Why is Ag+ a soft acid?

A soft species is typically large, polarizable and has a low charge (magnitude). The silver cation is not highly charged (this supports softness) however, it is anomalously small due to high nuclear charge and poor shielding by the d-orbitals. Consequently, shouldn't it also be less polarizable?


It does have a tendency to form bonds that are significantly covalent in nature which backs up the assertion that $\ce{Ag^+}$ is soft; however, I don't understand why. 




According to HSAB theory, Soft Acids have:


large atomic/ionic radius; 


low or zero oxidation state bonding;


high polarizability;


low electronegativity.


The ion $\ce{Ag+}$ satisfies all the aforementioned properties. 
I don't agree with you when you say it is anomalously small due to high nuclear charge and poor shielding by the d-orbitals. The effective nuclear charge has nothing to do here.


According to <http://www.webelements.com/silver/atom_sizes.html>: The radius of this ion is $114\ \mathrm{pm}$ when it's in 4-coordinate, tetrahedral geometry and it's $116\ \mathrm{pm}$ when it's 4-coordinate, square-planar geometry. Please just compare these radii with the radius of $\ce{Al^3+}$ ion : $53.5\ \mathrm{pm}$ which is considered as a hard acid.
I hope it's clear now!


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probably it is related to its filled $\mathrm{d}$ orbitals. Filled or nearly filled orbital elements counted as soft acids, and $\ce{Ag+}$ has totally filled $\mathrm{d}$ orbitals, and it can donate to ligands, making ligand-$\pi$ bonding, which causes stronger interaction.


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