Are the prefixes bis-, tris- etc. used only for ligands that are bidentate in complex nomenclature?


For naming coordination compounds, the multiplicative prefixes ‘di’, ‘tri’, ‘tetra’, ‘penta’, etc., are used in the case of simple ligands, for example:

$\ce{[PtCl4]^2-}$  tetrachloridoplatinate(2−)

The alternative multiplicative prefixes ‘bis()’, ‘tris()’, ‘tetrakis()’, ‘pentakis()’, etc. are not only used for bidentate ligands. They are generally used with composite ligand names or in order to avoid ambiguity, for example:

$\ce{[Pt(PPh3)4]}$  tetrakis(triphenylphosphane)platinum(0)

The corresponding section in the current version of Nomenclature of Inorganic Chemistry – IUPAC Recommendations 2005 (Red Book) reads as follows:

IR- Number of ligands in a coordination entity

Two kinds of multiplicative prefix are available for indicating the number of each type of ligand within the name of the coordination entity (…).

(i) Prefixes di, tri, etc. are generally used with the names of simple ligands. Enclosing marks are not required.

(ii) Prefixes bis, tris, tetrakis, etc. are used with complex ligand names and in order to avoid ambiguity. Enclosing marks (…) must be placed around the multiplicand.

The use of multiplicative prefixes is also explained in the IUPAC Technical Report Brief guide to the nomenclature of inorganic chemistry. Pure Appl. Chem. 2015, 87(9–10), 1039–1049 as well as in the corresponding four-sided lift-out document, which is available as supplementary material.

Composite ligand names include ligand names which are substituted or which themselves contain multiplicative prefixes or locants.

For example, the normal prefix is used in ‘diammine’ $(\ce{(NH3)2})$, but the alternative multiplicative prefix is used in ‘bis(methylamine)’ $(\ce{(CH3NH2)2})$ to make a distinction from ‘dimethylamine’ $(\ce{(CH3)2NH})$.

Note that the use of the alternative multiplicative prefixes is not limited to coordination compounds. They are generally used for compositional names in inorganic chemistry when a simple multiplicative prefix may be misinterpreted. For example,
‘tris(iodide)’ is used for $\ce{(I- )3}$ (rather than ‘triiodide’, which is used for $\ce{I3-}$);
‘bis(phosphate)’ is used for $\ce{(PO4^3- )2}$ (rather than ‘diphosphate’, which is used for $\ce{P2O7^4-}$.

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    $\begingroup$ +1 for the link to the new technical report and the Red Book. $\endgroup$ – Geoff Hutchison Feb 13 '16 at 18:41

Both the prefixes bis-, tris-, tetrakis- etc. and the prefixes di-, tri-, tetra- etc. are used to describe how many times a certain single ligand attaches to a central metal — and this usage is no different here than in typical molecule nomenclature. The ‘standard’ form one would use is di-, tri-, tetra- etc.

However, sometimes this can lead to ambiguity or confusion. Consider for example the triphosphate ion $\ce{P3O10^5-}$. Its structure is described well as $\ce{(O3P-O-PO2-O-PO3)^5-}$ — it is a single molecule consisting of three phosphorus atoms with oxygens in between and each phosphorus having a total of four oxygens surrounding it. Triphosphate can form stable chelate complexes with calcium ions and is therefore often added to washing powder. Let us assume a complex of calcium and triphosphate described by the formula $\ce{[Ca(P3O10)]^3-}$. (I don’t know if this is an actually existing one, but it can serve well as an example.) If we had to name it, we would arrive at triphosphato-calciate.

Now, of course, simple phosphate ions can also coordinate metals. It is totally conceivable that three phosphate ions coordinate a single calcium ion to give a complex represented by the following formula $\ce{[Ca(PO4)3]^7-}$. (Actually, this in itself is rather unlikely due to the high negative charge — using hydrogen phosphate would be a much more likely example but wouldn’t serve well enough to highlight the problem. Please note again that I don’t claim any of these complexes to actually exist, I am merely using them as convenient examples.) We now have three phosphato-ligands so naively we could name the complex triphosphato-calciate again.

This is where the ambiguity-prevention squad makes their glorious entry. To make it clear that we are dealing with three distinct phosphato ligands, we can switch to the prefix tris- which is reserved for multiple distinct occurrances of a single species. So a better name for this second complex $\ce{[Ca(PO4)3]^7-}$ would be tris-phosphato-calciate.

After ambiguity, let’s turn to confusion. Triphenylphosphane $\ce{(C6H5)3P}$, often abbreviated $\ce{PPh3}$, is a common ligand for catalysts used in organic chemistry. It consists of three phenyl rings connected to a phosphorus atom; the donated lone pair is phosphorus’. Consider Wilkinson’s catalyst $\ce{[RhCl(PPh3)3]}$. Naming this one would result in chloridotri(triphenylphosphano)rhodium(I) as an initial guess. Note the double-tri in the middle causing possible confusion. Instead, the prefix is changed to tris- which again means three occurrances of the same distinct species — in this case, triphenylphosphane. So the commonly used name for Wilkinson’s catalyst is chlorido-tris(triphenylphosphano)-rhodium(I).

As you can see in the final example, triphenylphosphane is a monodentate ligand not able to chelate. Yet tris- (or in other compounds: bis-, tetrakis- etc.) is used. So it is wrong to think of that set of names only for polydentate ligands. In fact, trisphosphato explicitly refers to three single phosphate ligands rather than a much more polydentate triphosphate moiety.


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