Reaction of 2-chloro-1,3,2-diazaarsolenes and -diazaphospholenes with Tl[Co(CO) 4 ] gives instable complexes of type [Co(ER 2 )(CO) 4 ] which decarbonylated to yield [Co(ER 2 )(CO) 3 ]. Spectroscopic and X-ray diffraction studies revealed that the tetracarbonyl complexes can be formulated as ion pair for E ϭ P and as covalent metalla-arsine for E ϭ As, and the tricarbonyl complexes as carbene-like species with a formal EϭCo double bond. A similar reactivity towards Tl[Co(CO) 4 ] was also inferred for 1,3,2diazastibolenes although the products were not isolable and their constitution remained uncertain. Evaluation of structural and com-1403 putational data suggests that the weak and polarized CoϪAs bond in [Co(AsR 2 )(CO) 4 ] can be characterized as an "inverse" MǞL donor-acceptor bond. The computational studies disclosed further η 2 (EN)-coordination of the EN 2 C 2 heterocycle as an alternative to the formation of a carbene-like structure for [Co(ER 2 )(CO) 3 ]. The η 2 -complex is less stable for E ϭ P but close in energy for E ϭ As and more stable than the carbene-like complex for E ϭ Sb.Scheme 2 (R ϭ tBu (1a, 3a, 4b), Mes (1b, 4b-9b)).

Novel pincer-type, pyridine-bridged bis(benzimidazolylidene)-palladium complexes 5-7 were synthesised from cheap commercial precursors under microwave assistance. Although simple in structure, carbene complexes 5a,b are efficient low-molecular-mass metallogelators. They gelate not only a broad variety of protic and aprotic organic solvents, but also different types of customary ionic liquids (such as imidazolium, pyridinium, pyrazolidinium, piperidinium and ammonium salts) at concentrations as low as 0.5 mg mL(-1). The morphologies of the resulting 3D gel networks composed from long and thin fibres were studied by TEM and light microscopy for a selection of organic and ionic liquids. The achiral gelators are able to induce the formation of helical fibres. The thermal stability of the gel samples increases with the gelator concentration as demonstrated by thermoreversible DSC studies. Temperature-dependent NMR and X-ray diffraction studies, as well as comparisons with pincer complex analogues bearing shorter alkyl chains, suggest that the 3D networks responsible for gelation are based on non-covalent interactions, such as pi-stacking, van der Waals interactions, and hydrogen and metal-metal bonding. Ionic liquids and gels obtained from them and 5a,b display comparable high conductivities, which characterises pyridine-bridged bis(benzimidazolylidene)-palladium pincer complexes as air-stable metallo gelators that efficiently immobilise ionic liquids in low gelator concentration indicating--beyond catalysis--their potential applications in electrochemical devices.

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Synthesis of azaphosphiridine complexes 3a-e was achieved via thermal group transfer reaction using 2H-azaphosphirene complex 1 and N-methyl C-aryl imines 2a-e (i) or via reaction of transient Li/Cl phosphinidenoid complex 5 (prepared from dichloro(organo)phosphane complex 4) using 2a-c (ii), respectively. Reaction of complexes 3a,d and trifluoromethane sulfonic acid in the presence of dimethyl cyanamide led to a highly bond- and regioselective ring expansion yielding 1,3,4sigma3lambda3-diazaphosphol-2-ene complexes 8a,d after deprotonation with NEt3. 31P NMR reaction monitoring revealed that protonation of complex 3a yields the azaphosphiridinium complex 6a, unambiguously identified by NMR spectroscopy at low temperature. All isolated products were characterized by multinuclear NMR spectroscopy, IR and UV/Vis (for 3a,d, 6a, 8a,d), MS and single-crystal X-ray crystallography in the cases of complexes 3b-d, 8a and 8d. DFT studies on the reaction mechanism and compliance constants of the model complex of 6a are presented.