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How do you know the number of ligands surrounding a metal ion?

Example: the obtained complex of $\ce{Ni^2+}$ ion in $\ce{NH3}$ solution is written as $\ce{[Ni(NH3)6]^2+}$ but $\ce{Cu^2+}$ in $\ce{NH3}$ solution is $\ce{[Cu(NH3)4]^2+}$.

So how do you find the number for different metal ions like $\ce{Ag^2+}$?

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    $\begingroup$ The coordination number (number of ligands) changes depending on the type of ligand. In this case ammonia. $\endgroup$ – Ali Caglayan May 24 '15 at 19:48
  • $\begingroup$ I suspect you may have erroneously tried to use the silver dication $\ce{Ag^{2+}}$ as a standard example of transition metal ion by extrapolating the most common aqueous copper ion, $\ce{Cu^{2+}}$. In reality, $\ce{Ag^{2+}}$ is rather unstable, and silver is much more commonly found as a monocation, $\ce{Ag+}$. $\endgroup$ – Nicolau Saker Neto May 24 '15 at 22:53
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    $\begingroup$ @NicolauSakerNeto $\ce{Ag^{2+}}$ is perfectly stable in proper neighborhood. In fact $\ce{Ag+}$ disproportionate giving $\ce{Ag^{2+}}$ and metallic silver in some conditions. $\endgroup$ – permeakra May 24 '15 at 23:28
  • $\begingroup$ @permeakra Quite true, I didn't mean to imply it wasn't stable ever. To my understanding $\ce{Ag^{2+}}$ is stable in a narrower range of conditions, though, or at least doesn't overlap much with "common" conditions, for some reasonable definition of common. Coordination compounds in aqueous media with $\ce{Ag^{2+}}$ ions are relatively scarce, I would believe. $\endgroup$ – Nicolau Saker Neto May 24 '15 at 23:37
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    $\begingroup$ @NicolauSakerNeto you are incorrect in it. $\ce{Ag^{2+}}$ is quite stable in bipiridyl complexes and again, $\ce{Ag+}$ disproportionate in presence of some ligands pubs.acs.org/doi/abs/10.1021/ja00775a074 $\endgroup$ – permeakra May 24 '15 at 23:44
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To my knowledge, there is no "rule" to predict the coordination number of a given metal ion. There are, however, guidelines, which are nicely summarized here.

  • Larger ions can accommodate more ligands
  • Bulky ligands will reduce the overall coordination number
  • Highly charged ions will tend to accept a greater number of Lewis Bases

With your example, we can make an educated guess as to the coordination number of Ag2+ using periodic trends. It is in the same group as copper and will be bigger since it is lower on the periodic table. Based on the three points above, I would predict that Ag2+ could have a coordination number greater than 4. It turns out that (distorted) octahedral coordination of the silver dication is not all that uncommon.

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1st-row $d$-element + 2nd-row $p$-element - usually octahedron except for $\ce{Ni(II),Cu(II),Co(I)}$ (often a square or a distorted octahedron with two weakly bound ligands) and $\ce{Zn}$ (usually a tetrahedron). 1st-row $d$-element + 3+-row $p$-element - usually a tetrahedron.

2nd/3rd row $d$-element are somewhat more complicated as they allow higher coordination number up to 9 ($\ce{ReH9^{2-}}$), but the situation is so much influenced by electron configuration, ligands and metal-metal interactions here that simple rules loose a lot of meaning.

In short, the coordination number is usually a compromise between 18-electron rule and size of ligands. For example, $\ce{V(CO)6}$ should dimerize, giving c.n. of 7 to vanadium, but can't do so because of size constrains. Somewhat similar happens with bulky alcoholats of titanium, that tend to include $\ce{Ti(IV)}$ in octahedral coordination, but lower c.n. is observable in case of tritox (tris-tert-butyl) alcoholat.

Though it is generally not recommended to guess the c.n., it is always better to google for some reference compounds.

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