EXAMPLE 5.7 Interpreting potential data to identify bonding trends in complexes

Question

EXAMPLE 5.7 Interpreting potential data to identify bonding trends in complexes Ruthenium is located immediately below iron in the periodic table. The following reduction potentials have been measured for species of Ru in aqueous solution. What do these values suggest when compared to their Fe counterparts? [Ru(OH2)6]3++ e-[Ru(OH2W" [Ru(CN)6]3-+ e- [Ru(CN)6]4- E+=+0.25V E+ = +0.85V Answer We can answer this question by noting that if complexation by a certain ligand causes the reduction potential of a metal ion to shift in a positive direction, then the ne stabilizing the reduced metal ion. In this case we see that CN-stabilizes Ru(U) with respect to Ru(III) This behaviour is in stark contrast to the behaviour of Fe (see the preceding briefillustration) where we noted that CN- stabilizes Fe(IlI), a result more in keeping with Fe-CN bonds being more ionic. The ontrasting effects for species having identical charges suggest that the bonding between CN- and Rull) is particularly strong. w ligand must be Self test 5.7 The ligand bpy (1) forms complexes with Fel(lI) and Fe(I). The standard potential of the lfelbpy),1 /[Fe(bpy),]2* couple is + 102 V. Does bpy bind preferentially to Fel) or Fe()?

Explanation / Answer

the ligand bipyridine has a special affinity to Fe2+, and its complex have intense red coloration, whilewithFe3+,it forms almost colourless solution.On coordintin with Fe2+, the ligand assume a helical conformation inwhich the bipyridine rings are inclined at an angle of 540 to molecular axis and it is useful for determination of Fe2+ also.

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