Low spin, diamagnetic [CoI_6]^3- (which contains a Co^3+ ion) is an example of a
ID: 1020563 • Letter: L
Question
Low spin, diamagnetic [CoI_6]^3- (which contains a Co^3+ ion) is an example of a complex ion that is thermodynamically unstable with respect to ligand exchange with F^-. but is kinetically inert: [Col_6]^3- + 6 F rightarrow [CoF_6]^3' + 6| Why is [CoF_6]^3- more thermodynamically stable than [Col_6]^3-? Why is [Col_6]^3' kinetically inert? Compete the energy diagram below to illustrate the energy changes that occur in the reaction of [Col_6]^3- with F- that are consistent with the statements above? The reaction of [Col_6]^3- with F generates a reactive intermediate. Draw the structure of this intermediate. (If [Col_6]3- is reduced by one electron to [Col_6]^4- (which now contains a high spin Co^2+ ion), ligand exchange reactions become much more rapid than those of [Col_6]^3-. Explain this observation.Explanation / Answer
1. a) Why [CoF6]3- is more thermodynamically stable than [CoI6]3-
According to Pearson’s HSAB concept, the Lewis acids and bases can be further divided into hard or soft or border line types.
Hard Lewis acids are characterized by small ionic radii, high positive charge, strongly solvated, empty orbitals in the valence shell and with high energy LUMOs.
Soft Lewis acids are characterized by large ionic radii, low positive charge, completely filled atomic orbitals and with low energy LUMOs.
Hard Lewis bases are characterized by small ionic radii, strongly solvated, highly electronegative, weakly polarizable and with high energy HOMOs.
Soft Lewis bases are characterized by large ionic radii, intermediate electronegativity, highly polarizable and with low energy HOMOs.
The Border line Lewis acids and bases have intermediate properties.
HSAB Principle: According to HSAB concept, hard acids prefer binding to the hard bases to give ionic complexes, whereas the soft acids prefer binding to soft bases to give covalent complexes.
1) The large electronegativity differences between hard acids and hard bases give rise to strong ionic interactions.
2) The electronegativities of soft acids and soft bases are almost same and hence have less ionic interactions. i.e., the interactions between them are more covalent.
3) The interactions between hard acid - soft base or soft acid - hard base are mostly polar covalent and tend to be more reactive or less stable. The polar covalent compounds readily form either more ionic or more covalent compounds if they are allowed to react.
[CoF6] 3- is more termodynamycally, because both Co+++ and F- are chracterised by strong ionic radii high charge density results in almost same and hence have less ionic interactions. i.e., the interactions between them are more covalent and resulted in stable complex.
Whereas in in [CoI6]3- interaction is, hard Co3+ ion with large ionic radii and highly polarisable I- inos resulting in HA-SB (Hard acid and soft base) combination, hence interactions are re mostly polar covalent and tend to be more reactive or less stable.
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