Since the discovery of a stable "bottleable" N-heterocyclic carbene (NHC), there has been a spectacular explosion of interest in this ligand class. This interest stems from a desire to understand the fundamentals of the structure and bonding of these systems, but also because of their numerous and emerging applications in small molecule activation, homogeneous catalysis and Lewis acid-catalysed reactions. In this Tutorial Review, we introduce the reader to NHCs, cover general synthetic methods to prepare anionic tethered NHCs and their metal complexes, and discuss emerging applications in reactivity and catalytic studies.

The synthesis of lithium(I) complexes of the neutral and anionic salts of a tridentate aminobis(Nheterocyclic carbene) ligand, L, are reported (L ) N{CH 2 CH 2 (1-C[NCHCHNMes])} 2 ), along with the X-ray diffraction determined molecular structure of the imidazolium proligand [H 2 N{CH 2 CH 2 (1-HC-[NCHCHNMes])} 2 ][Br] 3 (H 4 LBr 3 ). The transamination reaction between the lithium chloride adduct of HL and Y[N{SiMe 3 } 2 ] 3 affords yttrium amidocarbene complexes, while the transamination reaction between the imidazolium proligand and Ti(NMe 2 ) 4 or Zr(NMe 2 ) 4 affords a titanium or zirconium amidocarbene complex, respectively. The molecular structure of one of the yttrium products, Y(L)-(N{SiMe 3 } 2 )Cl, is reported. The reactivity of a selection of the complexes with phosphine oxides is compared.Supporting Information Available: CIF files giving crystallographic data for compounds 1a and 4. This material is available free of charge via the Internet at http://pubs.acs.org. OM0701874

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Magnesium dications bind strongly to a tridentate anionic dicarbene ligand L = [N{CH(2)CH(2)(CNCHCHNMes)}(2)] forming dinuclear and trinuclear Mg complexes with some particularly short Mg-C bonds. Treatment of the proligand H(4)LCl(3) with three equivalents of methyl magnesium chloride or benzyl magnesium chloride affords Mg(3)(HL)Cl(6) in high yield. A suspension of in thf was heated to 80 degrees C for 2 h to afford Mg(2)(L)Cl(3), consistent with the loss of one equivalent of MgCl(2), and the deprotonation of the remaining acidic NH, lost as HCl gas. Treatment of Mg(3)(HL)Cl(6) with one equivalent of KC(8) results in deprotonation of the ligand amine NH to afford Mg(3)(L)Cl(5); treatment with a second equivalent forms the radical anion of the complex, K[Mg(3)(L)Cl(5)], which decomposes upon storage, precluding its structural characterisation. The acidic NH proton of the ligand in Mg(3)(HL)Cl(6) can also be removed by deprotonation with Li{N(SiMe(3))(2)}; additional equivalents of which also exchange the magnesium-bound chlorides for silylamido ligands, affording Mg(2)(L)Cl(2)N'' and Mg(2)(L)Cl(N'')(2), which have both been characterised by single-crystal X-ray diffraction studies.

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