Nitrogen-15 NMR chemical shifts in metal-ammine complexes. II. Cobalt(III) ammine and amine complexes.

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15N nuclear magnetic resonance spectra were measured for [CoX(15NH3]n+ (X=CN−, 15NH3, H2O, and Br−) and cis-[CoX2(en-15N,15N)2]n+ (X2=(CN−)2, (NO2−)2, en-15N,15N, ox2−, and (H2O)2; en-15N,15N and ox2− represent the 15N-enriched ethylenediamine and oxalate ion, respectively). Two sharp signals of 15N NMR were observed for [CoX(15NH3)5]n+ except for the cases of X=15NH3 and NO2−. The signals of 15N NMR in cis-[CoX2(en-15N,15N)2]n+, were assigned on the basis of the selectively proton irradiated spectra at frequencies which had been already assigned to each amino proton. The chemical shifts of 15N NMR are correlated to the Co–N bond strength; it is expected that the stronger is the Co–N bond, the greater is the shift of the 15N NMR signal to a higher field. When we express [CoX(15NH3)5]n+ and cis-[CoX2(en-15N,15N)2]n+ as [CoXN5]n+ and cis-[CoX2N4]n+, the effect on the 15N NMR chemical shifts by the substituent X is roughly additive.
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