Reaction of the trivalent uranium complex [(((ArO)-Ar-Ad,Me)(3)N)U(DME)] with one molar equiv [Na(OCAs)(dioxane)(3)], in the presence of 2.2.2-crypt, yields [Na(2.2.2-crypt)][{(((ArO)-Ar-Ad,Me)(3)N)U-IV(THF)}(-O){(((ArO)-Ar-Ad,Me)(3)N)U-IV(CAs)}] (1), the first example of a coordinated (1)-cyaarside ligand (CAs-). Formation of the terminal CAs- is promoted by the highly reducing, oxophilic U-III precursor [(((ArO)-Ar-Ad,Me)(3)N)U(DME)] and proceeds through reductive C-O bond cleavage of the bound arsaethynolate anion, OCAs-. If two equiv of OCAs- react with the U-III precursor, the binuclear, -oxo-bridged U-2(IV/IV) complex [Na(2.2.2-crypt)](2)[{(((ArO)-Ar-Ad,Me)(3)N)U-IV}(2)(-O)(-AsCAs)] (2), comprising the hitherto unknown :(1),(1)-coordinated (AsCAs)(2-) ligand, is isolated. The mechanistic pathway to 2 involves the decarbonylation of a dimeric intermediate formed in the reaction of 1 with OCAs-. An alternative pathway to complex 2 is by conversion of 1 via addition of one further equiv of OCAs-.