Schiff base ligand forming complex with metal in zero oxidation state
Dear Sengottuvelan Balasubramanian complexes of Schiff base ligands with transition metals in the oxidation state zero are not very common. Quite often Schiff base liagnds such as the famous "salen" have –OH fundtional groups which are deprotonated upon complexation with transition metals so that the metals in the resulting complexes have higher oxidation states. When looking for M(0) compleses of Schiff base ligands you should especially search for metal carbonyl-derived complexes. For example, please have a look a the following relevant literature reference:
Metal carbonyl complexes with Schiff bases derived from 2-pyridinecarboxaldehyde: Syntheses, spectral, catalytic activity and antimicrobial activity studies
Article Metal carbonyl complexes with Schiff bases derived from 2-py...
Fortunately this article is freely available as public full text on RG. See particularly compound 6 which contains the neutral Schiff base ligand and is thus a zerovalent metal complex. I hope this answers your question. Good luck with your work and best wishes, Frank Edelmann
Dear Prof.Edelmann, thank you for the J.Mol.Struct article in which the metal hexacarbonyl acts as a precursor complex. The substitution of metal carbonyls by other ligands to form a number of organometallic complexes has been reported. The labile nature of carbonyl ligand facilitates easy substitution. The presence of carbonyl as a co-ligand not only stabilizes metal in zero oxidation state but also in negative oxidation states. I am not totally convinced by the structural assignment made to complexes (1) and (4) in which Cr exists in zero oxidation state when all the carbonyl ligands have been displaced. The purification of the complexes by washing with hot petroleum ether and recrystallization from DMF does not rule out the presence of polynuclear species. Perhaps crystal structure / XPS will clearly establish the nature of the central metal (ion ?)
Hello Sengottuvelan Balasubramanian many thanks for your kind response. I fully agree with you in your skepticism about the paper cited in my first answer. In my opinion it is already suspicious to publish a paper on the "Journal of Molecular Structure" without a single crystal structure determination! You are also absolutely right in that the structural assignment of compounds (1) and (4) as zero-valent chromium complexes is clearly wrong. I'm convinced that in both complexes the ligand is deprotonated and we have here complexes of chromium(III) and chromium(II), respectively. Obviously the reviewers of this manuscript did a bad job.
Good luck with your work and best wishes, Frank Edelmann
Thanks for your understanding. The carbonyls are pi- acid ligands and are capable of forming M-L back bonding which is not available in the azomethine ligand. Even SciFinder search did not yield any result for this question. That was the reason I posted this question in this forum
Article Metal carbonyl complexes with Schiff bases derived from 2-py...
The question is not with respect to metal carbonyl complexes. There are thousands of examples available in the literature. It is the direct bonding between M(0) and the azomethine ligand
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