Reaction of lanthanum with Hg(C6F5)2 and bulky N,N'-bis(2,6-diisopropylphenyl)formamidine (HDippForm) in tetrahydrofuran gives [LaF{DippForm}2(THF)] with a rare terminal Ln-F bond, and a high yield of a novel functionalized formamidine, DippForm((CH2)4OC6F4H-o).

The development of alkali-metal amidinate reagents, in particular formamidinates, has proceeded hand-in-hand with fundamental advances in transition-metal bonding, e.g. metal-metal bonding, and the progressive departure from cyclopentadienyl support ligands in early transition-metal catalysis. This highly personalised account highlights the coordinative versatility of one alkali-metal amidinate subclass; the bis(aryl)formamidinates. These compounds have proven invaluable during transition-metal studies but were considered unworthy of investigation in their own right prior to our work.

Reactions of a range of the readily prepared and sterically tunable N,N'-bis(aryl)formamidines with lanthanoid metals and bis(pentafluorophenyl)mercury (Hg(C6F5)2) in THF have given an extensive series of tris(formamidinato)lanthanoid(III) complexes, [Ln(Form)3(thf)n], namely [La(o-TolForm)3(thf)2], [Er(o-TolForm)3(thf)], [La(XylForm)3(thf)], [Sm(XylForm)3], [Ln(MesForm)3] (Ln=La, Nd, Sm and Yb), [Ln(EtForm)3] (Ln=La, Nd, Sm, Ho and Yb), and [Ln(o-PhPhForm)3] (Ln=La, Nd, Sm and Er). [For an explanation of the N,N'-bis(aryl)formamidinate abbreviations used see Scheme 1.] Analogous attempts to prepare [Yb(o-TolForm)3] by this method invariably yielded [{Yb(o-TolForm)2(mu-OH)(thf)}2], but [Yb(o-TolForm)3] was isolated from a metathesis synthesis. X-ray crystal structures show exclusively N,N'-chelation of the Form ligands and a gradation in coordination number with Ln3+ size and with Form ligand bulk. The largest ligands, MesForm, EtForm and o-PhPhForm give solely homoleptic complexes, the first two being six-coordinate, the last having an eta1-pi-Ar--Ln interaction. Reaction of lanthanoid elements and Hg(C6F5)2 with the still bulkier DippFormH in THF resulted in C--F activation and formation of [Ln(DippForm)2F(thf)] (Ln=La, Ce, Nd, Sm and Tm) complexes, and o-HC6F4O(CH2)4DippForm in which the formamidine is functionalised by a ring-opened THF that has trapped tetrafluorobenzyne. Analogous reactions between Ln metals, Hg(o-HC6F4)2 and DippFormH yielded [Ln(DippForm)2F(thf)] (Ln=La, Sm and Nd) and 3,4,5-F3C6H2O(CH2)4DippForm. X-ray crystal structures of the heteroleptic fluorides show six-coordinate monomers with two chelating DippForm ligands and cisoid fluoride and THF ligands in a trigonal prismatic array. The organometallic species [Ln(DippForm)2(C[triple chemical bond]CPh)(thf)] (Ln=Nd or Sm) are obtained from reaction of Nd metal, bis(phenylethynyl)mercury (Hg(C[triple chemical bond]CPh)2) and DippFormH, and the oxidation of [Sm(DippForm)2(thf)2] with Hg(C[triple chemical bond]CPh)2, respectively. The monomeric, six-coordinate, cisoid [Ln(DippForm)2(C[triple chemical bond]CPh)(thf)] complexes have trigonal prismatic geometries and rare (for Ln) terminal C[triple chemical bond]CPh groups with contrasting Ln--C[triple chemical bond]C angles (Ln=Nd, 170.9(4) degrees; Ln=Sm, 142.9(7) degrees). Their formation lends support to the view that [Ln(DippForm)2F(thf)] complexes arise from oxidative formation and C--F activation of [Ln(DippForm)2(C6F5)] intermediates.

