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Why Group 2B metals (Zn, Cd, Hg) can form complexes with Ammonia ($\ce{NH3}$), Cyanide ($\ce{CN-}$) etc. and not with ligands such as carbonyl ($\ce{CO}$), nitrosyl ($\ce{NO}$), and olefins? What I think is that M(2B)-$\ce{NH3}$/$\ce{CN-}$ complexes are stable without any pi-bonding whereas $\ce{CO}$/$\ce{NO}$ complexes require pi-bonding for stability. Backbonding (pi-bonding) is what stabilizes Carbonyl, Nitrosyl and olefin complexes. Group 2B metals (Zn, Cd, Hg) have filled 3d orbital and cannot accept electrons from ligands and do not form stable complexes with aforementioned ligands. Am I correct? Please explain this further if possible. Now, $\ce{Cu (1)}$, group 1B, also has a filled d-orbital (3d10), but its complexes with Carbonyl do exist such as $\ce{[Cu(CO)n]+}$ where n=1,2,3. Why is it so?

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2 Answers 2

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Zinc actually has been incorporated into a room-temperature carbonyl complex embeeded into a zeolite matrix [1]. Ordinarily, G12 metal centers have their $d$ orbitals too tightly bound to serve as pi donors to the pi-accepting carbon monoxide, but in the zinc-zeolite compound the oxo ligands provide that function and thus enable the zinc to accept carbonyl ligands.

Reference

  1. Guodong Qi, Qiang Wang, Yueying Chu, Jun Xu, Anmin Zheng, Jihu Su, Jiafu Chen, Chao Wang, Weiyu Wang, Pan Gaoa and Feng Deng, "Room temperature stable zinc carbonyl complex formed in zeolite ZSM-5 and its hydrogenation reactivity: a solid-state NMR study", Chem. Commun., 2015,51, 9177-9180. https://doi.org/10.1039/C5CC02601F
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In addition to Oscar's zinc complex, I want to point out the very first thermally stable cadmium carbonyl complex, $\ce{[Cd(CO)3(C6H3Cl)]4·2H2O}$. It has been synthesized with an overall yield of 35%.

Ref.: Subrata K Dey, Shyamapada Shit, Sankar P Dey, Samiran Mitra, K M Abdul Malik, First Report on Thermally Stable Cadmium Carbonyl Complex Containing an Interesting Chloroaryl Bridge: Isolation and Characterization, Chemistry Letters, Volume 40, Issue 8, 2011, Pages 810–812, DOI: 10.1246/cl.2011.810


FWIW, zinc(II) is also shown to form a series of divinylzinc π-complexes, $\ce{Zn[C(Me)CH2]2}$ and $\ce{Zn[C(H)CMe2]2}$, have been synthesized and isolated as white crystalline solids in 66 and 72% yield, respectively.

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Ref.: Unprecedented Alkene Complex of Zinc(II):  Structures and Bonding of Divinylzinc Complexes, Alfred Wooten, Patrick J. Carroll, Aaron G. Maestri, and Patrick J. Walsh, Journal of the American Chemical Society 2006 128 (14), 4624-4631, DOI: 10.1021/ja058700x

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