Table 1. Data Available for the Chelate Complexes Discussed in the Text (in the Order of Appearance): Chemical Analysis (CA), IR, MS, NMR, Crystal Structure Determination (CSD), and Further Data (Brackets Denote That Only Selected Values Are Given) compound CA IR MS NMR CSD further data 1Na(THF) 29 2aK 33 2bK 33 13 65

The half-sandwich methylimido complexes [Nb(q-C,R,)(NMe) CI,] (R = H l a , or M e l b ) have been prepared by treatment of [Nb(q-C,R,) CI,] with NMe(SiMe,), in acetonitrile. The sterically hindered alkyland aryl-imido analogues [Nb(q-C,H,)(NR)CI,] (R = But lc, or C6H,Pri2-2,6 Id), [Nb(q-C,Me,)-(N-C6H,Pri,-2,6)CI,] l e and the tantalum compound [Ta(q-C,Me,)(N-C,H,Pri,-2,6)C12] 1 f are obtained by treatment of [M(q-C,R,)CI,] with t w o equivalents of NHR(SiMe,) in chlorocarbon solvent. Crystal structures of l a , lc, I d and I f show that these complexes are mononuclear with quasi-linear imido ligands. The metal-nitrogen bond distances range from 1.744(3) t o 1.780(5) A consistent with pseudo triple bonds. Compounds la-le react with tertiary phosphines t o give eighteen-electron adducts of the type [Nb(q-C,H,)(NR)CI,(PR',)] 2a-2e. The crystal structure of [Nb(q-C,H,)(NMe)CI,(PMe,)]shows a distorted four-legged piano-stool geometry in which the phosphine binds cis to the methylimido and cis t o one of the chloride ligands. Alkoxide and aryloxide derivatives of the type [Nb(q-C,R,) (NR')(OR"),] may be prepared via reactions of the dichlorides 1 with 2 equivalents of LiOR" in Et,O. Fenske-Hall quantumchemical calculations indicate that there exists a close relationship between the Nb(q-C,H,) (NR) fragment and the bent metallocene moiety M(T~-C,H,)~ for the Group 4 elements.

N-Heterocyclic carbenes (NHCs) are extremely valuable as nucleophilic organocatalysts. They are widely applied as ligands in transition-metal catalysed reactions, where they are known as particularly potent s-donors. They are commonly viewed as workhorses exhibiting reliable, but undramatic, chemical behaviour. The N / C carbene p-donation stabilises NHCs at the expense of low reactivity towards nucleophiles. In contrast to NHCs, stable (alkyl)(amino)carbenes exhibit spectacular reactivity, allowing, for example, the splitting of hydrogen and ammonia and the fixation of carbon monoxide. NHCs have been judged to be electronically not suitable for showing similar reactivity. Here, we demonstrate that a ferrocene-based NHC is able to add ammonia, methyl acrylate, tert-butyl isocyanide, and carbon monoxide-reactions typical of (alkyl)(amino)carbenes, but unprecedented for diaminocarbenes. We also show that even the simplest stable diaminocarbene, C(NiPr 2 ) 2 , adds CO. This reaction affords a b-lactam by a subsequent intramolecular process involving a C-H activation. Our results shed new light on the chemistry of diaminocarbenes and offer great potential for synthetic chemistry and catalysis.

Thin films of 1,3-diethylbenzimidazol-2-ylidene (BIEt) were fabricated from THF solution on solid gold substrates and characterised by high-resolution X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. The surface-analytical data are in accord with the formation of self-assembled monolayers of BIEt molecules exhibiting an approximately vertical orientation on the substrate. The crystal structure of (BIEt)2 was also determined.

The new tridentate thioether ligands PhSi(CH2SMe)3 (1) and Ph-p-C6H4Si(CH2SMe)3 (2) have been synthesised and used for the preparation of the chelates fac-[W(kappa3-1)(CO)3] and fac-[W(kappa3-2)(CO)3], which were characterised by single-crystal X-ray diffraction. 1 and 2 were used as tripodal adsorbate molecules for the fabrication of self-assembled monolayers (SAMs) on gold. Film formation from solution was investigated in situ by second harmonic generation (SHG) and ellipsometry, which revealed a two-stepped process (fast adsorption, followed by slow film ordering). SAMs of 2 on gold were further investigated by ex situ methods, viz. high-resolution X-ray photoelectron spectroscopy (HRXPS), Fourier transform infrared reflection absorption spectroscopy (FTIRRAS), and scanning tunneling microscopy (STM). The latter two methods indicated dense packing of the tripodal anchor groups on the surface, with a substantially lower density of the biphenyl pricks. HRXPS showed three different binding states of sulfur, including a standard thiolate-type and a coordination-type state.