An [AsCCAs] ligand featuring a central alkyne and two flanking arsenic donors was employed for the synthesis of a trihydrido rhenium complex, while the corresponding phosphorus ligand was shown to be less suited. The reactivity of the former trihydride [AsCCAs]ReH 3 (3) was examined in detail, which revealed that two alternative reaction channels may be entered in dependence of the substrate. Upon reaction of 3 with PhC�CPh, ethylene, and CS 2 , monohydrides of the general formula [AsCCAs]Re(L)H with L = η 2 -PhC�CPh (4), η 2 -H 2 C�CH 2 (5), and η 2 -CS 2 (6) were formed along with H 2 . In contrast, insertion products of the type [AsCCAs]Re(X)H 2 (7−9) were obtained upon treatment of 3 with CyN�C�NCy, PhN�C�O, and Ph 2 C�C�O, while CO 2 failed to react with 3 under identical reaction conditions. Given that several productive reactions between CO 2 and hydrido rhenium carbonyls have been reported in the literature, 3 was further derivatized by introducing CO and t BuNC coligands, respectively. This led to the isolation of trans-[AsCCAs]ReH(CO) 2 (trans-10) and trans-[AsCCAs]ReH(CN t Bu) 2 (trans-11), which were shown to thermally isomerize to the corresponding cis-configured products, cis-10 and cis-11. Interestingly, only the cis-complexes were found to react with CO 2 , which was rationalized by evaluating the relative nucleophilicities of the hydrides in cis-10, trans-10, cis-11, and trans-11 via Fukui analysis. The formates cis-[AsCCAs]Re(OCHO)(CO) 2 (12) and cis-[AsCCAs]Re(OCHO)(CN t Bu) 2 (13) were isolated and shown to contain κ 1 -O-coordinated formate moieties. Treatment of 12 with [LutH]Cl/B(C 6 F 5 ) 3 (or with Ph 3 SiCl) led to the liberation of [LutH][OCHO•••B(C 6 F 5 ) 3 ] (or triphenylsilyl formate) with concomitant formation of the expected chloro complex cis-[AsCCAs]ReCl(CO) 2 ( 14). In a closed synthetic cycle, hydride 12 was regenerated from the latter chloride using NaBEt 3 H as a hydride source.

In this work, the change of reactivity induced by the introduction of two para ‐ethynyl substituents (CCSi( i Pr) 3 or CCH) to the organic electron‐donor 1,2,4,5‐tetrakis(tetramethylguanidino)‐benzene is evaluated. The redox‐properties and redox‐state dependent fluorescence are evaluated, and dinuclear Cu I and Cu II complexes synthesized. The Lewis‐acidic B(C 6 F 5 ) 3 substitutes the proton of the ethynyl −CCH groups to give new anionic −CCB(C 6 F 5 ) 3 − substituents, leading eventually to a novel dianionic strong electron donor in its diprotonated form. Its two‐electron oxidation with dioxygen in the presence of a copper catalyst yields the first redox‐active guanidine that is neutral (instead of cationic) in its oxidized form.