Using the possibility of tuning the metal ± metal distance in pyrazolatebased bimetallic complexes by the length of the chelating side arms attached to the heterocycle, we have synthesized related dinickel(ii) complexes in which either a hydroxide (1) or a H 3 O 2 unit (2) spans the two metal centers within the bimetallic pocket. Complex 1 exhibits strong antiferromagnetic coupling mediated by the bridging hydroxide unit (J À 56.9 cm À1 ), while 2 shows Curie ± Weiss behavior over a wide temperature range. The H 3 O 2 unit in 2 acts like the resting form of an active metalbound hydroxide and allows the study of cooperative transformations of small molecules at a dinickel(ii) site relevant to the active site of the metalloenzyme urease. Firstly, it mediates the hydration of various nitriles to give the respective m-amidato-N,O-bridged complexes 3 a ± c. Reaction of 2 with urea affords complex 5 with an N,O-bridging deprotonated urea, which when heated releases ammonia to yield the cyanato-bridged dinickel(ii) compound 4. This product is also obtained when 2 is treated with dimethylcyanamide. Unactivated amides dimethylformamide (DMF) and dimethylamide (DMA) are hydrolyzed by 2 only sluggishly, while the stoichiometric hydrolysis of esters such as ethyl acetate and ethyl propionate runs more cleanly to afford the carboxylato-bridged bimetallic complexes 10 b and c. Likewise, hydrolytic cleavage occurs with g-butyrolactone to give complex 10 d. We propose that these reactions proceed by cooperative action of the two metal ions within the bimetallic pocket of the dinickel(ii) core. Possible alternatives are addressed by investigation of analogous mononuclear nickel(ii) complexes 8 a and b in conjunction with a corresponding dimeric compound 9 and by the reaction of 2 with an excess of acetate to yield the dinickel(ii) complex 11, in which a formerly chelating side arm has been driven out of the coordination sphere. Complexes 1´(BPh 4 ) 2 , 2( BPh 4 ) 2 , 3c´(ClO 4 ) 2 , 4´(BPh 4 ) 2 , 8 b( ClO 4 ) 2 , 9´(ClO 4 ) 2 , 10 d´(ClO 4 ) 2 , and 11´(ClO 4 ) have been characterized structurally by X-ray crystallography.

Unique tetranickel(II) complexes have been synthesized which incorporate genuine mu3-1,1,3 or mu4-1,1,3,3 bridging azido ligands. Identification of two distinct isomeric mu4-1,1,3,3 binding modes confirms the structural flexibility of the quadruply bridging azide.

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Reaction of [U(Tren TIPS)(PH 2)] (1,T ren TIPS = N(CH 2 CH 2 NSiPr i 3) 3)w ith C 6 H 5 CH 2 Ka nd [U(Tren TIPS)-(THF)][BPh 4 ](2)afforded arare diuranium parent phosphi-nidiide complex [{U(Tren TIPS)} 2 (m-PH)] (3). Treatment of 3 with C 6 H 5 CH 2 Ka nd two equivalents of benzo-15-crown-5 ether (B15C5) gave the diuranium m-phosphido complex [{U(Tren TIPS)} 2 (m-P)][K(B15C5) 2 ](4). Alternatively,r eaction of [U(Tren TIPS)(PH)][Na(12C4) 2 ](5,1 2C4 = 12-crown-4 ether) with [U{N(CH 2 CH 2 NSiMe 2 Bu t) 2 CH 2 CH 2 NSi(Me)-(CH 2)(Bu t)}] (6)p roduced the diuranium m-phosphido complex [{U(Tren TIPS)}(m-P){U(Tren DMBS)}][Na(12C4) 2 ][ 7, Tren DMBS = N(CH 2 CH 2 NSiMe 2 Bu t) 3 ]. Compounds 4 and 7 are unprecedented examples of uranium phosphido complexes outside of matrix isolation studies,and they rapidly decompose in solution underscoring the paucity of uranium phosphido complexes.I nterestingly, 4 and 7 feature symmetric and asymmetric UPU cores,r espectively,r eflecting their differing steric profiles. Inrecent years there has been burgeoning interest in the synthesis and chemistry of uranium-ligand multiple bonds, [1] which stems from adesire to better understand the chemical bonding of uranium and to correlate this to observed physicochemical properties.H owever,m ost progress has been made regarding complexes where uranium engages in af ormal multiple bond to C-/N-/O-based donor ligands,a nd examples of second row-centered, and beyond, donor ligands generally continue to be rare. [2] Where uranium-phosphorus multiple bonding is concerned, [3] only two structurally authenticated phosphinidene complexes have been reported, [4] and investigations into uranium phosphido complexes are exceedingly rare and restricted to cryogenic matrix isolation and/or computational studies. [5] Thus,t here are no reports of uranium phosphido complexes on macroscopic scales under conditions that would permit further investigation ;i ndeed, the phosphido linkage,w hether terminal or m-bridging,r emains ar elatively rare structural motif even in transition-metal chemistry. [6] As part of our work on actinide-ligand multiple bonds, [7] we reported dithorium phosphido and arsenido complexes that are supported by the very sterically demanding triamido-amine ligand N(CH 2 CH 2 NSiPr i 3) 3 (Tren TIPS). [7a,d] Fort he ThPThd erivative this ligand combination produced as eem-ingly optimal balance of steric shielding of the ThPThc ore versus inter-Tren TIPS steric repulsion. We therefore considered whether the analogous diuranium complex might be accessible ;however,uranium has potentially deleterious and facile redox chemistry compared to the more redox-robust thorium, and is smaller than thorium by 0.05-0.18 Š, [8] so uranium with the same ligand set might well be too strained to form astable UPU linkage and could very easily decompose.H erein, however, we report two different methods for the bulk-scale preparation and subsequent characterization of diuranium m-phosphido complexes,u tilizing Tr en TIPS and the related Tr en DMBS (Tren D...

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The Fermi surface (FS) of the new organic metal (BEDT-TTF)4[Ni(dto)2] has been investigated by de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) experiments. In both quantum oscillations with two different frequencies Fα = 634 T and F β = 4245 T are observed. These results confirm the calculated FS, which consists of one-dimensional and twodimensional parts separated by a small energy gap. It is shown that the temperature and field dependence of the oscillation amplitudes can be well described by standard Lifshitz-Kosevich theory considering the magnetic breakdown by the so-called coupled network model. Beatings of the oscillation amplitudes are observed by field-dependent dHvA and SdH experiments. They can be explained by a slightly warped FS showing that this material represents a quasitwo-dimensional electronic system. The observed beating nodes in the dHvA and SdH signals appear at different magnetic fields. This fact is discussed in terms of additional scattering mechanisms.