Are M–N bonds indeed inherently weaker when N is a tertiary rather than a primary or secondary nitrogen atom?
Introduction
It is commonly accepted that metal–nitrogen bonds are inherently weaker when the nitrogen is a tertiary rather than a primary or a secondary nitrogen atom [1]. This notion is based on three main observations:
- 1.
The stability constants of complexes with tertiary amines are considerably lower than those with the analogous primary or secondary amines [2], [3], [4], [5], [6], [7], [8], [9], [10].
- 2.
The ligand field induced by tertiary amines, as deduced from the energies of the d→d transitions of their complexes, is considerably lower than that induced by the analogous primary or secondary amines [4], [6], [10], [11], [12], [13], [14]1. This phenomenon is usually significant only when several amines are ligated to the same central cation [2], [3].
- 3.
N-alkylation of amine ligands shifts the redox potentials of the couples Mn+1/nLm anodically relative to those of the corresponding non-alkylated complexes [4], [5], [6], [10], [11], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27].
Section snippets
Discussion
For this purpose the properties of first row transition metal complexes with the following ligands were studied recently:the results point out the following general conclusions.
1. N-methylation decreases the stability constants of the corresponding complexes, thus for all systems studied Kn(L2n−1)>Kn(L2n) is observed [2], [3], [4], [6], [7], [8], [9], [10], [16], [29], [30], [31]; where KjLi=[MjLi]/[Mjaq][Li] and j is the oxidation state of the central metal cation M.
2. The effect of N
Concluding remarks
The results obtained in recent years point out that N-alkylation of amine ligands affects the properties of their metal ion complexes via several, often opposing, effects:
1. It decreases the outer sphere solvation energy of the complexes.
2. It eliminates the M–N–H⋯O hydrogen bonds, thus turning the nitrogen a poorer σ donor.
3. It elongates the M–N bonds due to steric hindrance.
4. The alkyl substituents are electron donating groups which turn the nitrogen into a better σ donor.
5. It often induces
Note added in proof
Recently (H. Deng, P. Kebarbe, J. Am. Chem. Soc. 120 (1998) 2925) reported that ΔG°393 for the reaction CuL2+⇄Cu++2L in the gas phase equals 82.7, 93.1 and 98.7 kcal/mol for L=NH3; NH2–C3H7 and N(n-C4H9)3, respectively, i.e. N-alkylation stabilizes the complexes in the gas phase.
Acknowledgements
I am indebted to my many colleagues whose work is cited in this manuscript, without their hard work it could not have been written. I wish to thank The Alexander von Humboldt Stiftung for a research prize which enabled my stay at the University of Erlangen-Nurnberg, during which this manuscript was written. This study was supported in part by grants from The Israel Science Foundation administered by The Israel Academy of Sciences and Humanities and by grants from the Budgeting and Planning
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