Understanding and quantifying the electronic properties of ligands is important both from a fundamental point of view and also for their applications in various branches of chemistry. In this contribution, we present a comparative study on the donor/acceptor properties of 10 different triazolylidene-type mesoionic carbene ligands (MICs), with the aim of investigating and deciphering the effects of substituents on the donor/acceptor properties of such MIC ligands. Data from 77Se NMR chemical shifts of the triazoline selones, the 1 J C‑H coupling constants of the corresponding triazolium salts, and the Tolman electronic parameters (TEP) of the Ir-CO complexes are compared and contrasted. The data collected in this work show that the spread of both the TEP and the 1 J C‑H coupling constants is relatively narrow to be able to significantly differentiate between the donor properties of the various MICs. The 77Se NMR shifts of the triazoline selones on the other hand show a broad spread, and it is in principle well-suited to understand the π-accepting properties of the MICs as a function of substituents. However, this method can have other complications which are discussed in this work. DFT calculations are presented to spread light on the 77Se NMR shifts. Additionally, we present the first example of a AuI complex with mesoionic triazoline selones, a class of ligands whose utility has remained underexplored in transition metal chemistry.

Carbene‐based radicals are important for both fundamental and applied chemical research. Herein, extensive electrochemical investigations of nine different 1,2,3‐triazolylidene selenium adducts are reported. It is found that the half‐wave potentials of the first reduction of the selones correlate with their calculated LUMO levels and the LUMO levels of the corresponding triazolylidene‐based mesoionic carbenes (MICs). Furthermore, unexpected quasi‐reversibility of the reduction of two triazoline selones, exhibiting comparable reduction potentials, was discovered. Through UV/Vis/NIR and EPR spectroelectrochemical investigations supported by DFT calculations, the radical anion was unambiguously assigned to be triazoline centered. This electrochemical behavior was transferred to a triazolylidene‐type MIC‐gold phenyl complex resulting in a MIC‐radical coordinated AuI species. Apart from UV‐Vis‐NIR and EPR spectroelectrochemical investigations of the reduction, the reduced gold‐coordinated MIC radical complex was also formed in situ in the bulk through chemical reduction. This is the first report of a monodentate triazolylidene‐based MIC ligand that can be reduced to its anion radical in a metal complex. The results presented here provide design principles for stabilizing radicals based on MICs.

The control of catalytic properties through ligand design is at the heart of homogeneous catalysis. Herein we report on ligand effects in six 1,3,4-substituted 1,2,3-triazolylidene gold complexes as precatalysts for the intermolecular hydroamination catalysis of alkynes with amines. Mainly silver-free protocols have been followed for the synthesis of the complexes and for the catalytic reactions. All of the novel gold complexes have been fully characterized, one of them being, to the best of our knowledge, the first 1,2,3-triazolylidene-based mesoionic carbene (MIC) gold complex with a weakly coordinating second ligand. In contrast to expectations, the gold complexes bearing the most strongly electron donating MICs did not display the best conversion in catalysis. According to our investigations the stability of the complex is a determining factor in the present case. The MIC gold bistriflimide complex shows activity in hydroamination catalysis without the need for an activating additive.