The first anionic N-heterocyclic dicarbene, polymeric [:C{[N(2,6-Pr(i)(2)C(6)H(3))](2)CHCLi(THF)}](n) 1, containing both normal (C2) and abnormal carbene (C4) centers in the same five-membered imidazole ring (III), has been prepared by lithiation of the imidazole monocarbene, :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2). The dicarbene nature of 1 was unambiguously demonstrated by the formation of the group 13 Lewis acid adducts (THF)(2)Li:C{[N(2,6-Pr(i)(2)C(6)H(3))](2)CHC(LA)}, where LA = AlMe(3) [2·(THF)(2)] and BEt(3) [3·(THF)(2)].

In contrast to nitrogen, the lightest yet most ubiquitous Group 15 element, phosphorus and arsenic exhibit extensive allotropy. Along with the more stable polymeric forms, both white phosphorus and yellow arsenic are well-known tetrahedral allotropes. Indeed, P 4 and As 4 readily pyrolyze to afford P 2[1] and As 2 , [2,3] respectively, which are well-characterized energetically and spectroscopically in the gas phase, [4] but are only persistent at high temperatures. Cummins and co-workers demonstrated that it is possible to generate complexes of diphosphorus in condensed phases by mild thermal extrusion of P 2 from niobium diphosphaazide complexes.[5] We subsequently reported a new class of carbene-stabilized diphosphorus molecules, [LDPÀPDL] (LD = Nheterocyclic carbenes (NHCs)).[6] Most recently, Bertrand and co-workers synthesized cyclicA C H T U N G T R E N N U N G (alkyl)A C H T U N G T R E N N U N G (amino)carbene-stabilized P 1 and P 2 complexes by P 4 fragmentation. [7] The bond energy of N 2 (226 kcal mol À1) is nearly twice that of P 2 (116 kcal mol À1 ) [8] and thrice that of As 2 (83 kcal mol À1 ), [9] thus suggesting a diminished importance of p-p bonding among third and fourth period elements. [10] Consequently, unlike N 2 , which usually [11] exhibits bonding mode A (Scheme 1) in its complexes, the E 2 cores in the corresponding diphosphorus and diarsenic complexes may assume either bonding mode A (triply bonded dipnictogen) or B (singly bonded dipnictinidene; Scheme 1). For example, diarsenic, like diphosphorus, has been known to function as a four-, six-, or eight-electron-donor ligand (C-G, Scheme 1) in transition-metal (M)-carbonyl complexes.[1, 2, 12, 13] An E 2 (E = P or As) fragment with an A-type structure may be extracted from C-F, [1] whereas the dipnictinidene core (B) has been observed in G. [2,13] Is it possible to prepare a carbenestabilized diarsenic analogue of H (Scheme 1)?We recently demonstrated that NHCs can effectively stabilize highly reactive molecules. [14][15][16][17] Prominent examples of carbene-stabilized complexes not only include diphosphorus, [6] [18,19] and disilicon ([LDSi=SiDL]), [20] but this strategy also succeeded in the recent preparation of a neutral metalloaromatic Ga 6 octahedron ([LDGaA C H T U N G T R E N N U N G (Ga 4 Mes 4 )GaDL]). diborenes ([LD(H)B=B(H)DL]),[21] Herein, we now report the syntheses, X-ray structures, and computations of carbenestabilized arsenic derivatives of AsCl 3 ([L The hypervalent carbene-stabilized arsenic chloride compound 1 was prepared, in nearly quantitative yield, by the treatment of AsCl 3 with the carbene ligand (L 1 D) [22] (Scheme 2). Subsequent potassium/graphite reduction of 1 affords the carbene-stabilized diarsenic complex, 2, as airsensitive red crystals (Scheme 2

The lithiated N-heterocyclic carbene−phosphinidene adduct L′:P−H (3; L′: = :C{[N(2,6-Pri 2C6H3)]2CHCLi(THF)3}) unexpectedly resulted from the reaction of lithium metal with the carbene-stabilized diphosphorus species L:P−P:L (2; L: = :C{N(2,6-Pri 2C6H3)CH}2). Compound 2 was previously prepared by the potassium graphite reduction of L:PCl3 (1).

Oxidation of carbene-stabilized diarsenic, L:As-As:L [L: = :C{N(2,6-(i)Pr(2)C(6)H(3))CH}(2)] (1), with gallium chloride in a 1:4 ratio in toluene affords the dicationic diarsene complex [L:As═As:L](2+)([GaCl(4)](-))(2) (2(2+)[GaCl(4)](2)), while oxidation of 1 with GaCl(3) in a 1:2 ratio in Et(2)O yields the monocationic diarsenic radical complex [L:AsAs:L](•+)[GaCl(4)](-) (2(•+)[GaCl(4)]). Strikingly, complex 2(•+) is the first arsenic radical to be structurally characterized in the solid state. The nature of the bonding in these complexes was probed computationally and spectroscopically.

Reaction of carbene-stabilized disilicon L:Si═Si:L (L: = :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2), 1) with BH(3)·THF results in facile cleavage of the silicon-silicon double bond and the formation of two quite different "push-pull" stabilized products with borane- and carbene-coordinated silylene moieties: 2, containing a parent silylene (:SiH(2)); and 3, containing a unique three-membered cyclosilylene.