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The synthesis and molecular structures of the cobalt(II) N-heterocyclic carbene (NHC) complexes [(NHC)Co{N(SiMe3)2}2], where NHC = 1,3-bis(diisopropylphenyl)imidazolylidene (IPr) (6), 1,3-bis(mesityl)imidazolylidene (IMes) (7), and 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu) (8), are reported. Complexes 6-8 are rare examples of three-coordinate cobalt NHC complexes. The steric congestion within the coordination environments of the cobalt(II) centers in 6 and 7 results in the longest Co-C(NHC) distances currently known. Investigating the thermal stability of 6-8, we have found that the steric congestion in 6 is such that heating the complex to reflux in toluene prompts a rearrangement from the normal, C2-bonding mode of the IPr ligand to the corresponding "abnormal" or mesoionic bonding mode. The rearrangement results in formation of [(aIPr)Co{N(SiMe3)2}2] (9), which is the first cobalt complex of an abnormal NHC ligand. The Co-C bond in 9 is 0.06 Å shorter than the analogous bond in 6, suggesting that, although the rearrangement occurs due to the spatial demands of the IPr ligand, there is also a thermodynamic driving force in terms of the Co-C bond. In contrast to the case for 6, complex 7 is stable with respect to the normal-to-abnormal rearrangement. Refluxing complex 8 in toluene results in activation of a tert-butyl substituent, which is eliminated as isobutene, followed by formation of the 1-tert-butylimidazole complex [(tBuIm)Co{N(SiMe3)2}2] (10). [ABSTRACT FROM AUTHOR]

Chloride abstraction from the diamido-bismuth compound Bi(Me2Si{NAr}2)Cl (l, Ar = 2,6-i-Pr2C6H3) using MC13 (M = Al, Ga) is a facile route to cationic species. Stoichiometric reactions afford the tetrachlorometallate salts [Bi(Me2Si{NAr}2)][MCl4] (2a, M = Ah 3a, M = Ga), whereas reaction with 0.5 equiv of the group 13 reagent gives the μ-chlorido bridged cations [{Bi(Me2Si{NAr}2)}2(μ-Cl)][MCl4] (2b, M - Al; 3b, M = Ga). The crystal structure of 2a shows a formally two-coordinate bismuth cation, with a Bi--Cl contact to the [A1C14]- anion, whereas the structure of 3b shows a total of three Bi--Cl contacts to [GaCl4]-. Both species associate as {1:1 }2 dimers in the solid state through additional Bi....Cl interactions. Attempted preparation of cationic complexes using either NaBR4 (R = Ph, Et) or [HNEt3] [BPh4] were unsuccessful Instead of forming the borate salts, the neutral compounds Bi(Me2Si{NAr}2)R (4, R = Et; 5, R = Ph) were isolated as a result of aryl/alkyl transfer from boron to bismuth. [ABSTRACT FROM AUTHOR]

The 'normal' three-coordinate iron–NHC complex [(IPr)Fe(N″)2] (N″ = N(SiMe3)2) rearranges to its abnormal NHC analogue [(aIPr)Fe(N″)2] (6) on heating, providing a rare abnormal iron-aNHC complex, and the first such three-coordinate complex. The tert-butyl-substituted complex [(ItBu)Fe(N″)2] (4) undergoes a thermal decomposition that has not previously been observed in iron-NHC chemistry, resulting in the bis(imidazole) complex [(tBuIm)2Fe(N″)2] (7). A mechanism that involves consecutive C–H and C–N activation is proposed to account for the formation of 7. [ABSTRACT FROM AUTHOR]

Structurally characterized lithiumpentadienyls are very rare.We report the molecular structures of two lithium pentadienyls, [(pmdeta)Li{1-(R3Si)C5H6}] (3) and [(pmdeta)Li{1,5-(R3Si)2C5H5}] (4) (R3Si = Me2(NMe2)Si, pmdeta = N,N,N′,N″,N″-pentamethyldiethylenetriamine).In 3, the pentadienyl carbons adopt an η3coordination mode, whereas in 4η1coordination is found. The origins of the different coordinationmodes are discussed. [ABSTRACT FROM AUTHOR]