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(CN)6]3-+ e- [Ru(CN)6]4- 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 new ligand must be 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 brief illustration) where we noted that CN stabilizes Fe(II), a result more in keeping with Fe-CN bonds being more ionic. The contrasting effects for species having identical charges suggest that the bonding between CN- and Ru(ll) is particularly strong. Selftest 5.7 The ligand bpy (1) forms complexes with Fe(lI) and Fe(l). The standard potential of the elbpy),J /lFe(bpy),1+ couple is +1.02 V. Does bpy bind preferentially to Fe(ll) or Fe(ll)?

Explanation / Answer

self test: 5.7

[Fe(bpy)3]3+ +e- -------> [Fe(bpy)3]2+ E0 = 1.02V

A positive E0 value shows that forward reaction is preffered over backward reaction. Thus, bpy preferentially bind to Fe(II)

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