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.

The coordination chemistry of the new NNP pincer ligand framework (QuiNacNacP) is explored with cobalt. Upon treatment of the cobalt(II) complex Co[QuiNacNacP]Cl with KC8, the formation of cobalt(I) dinitrogen complex Co[QuiNacNacP]N2 was observed. Co[QuiNacNacP]N2 crystallizes as a square planar (S = 0) complex with an essentially unactivated N2 ligand. In solution, the dinitrogen complex is in equilibrium with the paramagnetic T‐shaped complex Co[QuiNacNacP] (S = 1). The ability of Co[QuiNacNacP]Cl to act as a catalyst precursor in the reductive silylation of dinitrogen was also briefly explored. Reaction of ≈ 1000 equivalents KC8 with ≈ 1500 equivalents Me3SiCl (relative to Co[QuiNacNacP]Cl) under 1 atm of N2 furnished roughly 40 equivalents of N(SiMe3)3.

A flexible [PNP] ligand (forming six-membered chelates upon coordination) was employed for the synthesis of niobium(IV) and tantalum(IV) complexes. Starting from the protioligand (H[PNP]) and the tetrachlorides NbCl4(thf)2 or TaCl4, the trichloro complexes [PNP]MCl3 (1-M, M = Nb, Ta) were prepared via in situ deprotonation using LiN(SiMe3)2. Attempts to alkylate complexes 1-M (M = Nb, Ta), however, were impeded due to their limited solubility in nonchlorinated solvents. Hence, the lithiated ligand was reacted with (thf)2Cl3M=CHCMe2Ph (M = Nb, Ta) to afford the d 0-configured alkylidene complexes 2-Nb and 2-Ta, respectively. These meridionally coordinated and freely soluble complexes were found to be stable in nonchlorinated and chlorinated solvents, even at elevated temperature. Reduction of compounds 2-M with magnesium anthracene (2-Nb and 2-Ta) or sodium naphthalide (2-Ta) afforded the corresponding d 1-configured alkylidenes (3-Nb and 3-Ta), although the reduced niobium derivative was found to decompose rapidly. A 10-line and an 8-line EPR pattern was detected for 3-Nb (I(93Nb) = 9/2) and 3-Ta (I(181Ta) = 7/2), respectively, confirming the metalloradical character of each complex. For 3-Ta, the molecular structure was elucidated by single crystal X-ray diffraction. With the molecular structure of this d 1-configured (open-shell) tantalum alkylidene ascertained, its electronic structure was examined by DFT and CASSCF calculations.