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Is the correct formula for diamminechloridonitrito-n-platinum(II) $\ce{[Pt(NH3)2Cl(NO2)]}$ or $\ce{[PtCl(NH3)2(NO2)]}$?

I was taught that the sequence of symbols within the coordination entity should be written in the following order:

  1. The symbol of the central metal atom is written first.
  2. The ligands are written afterwards and should be mentioned in alphabetical order according to the first symbol of their formulae. For example, for the coordination compound $\ce{[Co(H2O)2(NH3)4]Cl2}$, $\ce{H2O}$ is cited first and then $\ce{NH3}$. (As H comes before N)

However, it is given in my textbook that the formula for diamminechloridonitrito-n-platinum(II) is $\ce{[Pt(NH3)2Cl(NO2)]}$, where $\ce{NH3}$ is followed by $\ce{Cl}$.

I assumed that the correct formula would be $\ce{[PtCl(NH3)2(NO2)]}$ ($\ce{Cl}$ followed by $\ce{NH3}$) instead. What am I missing here?

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  • $\begingroup$ I believe your proposed formula is correct simply because the anionic ligands must be written before the neutral ligands. The alphabetical order applies in case of two ligands of the same type such as two neutral ligands. Source : chem.libretexts.org/Bookshelves/Inorganic_Chemistry/… $\endgroup$ – shattereddonuts Dec 19 '20 at 15:18
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    $\begingroup$ Diamine should have two ammonia molecules, so either formula you posted doesn't match. I can't find a structure for that complex, only some exercises from those Indian exams that pay more attention to memorization of garbage rules than understanding of chemistry. Knowing the structure would certainly help... Otherwise, from my point of view, it doesn't matter. $\endgroup$ – Martin - マーチン Dec 20 '20 at 13:16
  • $\begingroup$ @Martin-マーチン Whoops, my bad! You're right, the formula should have two ammonia molecules. I corrected it. And as for the Indian exams, I totally agree with you! The education system here encourages rote memorization instead of long term understanding. Unfortunately, this problem isn't just limited to chemistry... $\endgroup$ – Syra Dec 22 '20 at 4:30
  • $\begingroup$ in Indian exams it doesn't matter. we just have to memorize the rules for nomenclature and represent the compound the way we want(chlorine can be before or after, it doesn't matter). At our level the best we have to do is to learn to draw structures remember the(non-useful VBT) and just learn to feel their presence in the daily life and their application, presence and that is all $\endgroup$ – user99515 Dec 22 '20 at 15:46
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So I did some digging, and here's what I found:

According to the current version of Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005,

IR-9.2.3.1 Sequence of symbols within the coordination formula

(i) The central atom symbol(s) is (are) listed first.

(ii) The ligand symbols (line formulae, abbreviations or acronyms) are then listed in alphabetical order (see Section IR-4.4.2.2).Thus, CH3CN, MeCN and NCMe would be ordered under C, M and N respectively, and CO precedes Cl because single letter symbols precede two letter symbols. The placement of the ligand in the list does not depend on the charge of the ligand.

(iii) More information is conveyed by formulae that show ligands with the donor atom nearest the central atom; this procedure is recommended wherever possible, even for coordinated water.

So in this case, $\ce{Cl}$ should come before $\ce{NH3}$ and the correct formula would be $\ce{[PtCl(NH3)2(NO2)]}$.

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  • $\begingroup$ But if chlorine is not a ligand, things change. Anyway, good research. $\endgroup$ – Martin - マーチン Dec 22 '20 at 19:26

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