now used in computer monitors and in the screens on mobile phones and personal digital assistants (PDAs). One molecule used successfully in OLEDs is the aluminum(III) complex of 8-hydroxyquinoline. By incorporating different substituents, different wavelengths of light are emitted.
8-hydroxyquinoline (C9H6NO) complex of Al3+
This water-insoluble compound is also used in the gravimetric analysis for aluminum in a sample.
Assuming octahedral coordination around the Al3+ ion, how many stereoisomers are possible for the complex (C9H6NO)3Al? Sketch the structure of at least one of the stereoisomers.
Model Answer
The complex has 2 diastereoisomers and each is optically active (has an enantiomer). The ligand, which binds through N and O atoms, is depicted as N–O.