Back-bonding between an electron-poor, high-oxidationstate metal and poor -acceptor ligand in a uranium(V)-dinitrogen complex . Back-bonding between an electron-poor, high-oxidation-state metal and poor -acceptor ligand in a uranium(V)-dinitrogen complex. Nature Chemistry,11,[806][807][808][809][810][811] Back-bonding between an electron-poor, high-oxidation-state metal and poor π-acceptor ligand in a uranium(V)-dinitrogen complex Abstract A fundamental bonding model in coordination and organometallic chemistry is the synergic, donor-acceptor interaction between a metal and a neutral π-acceptor ligand where the ligand σdonates to the metal, which π-back-bonds to the ligand. This interaction typically involves a metal with an electron-rich, mid-, low-, or even negative, oxidation state and a ligand with a π* orbital.Here, we report that treatment of a uranium-carbene complex with an organo-azide produces a uranium(V)-bis(imido)-dinitrogen complex, stabilised by a lithium counter-ion. This complex, which has been isolated in crystalline form, involves an electron-poor, high-oxidation-state uranium(V) 5f 1 ion that is π-back-bonded to the poor π-acceptor ligand dinitrogen. We propose that this is made possible by a combination of cooperative heterobimetallic uranium-lithium effects and the presence of suitable ancillary ligands rendering the uranium ion unusually electron-rich. This electron-poor back-bonding could have implications for the field of dinitrogen activation.

Reaction of [U{C(SiMe3)(PPh2)}(BIPM)(μ‐Cl)Li(TMEDA)(μ‐TMEDA)0.5]2 (BIPM=C(PPh2NSiMe3)2; TMEDA=Me2NCH2CH2NMe2) with [Rh(μ‐Cl)(COD)]2 (COD=cyclooctadiene) affords the heterotrimetallic UIV−RhI 2 complex [U(Cl)2{C(PPh2NSiMe3)(PPh[C6H4]NSiMe3)}{Rh(COD)}{Rh(CH(SiMe3)(PPh2)}]. This complex has a very short uranium–rhodium distance, the shortest uranium–rhodium bond on record and the shortest actinide–transition metal bond in terms of formal shortness ratio. Quantum‐chemical calculations reveal a remarkable RhnormalI→→ UIV net double dative bond interaction, involving RhI 4dnormalz2 ‐ and 4dxy/xz‐type donation into vacant UIV 5f orbitals, resulting in a Wiberg/Nalewajski–Mrozek U−Rh bond order of 1.30/1.44, respectively. Despite being, formally, purely dative, the uranium–rhodium bonding interaction is the most substantial actinide–metal multiple bond yet prepared under conventional experimental conditions, as confirmed by structural, magnetic, and computational analyses.

Reaction of [U{C(SiMe 3 )(PPh 2 )}(BIPM)(m-Cl)Li-(TMEDA)(m-TMEDA) 0.5 ] 2 (BIPM=C(PPh 2 NSiMe 3 ) 2 ; TMEDA = Me 2 NCH 2 CH 2 NMe 2 )w ith [Rh(m-Cl)(COD)] 2 (COD = cyclooctadiene) affords the heterotrimetallic U IV À.This complex has avery short uranium-rhodium distance,t he shortest uranium-rhodium bond on recordand the shortest actinide-transition metal bond in terms of formal shortness ratio.Q uantum-chemical calculations reveal ar emarkable Rh I! ! U IV net double dative bond interaction, involving Rh I 4d z 2 -and 4d xy/xz -type donation into vacant U IV 5f orbitals,r esulting in aW iberg/Nalewajski-Mrozek U À Rh bond order of 1.30/1.44, respectively.D espite being,f ormally,p urely dative,t he uranium-rhodium bonding interaction is the most substantial actinide-metal multiple bond yet prepared under conventional experimental conditions,a s confirmed by structural, magnetic,a nd computational analyses.

We report the preparation of a range of alkali metal uranyl(VI) tri- bis(silyl)amide complexes [{M(THF) }{(μ-O)U(O)(N″)}] (1M) (N″ = {N(SiMe)}, M = Li, Na, x = 2; M = K, x = 3; M = K, Rb, Cs, x = 0) containing electrostatic alkali metal uranyl-oxo interactions. Reaction of 1M with 2,2,2-cryptand or 2 equiv of the appropriate crown ether resulted in the isolation of the separated ion pair species [U(O)(N″)][M(2,2,2-cryptand)] (3M, M = Li-Cs) and [U(O)(N″)][M(crown)] (4M, M = Li, crown = 12-crown-4 ether; M = Na-Cs, crown = 15-crown-5 ether). A combination of crystallographic studies and IR, Raman and UV-vis spectroscopies has revealed that the 1M series adopts contact ion pair motifs in the solid state where the alkali metal caps one of the uranyl-oxo groups. Upon dissolution in THF solution, this contact is lost, and instead, separated ion pair motifs are observed, which is confirmed by the isolation of [U(O)(N″)][M(THF) ] (2M) (M = Li, n = 4; M = Na, K, n = 6). The compounds have been characterized by single crystal X-ray diffraction, multinuclear NMR spectroscopy, IR, Raman, and UV-vis spectroscopies, and elemental analyses.

Two families of lanthanide(iii) phospholyl borohydride complexes are reported (carbon = grey, hydrogen = white, oxygen = red, boron = yellow, phosphorus = magenta, potassium = blue, lanthanides = teal; only BH4 hydrogens are shown for clarity).