13C relaxation times, T1, in six-coordinate cobalt(III) complexes: Study of the axial CoN (heterocyclic) bond

✍ Scribed by Jack Huet; A. Gaudemer

• Publisher: John Wiley and Sons
• Year: 1982
• Tongue: English
• Weight: 677 KB
• Volume: 20
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270200103

✦ Synopsis

Carbon

-13 relaxation times, Tl, have been measured for ten cobalt(III)-cyclohexanedione dioxime complexes: CH,CH,-Co(Niox),-pR-pyridine [R = H, N(CH,),, CH,, C,H,, C(CH,),, Cl, Br, CN and COCH,] and CH,CH,-Co(Niox),-3-N-methylimidazole. The values obtained have been rationalized by making assumptions on the length of the metal-heterocyclic nitrogen bond. The internal rotation around the axial Co-N (heterocyclic) bond is faster for the 3-N-methylimidazole ligand than for the pyridine ligands. Correlations of the Tl values with the a-donor and ?r-acceptor character of the pyridine ligands were attempted. The interpretation of the results suggests the existence of ?r-back-bonding from the metal to the N-1 pyridine nitrogen atom, in agreement with the results of other workers. This conclusion, however, was not supported by the use of the para-<: chemical shift as a criterion for back-bonding in pyridine-transition metal complexes.