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How would we assign denticity to bridging ligands?

For eg, consider $\ce{NH^-_2}$, it has 2 lone pairs and can hence act as a bridging ligand, making 2 metal-ligand bonds. So I thought it should be bidentate. However, my professor says it is monodentate, why so?

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  • $\begingroup$ My professor told us bridging ligands are monodentate, including mu-hydroxo ligands $\endgroup$ – K. Chopra Sep 9 '20 at 9:09
  • $\begingroup$ I was wrong. Jan's answer clears that up. Bridging ligands with a single donor atom are monodentate. $\endgroup$ – Aniruddha Deb Sep 9 '20 at 12:26
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    $\begingroup$ Does this answer your question? Why Cl⁻ can't act as bidentate ligand? $\endgroup$ – Mithoron Sep 10 '20 at 0:26
  • $\begingroup$ I think these two questions are sufficiently different to not warrant a dupe-closure. This one asks ‘What is the denticity of bridging ligands’ while the other asks ‘can an atom with multiple lone pair donate twice (I assume, although it isn’t implicit, to the same atom)?’. Full disclosure: I have a horse in the race as I have answered here. $\endgroup$ – Jan Sep 10 '20 at 11:10
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It is important to clearly separate the terms denticity, hapticity and bridging.

  • An atom or a ligand is bridging is one single atom bonds to two different acceptors.

  • A ligand (but never a single atom) can be multidentate or multihaptic if two atoms of that ligand can bond to the same acceptor.

A ligand such as the amide anion cannot be multidentate or multihaptic as it only has one coordinating atom: nitrogen. This nitrogen can coordinate to two different metals (in a μ-amido complex) but it still remains a monodentate ligand.

A ligand such as ethylenediamine or en for short ($\ce{H2N-CH2-CH2-NH2}$) can coordinate to the same central atom via each of the two nitrogen atoms. This is a bidentate ligand as it has two ‘teeth’ in the central atom.

A ligand such as ethene ($\ce{C2H4}$) can coordinate to a single central atom in a bihaptic (η2) fashion. Drawing shortest lines from nucleus to nucleus, both carbon atoms would be the same distance from the acceptor atom (in reality, it is better thought of the π system donating). Hapticity is reserved for such cases in which a continuous set of atoms coordinate to a central metal.

A ligand can be both multihaptic and multidentate. An example would be dibenzylideneacetone or dba ($\ce{Ph-CH=CH-C(=O)-CH=CH-Ph}$), in which both of the $\ce{C=C}$ double bonds act as a bihaptic ligand. (Due to geometry constraints, dba usually coordinates to two different acceptors rather than to a single one in a chelate fashion.)

Finally, it is possible for a ligand such as en to be both bidentate and briding, if either of the nitrogen atoms coordinates to two different metals. This requires deprotonation of at least one nitrogen.

The bottom line is: briding ligands can be monodentate or multidentate; they can be monohaptic or multihaptic. Only bridging ligands based on a single donor atom must be monodentate.

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