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New reactive, divalent lanthanoid formamidinates [Yb(Form)(2)(thf)(2)] (Form=[RNCHNR]; R=o-MeC(6)H(4) (o-TolForm; 1), 2,6-Me(2)C(6)H(3) (XylForm; 2), 2,4,6-Me(3)C(6)H(2) (MesForm; 3), 2,6-Et(2)C(6)H(3) (EtForm; 4), o-PhC(6)H(4) (o-PhPhForm; 5), 2,6-iPr(2)C(6)H(3) (DippForm; 6), o-HC(6)F(4) (TFForm; 7)) and [Eu(DippForm)(2)(thf)(2)] (8) have been prepared by redox transmetallation/protolysis reactions between an excess of a lanthanoid metal, Hg(C(6)F(5))(2) and the corresponding formamidine (HForm). X-ray crystal structures of 2-6 and 8 show them to be monomeric with six-coordinate lanthanoid atoms, chelating N,N'-Form ligands and cis-thf donors. However, [Yb(TFForm)(2)(thf)(2)] (7) crystallizes from THF as [Yb(TFForm)(2)(thf)(3)] (7a), in which ytterbium is seven coordinate and the thf ligands are "pseudo-meridional". Representative complexes undergo C-X (X=F, Cl, Br) activation reactions with perfluorodecalin, hexachloroethane or 1,2-dichloroethane, and 1-bromo-2,3,4,5-tetrafluorobenzene, giving [Yb(EtForm)(2)F](2) (9), [Yb(o-PhPhForm)(2)F](2) (10), [Yb(o-PhPhForm)(2)Cl(thf)(2)] (11), [Yb(DippForm)(2)Cl(thf)] (12) and [Yb(DippForm)(2)Br(thf)] (16). X-ray crystallography has shown 9 to be a six-coordinate, fluoride-bridged dimer, 12 and 16 to be six-coordinate monomers with the halide and thf ligands cis to each other, and 11 to have a seven-coordinate Yb atom with "pseudo-meridional" unidentate ligands and thf donors cis to each other. The analogous terbium compound [Tb(DippForm)(2)Cl(thf)(2)] (13), prepared by metathesis, has a similar structure to 11. C-Br activation also accompanies the redox transmetallation/protolysis reactions between La, Nd or Yb metals, Hg(2-BrC(6)F(4))(2), and HDippForm, yielding [Ln(DippForm)(2)Br(thf)] complexes (Ln=La (14), Nd (15), Yb (16)).

Treatment of N, NЈ-di(para-tolyl)formamidine (p-tolylformH) (1) with LiBu n in THF, DME or hexane/TMEDA leads to deprotonation of the amidine affording [Li 2 (p-tolylform) 2 (THF) 3 ]ؒ2THF (2), [Li(DME) 3 ][Li 2 (p-tolylform) 3 ] (3) and [{Li 2 (p-tolylform) 2 (TMEDA)} ∞ ] (4) respectively. Similar treatment of 1 with NaH or [Na{N(SiMe 3 ) 2 }] in THF or DME yields trinuclear [Na 3 (p-tolylform) 3 (THF) 4 ] (5) and dinuclear [{Na(p-tolylform)(DME)} 2 ] ( 6). All complexes were characterised by spectroscopy (NMR and IR) and X-ray crystallography. These show p-tolylform to be a versatile ligand for alkali metals, exhibiting a wide variety of binding modes, viz. µ-η 1 :η 1 in 2 and 3 (i.e. bridging mode), µ-η 2 :η 1 in 4 and 6 (i.e. bridging and chelating) and µ-η 2 :η 2 (i.e. bridging and double chelating) and µ 3 -η 2 :η 1 :η 1 (i.e. chelating and bridging between three metal centres) in 5. DALTON

Reaction of either LiInH 4 or [InH 3 (NMe 3 )] with the Arduengo-type carbene :CN(Mes)-C 2 H 2 N(Mes), Mes ) C 6 H 2 Me 3 -2,4,6, affords the indium trihydride complex [InH 3 {CN(Mes)-C 2 H 2 N(Mes)}]. This compound displays a remarkable thermal stability (dec 115 °C). In solution the products of decomposition depend on the solvent used. In toluene decomposition results in liberation of the free carbene and deposition of indium metal, and in tetrahydrofuran in the presence of indium metal hydrogen transfer occurs to give H 2 CN(Mes)C 2 H 2 N-(Mes) and indium metal, while in dichloromethane, chloride abstraction occurs from the solvent to yield [InCl 3 {CN(Mes)C 2 H 2 N(Mes)}]. A number of related compounds have also been prepared and their structures and properties investigated. These include [GaH 3 {CN-(Mes)C 2 H 2 N(Mes)}], which is the most stable gallium trihydride yet reported, and the indium hydride halide complex [InH 2 Cl{CN(Mes)C 2 H 2 N(Mes)}].

The ambient temperature reaction of the N-heterocyclic carbenes (NHCs) 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IDipp) with the triruthenium cluster [Ru3(CO)12], in a 3:1 stoichiometric ratio, results in homolytic cleavage of the cluster to quantitatively afford the complexes [Ru(CO)4(NHC)] (1; NHC = IMes, 2; NHC = IDipp). Reaction of the 2-thione or hydrochloride precursors to IMes, i.e. SIMes and IMes.HCl, with the same triruthenium cluster affords the complexes [Ru4(mu4-S)2(CO)9(IMes)2] (3) and [Ru4(mu4-S)(CO)10(IMes)2] (4) (3:1 and 2:1 reaction), and [{Ru(mu-Cl)(CO)2(IMes)}2] (7) (3:1 reaction) respectively. By contrast, the complex [Ru3(mu3-S)2(CO)7(IMeMe)2] (6), where IMeMe is 1,3,4,5-tetramethylimidazol-2-ylidene, is the sole product of the 2:1 stoichiometric reaction of SIMeMe with [Ru3(CO)12]. Compounds 1-4, 6 and 7 have been structurally characterised by single crystal X-ray diffraction.