chemistry help pls

Please could I have some help on this Question 4v page 17- I don't understand why Cd(h2o)6 is the least stable. Surely if Cd2+ exists as that it should be the most stable? Also they haven't given the kstab value for it so how do we compare?
paper: https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FChemistry%2FA-level%2FPast-Papers%2FCAIE%2FPaper-4%2FQP%2FMarch%202023%20(v2)%20QP.pdf
thanks!

Reply 1

Nitrotoluene

Original post by anonymous56754

First off, we've got [Cd(H2O)6]^2+(aq). Picture this: at the very heart, a cadmium ion, Cd^2+, surrounded by a team of six water molecules. Now, water's a pretty laid-back character in the ligand world; it interacts with the cadmium with a somewhat gentle grip. This hexaaqua setup is a common way for many positively charged metal ions with a 2+ charge to exist comfortably in a watery environment. It's like their default setting.
Then we stumble upon [Cd(OH)4]^2−(aq). Here, our Cd^2+ ion is linked up with four hydroxide ions, OH^−. Hydroxide is more active than water because it carries a charge, so this arrangement is more stable. Its negative charge means it can exert a stronger electrical pull on the positive cadmium ion.
Moving along, we encounter [CdCH3NH2)4]^2+(aq). In this scenario, four methylamine molecules (CH3NH2) are coordinated to the cadmium. Now, methylamine might be neutral like water, but it's got a special feature: its nitrogen atom. This nitrogen atom has a lone pair of electrons that it can donate to the cadmium much more readily than the oxygen atom in water. This enhanced electron donation creates a stronger coordinate bond, making this complex likely more stable than the one with water. It's like the nitrogen is offering a more generous gift of electrons.
Lastly, we have [CdCl4]^2−(aq), where four chloride ions (Cl^−) are bonded to the cadmium. Being negatively charged, these chloride ions also bring a significant attractive force to the party, strongly tugging on the Cd^2+ ion and contributing to the overall stability of the complex.
To really get to the bottom of these stability differences, we need to consider just how tightly these surrounding molecules, the ligands, are holding onto that central cadmium ion. As a general rule of thumb in the chemistry world, ligands that carry a negative charge or those that are exceptionally good at sharing their electrons tend to form more stable complexes. It's all about the strength of that bond.
Considering all these factors, the complex featuring methylamine, [Cd(CH3NH2)4]^2+(aq), is highly likely to be the most stable player in this lineup. The nitrogen in methylamine is a more generous electron donor compared to the oxygen in water, leading to a stronger, more tenacious interaction with the Cd^2+ ion.
On the flip side, the complex with water, [Cd(H2O)6]^2+(aq), is likely the least stable. Water, being a neutral ligand, doesn't have that extra electrical pull of a negative charge, nor is it as eager to donate electrons as methylamine.
Now, when we try to compare the chloride and hydroxide complexes directly without more specific data, it gets a bit difficult. Both Cl^− and OH^− are negatively charged, which signals a stronger interaction than neutral water. However, we can say with reasonable confidence that they are both more stable than the water complex.

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My 2 cents!

(edited 2 months ago)

Reply 2

TypicalNerd

Original post by anonymous56754

They’ve given you the Kstab values for Cd^2+ + {insert ligand} <=> {relevant complex} (+ water) on the previous page.

Since all of these Kstab values are large, the resulting complexes are all more stable than the precursor, Cd^2+ (which has been used as a simplified representation of the hexaaqua complex).

I’d have to disagree with Nitrotolene’s choice of the most stable ligand based on these values, despite how well-reasoned and generally accurate their reply is. The Kstab values indicate that the methylamine complex is actually the second most stable complex - the tetrahydroxocadmate(II) complex is about 150 x more stable than the [Cd(MeNH2)4]^2+ complex according to the data given earlier.

(edited 2 months ago)

Reply 3

anonymous56754

Original post by TypicalNerd

They’ve given you the Kstab values for Cd^2+ + {insert ligand} <=> {relevant complex} (+ water) on the previous page.
Since all of these Kstab values are large, the resulting complexes are all more stable than the precursor, Cd^2+ (which has been used as a simplified representation of the hexaaqua complex).
I’d have to disagree with Nitrotolene’s choice of the most stable ligand based on these values, despite how well-reasoned and generally accurate their reply is. The Kstab values indicate that the methylamine complex is actually the second most stable complex - the tetrahydroxocadmate(II) complex is about 150 x more stable than the [Cd(MeNH2)4]^2+ complex according to the data given earlier.

so is the hexa aqua ion generally always the least stable if all Kstab values are 10^x?

Reply 4

TypicalNerd

Original post by anonymous56754

so is the hexa aqua ion generally always the least stable if all Kstab values are 10^x?

If Kstab > 1, then you can say with confidence that the product is more stable than the reactant.

If all Kstab values are greater than 1, then the reactant is the least stable of all the complexes.