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Advanced A Level Inorganic Chemistry: ISOMERISM - transition metal complexes

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3. Isomerism and Stereochemistry of Transition metal complexes

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All my advanced A level organic chemistry notes

All my advanced A level isomerism and stereochemistry notes

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Case studies of structure, formation, properties and consequences - BUT this page was written before the detailed transition metal notes, so more details and examples will now be found on the individual metal pages and appendices.


3. Isomerism in Transition metal complexes

Case studies: 3.1 possibilities * 3.2 cis-platin * 3.3 ionisation isomerism

3.4 stereoisomerism in Co3+ complexes * 3.5 co-ordination isomerism


Case study 3.1 A general survey of some possible situations

(c) doc b

The spatial arrangement of the ligands in complex ions can allow the existence of isomers. (1) and (2) show two examples of how cis/trans geometrical isomers can exist with two different ligands (shown as red L's and blue L's) for co-ordination numbers 4 (square planar) and 6 (octahedral). (3) shows how optical isomers (enantiomers) can exist when three bidentate ligands are bonded to a central metal ion.

cisplatincis/trans isomers of [Pt(NH3)2Cl2]

(c) doc bcis/trans isomers of [Cr(H2O)4Cl2]+

(c) doc bcis/trans isomers of [Cr(NH3)4Cl2]+

In both cases (Z) and (E) isomers I presume? BUT I could not find a suitable study-reference in Google on assigning configurations for geometric/geometrical isomers using the E/Z notation.

Just as in the behaviour of natural or synthesised optically organic molecules like amino-acids or drugs, the stereochemistry can affect the pharmacological behaviour of inorganic compounds too.


Case study 3.2 Geometrical/Geometric isomerism in square planar platinum complexes

e.g. cis-diamminedichloroplatin(II), [Pt(NH3)2Cl2]0

Known as the cis-platin complex (cisplatin), molecular formula above (and left diagram below) is one of the most effective agents against cancers of the ovaries, bladder, and head and neck and helps as co-agent in the treatment of cancers of the cervix, lung and breast. Its biggest success has been in the treatment of testicular cancer, a form of cancer previously resistant to any therapy but now considered to be curable in most cases. The platinum stereochemistry is very important, with the trans isomer showing no anti-cancer activity compared to the cis-isomer which is pharmacologically active in the desired way.

(c) doc b based on [Pt(NH3)2Cl2]

(Z) and (E) isomers I presume?

The cis/trans isomers are neutral, square planer complexes of co-ordination number 4, exhibiting geometrical or E/Z isomerism. There are two of each of the monodentate ligands, namely the chloride ion and ammonia and the blue lines/dots show the 4 dative covalent bonds.


Case study 3.3 Ionisation/hydrate isomerism in Transition metal complexes

e.g. Four complexes of chromium(III) chlorides based on a Cr3+, 3Cl-'s and 6H2O's. Four different crystal forms can be made. The different colours arise from small differences in the electronic energy levels of the central transition metal ion caused by changes in the ligand arrangement.

(a) [Cr(H2O)6]3+(Cl-)3  (violet or grey-blue?),

(b) [CrCl(H2O)5]2+(Cl-)2.H2O  (pale green)

(c) [CrCl2(H2O)4]+Cl-.2H2O  (dark green, can also exist as cis or trans isomers - see case study 3.1 example 2 in diagram, L4 and L5 would be the two Cl ligands)

(d) [CrCl3(H2O)3]0.3H2O (brown?), the 0 can be omitted, it just signifies an overall electrically neutral complex, (a) to (c) are complex ions.


Case study 3.4 Examples of optical isomerism

Cobalt octahedral complexes. e.g.

[Co(H2NCH2CH2NH2)3]3+ consists of a Co3+ ion combined with three bidentate ligands (1,2-diaminoethane, dative bond N:=>Co, via the lone pair of electrons on the nitrogen). See (3) in case study 3.1 above.

[Co(H2NCH2CH2NH2)2NH3Cl]2+ consists of a Co3+ ion combined with two 1,2-diaminoethane bidentate ligands, one chloride ion and one ammonia molecule.

[Co(H2NCH2CH2NH2)2Cl2]+ consists of a Co3+ ion combined with two 1,2-diaminoethane bidentate ligands and two chloride ions.

All three of these can exist as 'mirror image' forms, i.e. enantiomers.

Case study 3.5 Co-ordination isomerism

[Co(NH3)6]3+[Cr(C2O4)3]3- or [Co(C2O4)3]3-[Cr(NH3)6]3+ 

Here in this Co3+/Cr3+ anionic-cationic complex the ligands attached to the central metal ion can be exchanged.

The two ligands involved are the electrically neutral ammonia molecule, :NH3, and the ethanedioate anion (2-), -OOC-COO- derived from the dicarboxylic acid, ethanedioc acid.


WHAT NEXT?

Part 10 3d block TRANSITION METALS: 10.1-10.2 Introduction 3d-block Transition Metals and Data * 10.3 Scandium * 10.4 Titanium * 10.5 Vanadium * 10.6 Chromium * 10.7 Manganese * 10.8 Iron * 10.9  Cobalt * 10.10 Nickel * 10.11 Copper * 10.12 Zinc * 10.13 Other Transition Metals e.g. Ag and Pt * Appendix 1. Hydrated salts, acidity of hexa-aqua ions * Appendix 2. Complexes & ligands * Appendix 3. Complexes and isomerism * Appendix 4. Electron configuration & colour theory * Appendix 5. Redox equations, feasibility, E * Appendix 6. Catalysis * Appendix 7. Redox equations * Appendix 8. Stability Constants and entropy changes * Appendix 9. Colorimetric analysis and complex ion formula * Extra Appendix 10 3d block - extended data * Appendix 11 Some 3d-block compounds, complexes, oxidation states & electrode potentials * Appendix 12 Hydroxide complex precipitate 'pictures', formulae and equations

OR All my advanced A level isomerism and stereochemistry notes

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