Bonding and 13C-NMR properties of coinage metal tris(ethylene) and tris(norbornene) complexes: Evaluation of the role of relativistic effects from DFT calculations

The pi-complexes of cationic coinage metal ions (Cu(I), Ag(I), Au(I)) provide useful experimental support for understanding fundamental characteristics of bonding and 13C-NMR patterns of the group 11 triad. Here, we account for the role of relativistic effects on olefin-coinage metal ion interaction for cationic, homoleptic tris-ethylene, and tris-norbornene complexes, [M(eta(2)-C2H4)(3)](+) and [M(eta(2)-C7H10)(3)](+) (M- Cu, Ag, Au), as representative case of studies. The M-(C=C) bond strength in the cationic, tris-ethylene complexes is affected sizably for Au and to a lesser extent for Ag and Cu (48.6%, 16.7%, and 4.3%, respectively), owing to the influence on the different stabilizing terms accounting for the interaction energy in the formation of coinage metal cation-pi complexes. The bonding elements provided by olefin -> M sigma-donation and olefin <- M pi-backbonding are consequently affected, leading to a lesser cova lent interaction going down in the triad if the relativistic effects are ignored. Analysis of the C-13-NMR tensors provides further understanding of the observed experimental values, where the degree of backbonding charge donation to pi(2)*-olefin orbital is the main influence on the observed high-field shifts in comparison to the free olefin. This donation is larger for ethylene complexes and lower for norbornene counterparts. However, the bonding energy in the later complexes is slightly stabilized given by the enhancement in the electrostatic character of the interaction. Thus, the theoretical evaluation of metal-alkene bonds, and other metal-bonding situations, benefits from the incorporation of relativistic effects even in lighter counterparts, which have an increasing role going down in the group.

Automated summary

The paper discusses the impact of relativistic effects on the interaction between cationic coinage metal ions and olefins, revealing its influence on bond strength and 13C-NMR patterns. The study finds that the effects of relativity on metal-alkene bonds become more significant as one goes down in the group of elements.