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.

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    $\begingroup$ Well, it's still big enough... $\endgroup$
    – Mithoron
    Feb 12 '15 at 13:28

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!

  • $\begingroup$ Hi, sorry for reopening this topic, but the K+ ion is considered hard and its radius according to your source is 152pm, which is larger than the silver+ ion, so what causes K+ to be hard? $\endgroup$
    – phi2k
    Mar 20 '18 at 15:09
  • $\begingroup$ Polarizability enters as well. The $4d$ electrons in silver ion are relatively polarizable and so soften this ion beyond what you might expect from ionic radius alone. Potassium ion does not have that. $\endgroup$ Nov 19 '19 at 23:53

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.

  • $\begingroup$ Neither zinc(II) nor copper(II) would qualify as soft in any HSAB definition of the word. $\endgroup$
    – Jan
    Nov 22 '19 at 14:38
  • $\begingroup$ Filled or nearly d orbitals is one of the factors should be considered to label an acid soft (rbmcollege.ac.in/sites/default/files/files/reading%20material/…) but not the sole condition. Several properties should be considered when deciding an acid is soft or hard. Zinc(II) and copper(II) are smaller in ionic radius (both around 70 pm, while Ag(I) around 110 pm) and also they are highly charged. $\endgroup$
    – yyyccc
    Nov 23 '19 at 19:50

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