The chemistry of 2‐phosphaethynolate is burgeoning, but there remains much to learn about this ligand, for example its reduction chemistry is scarce as this promotes P‐C‐O fragmentations or couplings. Here, we report that reduction of [U(TrenTIPS)(OCP)] (TrenTIPS=N(CH2CH2NSiPri3)3) with KC8/2,2,2‐cryptand gives [{U(TrenTIPS)}2{μ‐η2(OP):η2(CP)‐OCP}][K(2,2,2‐cryptand)]. The coordination mode of this trapped 2‐phosphaethynolate is unique, and derives from an unprecedented highly reduced and highly bent form of this ligand with the most acute P‐C‐O angle in any complex to date (P‐C‐O ∡ ≈127°). The characterisation data support a mixed‐valence diuranium(III/IV) formulation, where backbonding from uranium gives a highly reduced form of the P‐C‐O unit that is perhaps best described as a uranium‐stabilised OCP2−. radical dianion. Quantum chemical calculations reveal that this gives unprecedented carbene character to the P‐C‐O unit, which engages in a weak donor–acceptor interaction with one of the uranium ions.

Little is known about the chemistry of the 2-arsaethynolate anion, but to date it hase xclusivelyu ndergone fragmentationr eactions whenr educed. Herein, we report the synthesis of [U(Tren TIPS )(OCAs)] (2,T ren TIPS = N(CH 2 CH 2 NSiiPr 3 ) 3 ), which is the first isolable actinide-2-arsaethynolate linkage. UV-photolysis of 2 resultsindecarbonylation, but the putative [U(Tren TIPS )(As)] product was not isolated and instead only [{U(Tren TIPS )} 2 (m-h 2 :h 2 -As 2 H 2 )] (3)w as formed.I nc ontrast, reduction of 2 with [U(Tren TIPS )] gave the mixed-valence arsenido[ {U(Tren TIPS )} 2 (m-As)] (4)i nv ery low yield. Complex 4 is unstable which precluded full characterisation, but these photolytic and reductive reactions testify to the tendency of 2-arsaethynolate to fragment with CO release and As transfer.H owever,a ddition of 2 to an electride mixture of potassium-graphite and 2,2,2-cryptandg ives [{U(Tren TIPS )} 2 {m-h 2 (OAs):h 2 (CAs)-OCAs}][K(2,2,2-cryptand)] (5). The coordination mode of the trapped 2-arsaethynolate in 5 is unique, and derivesf rom an ew highly reduced and bent form of this ligand with the most acute O-C-As angle in any complex to date (O-C-As ff%1288). The trapping rathert han fragmentationo ft his highly reduced O-C-Asu nit is unprecedented, and quantumc hemical calculations reveal that reductionc onfers donor-acceptor character to the O-C-As unit.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi. Figure 6. Selectedcomputed a-spin molecularo rbitals of quin5 À À :a)a-HOMOÀ4( 457a, À1.794 eV), b) a-HOMOÀ3( 458a, À0.651 eV).

The chemistry of 2-phosphaethynolate is burgeoning, but there remains much to learn about this ligand, for example its reduction chemistry is scarce as this promotes P-C-Ofragmentations or couplings.Here,wereport that reduction of [U(Tren TIPS )(OCP)] (Tren TIPS