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Are there positively charged ligands which can bind to a central metal atom to form coordination compounds?

My thoughts:

I know that ligands are Lewis bases which donate a pair of electrons, and the central metal atom is usually a Lewis acid.

However, a positive charge on a species usually means it is electron-deficient. In that case, there would be no suitable electrons for it to bond to the central metal atom. So, even if positively charged ligands do exist, I think there would be very few. In such cases, it might bond with central atom which is electron rich forming the coordinate bond in the opposite way. Is that possible?

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  • $\begingroup$ The $\ce{NO+}$ is a very good example of a positive ligand. $\endgroup$ – Pritt says Reinstate Monica Aug 26 '17 at 6:11
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Aromatic cations like the tropylium cation ($\ce{C7H7+}$)[1] or the cyclopropenyl cation ($\ce{C3H3+}$)[2] can coordinate to metals, creating sandwich or half-sandwich compounds. The positive charge is shared by all carbon atoms of the rings.


  1. Anderson, J. E.; Maher, E. T.; Kool, L. B. Electrochemical and spectroelectrochemical properties of the titanium sandwich complexes (Cp)Ti(L) and (Cp*)Ti(L), where L is either cyclooctatetraene (C8H8) or the tropylium ion (C7H7+). Organometallics 1991, 10 (5), 1248–1254. DOI: 10.1021/om00051a009.

  2. Lichtenberger, D. L.; Hoppe, M. L.; Subramanian, L.; Kober, E. M.; Hughes, R. P.; Hubbard, J. L.; Tucker, D. S. Electron distribution and bonding in η3-cyclopropenyl-metal complexes. Organometallics 1993, 12 (6), 2025–2031. DOI: 10.1021/om00030a011.

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The nitroso ligand ($\ce{NO^+}$) is also one of the most commonly occurring positively charged ligands.

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I would answer as a tentative yes. Your reasoning is correct that a ligand, which behaves as a Lewis base, is unlikely to have a positive charge (and therefore be electron deficient). The earliest reference to a positively-charged ion behaving as a ligand (that I've found) is from the journal Inorganic Chemistry (1969, volume 8 page 2331) which describes the synthesis and characterization of Cu(II) and Mn(II) complexes containing a N-methyldabconium cation:

enter image description here

In this case the ligand is positively charged, but there is also a lone pair on the non-methylated nitrogen that is most likely forming the coordinative covalent bond with the metal center. It is this property (a sufficiently large ligand that has a localized positive charge that is remote from a region with Lewis base character) that would result in a positively charged ligand, and we don't need to invoke a switching of the metal roles (where the metal becomes the Lewis base) as you suggest. That said, one of the exciting aspects of Inorganic Chemistry is the constant effort to "break the rules".

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In addition to already mentioned cationic onium ligands, there is hydrazinium $\ce{H2N-NH3+}$ and its derivatives [1 -- 3].

Crystal structure of hydrazinium-trichloro-copper(II) $\ce{Cu(N2H5)Cl3}$ [3] shows $\ce{[CuN2Cl3]}$ plane with nearly $120^\circ$ angle $\ce{Cu-N-N}$:

$\color{#EEEEEE}{\Large\bullet}~\ce{H}$; $\color{#3050F8}{\Large\bullet}~\ce{N}$; $\color{#1FF01F}{\Large\bullet}~\ce{Cl}$; $\color{#C88033}{\Large\bullet}~\ce{Cu}$.

enter image description here

Bibliography

  1. Goedken, V. L.; Vallarino, L. M.; Quagliano, J. V. Inorg. Chem. 1971, 10 (12), 2682–2685. DOI 10.1021/ic50106a011.
  2. Govindarajan, S.; Patil, K. C.; Manohar, H.; Werner, P.-E. Dalton Trans. 1986, 1, 119–123. DOI 10.1039/DT9860000119.
  3. Bushuyev, O. S.; Arguelles, F. A.; Brown, P.; Weeks, B. L.; Hope-Weeks, L. J. Eur. J. Inorg. Chem. 2011 (29), 4622–4625. DOI 10.1002/ejic.201100465.
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$\ce{Na2[Fe(CN)5(NO)]}$ is one example.

This is because it is special compound exception and here the oxidation state of the $\ce{NO}$ ion is +1 rather than 0. Thus the oxidation state of the ferrate ion turns out to be two.

$\ce{NO}$ with +1 oxidation state is named as nitrosonium or nitrosyl, thus $\ce{NO + NO_x+}$ is the nitrosonium or nitrosyl cation.

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The nitrosonium ($\ce{NO+}$) and nitronium ($\ce{NO2+}$) ions both bear positive charges, but are capable of acting as Lewis bases/ligands. In $\ce{NO+}$, a lone pair on the nitrogen serves as the Lewis basic site for dative bond formation, and in $\ce{NO2+}$ an oxygen lone pair plays the same role.

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