N-Heterocyclic carbenes (NHCs) are widely used ligands and reagents in modern inorganic synthesis as well as in homogeneous catalysis and organocatalysis. However, NHCs are not always innocent bystanders. In the last few years, more and more examples were reported of reactions of NHCs with main-group elements which resulted in modification of the NHC. Many of these reactions lead to ring expansion and the formation of six-membered heterocyclic rings involving insertion of the heteroatom into the C-N bond and migration of hydrides, phenyl groups or boron-containing fragments. Furthermore, a few related NHC rearrangements were observed some decades ago. In this Perspective, we summarise the history of NHC ring expansion reactions from the 1960s till the present.

The first example of a catalytically active system for Suzuki-type cross-coupling reactions of perfluorinated arenes such as octafluorotoluene and decafluorobiphenyl is presented.

, which is a poor p acceptor owing to its high energy p* LUMO). The M(CO) n ± BE bond is therefore significantly stronger than the M(CO) n ± CO bond. The [Cr(CO) 5 (AE)] bond dissociation energy, for example, amounts to 41.8 (CO), 62.1 (BF), 72.1 (BNH 2 ), and 93.4 kcal mol À1 (BO À ). However, the high polarity of the BE ligands and the buildup of positive charge on BE suggest a low kinetic stability. Strategies for improving the kinetic stability of metal ± BE complexes are presented. Steric protection of the reactive BE frontier orbitals may be built into the ligand as in BNR 2 (with R potentially bulky) and into the metal fragment (by bulky ligands or m coordination). However, the electronic stabilization of the [M] ± BE bond seems to be just as important. We show that binuclear metal complex fragments, such Fe 2 (CO) 8 and Mn 2 (Cp) 2 (CO) 4 , have just the right frontier orbitals to accomplish this.

The reaction of [Ni2((i)Pr2Im)4(COD)] 1a or [Ni((i)Pr2Im)2(eta(2)-C2H4)] 1b with different fluorinated arenes is reported. These reactions occur with a high chemo- and regioselectivity. In the case of polyfluorinated aromatics of the type C6F5X such as hexafluorobenzene (X = F) octafluorotoluene (X = CF3), trimethyl(pentafluorophenyl)silane (X = SiMe3), or decafluorobiphenyl (X = C6F5) the C-F activation regioselectively takes place at the C-F bond in the para position to the X group to afford the complexes trans-[Ni((i)Pr2Im)2(F)(C6F5)]2, trans-[Ni((i)Pr2Im)2(F)(4-(CF3)C6F4)] 3, trans-[Ni((i)Pr2Im)2(F)(4-(C6F5)C6F4)] 4, and trans-[Ni((i)Pr2Im)2(F)(4-(SiMe3)C6F4)] 5. Complex 5 was structurally characterized by X-ray diffraction. The reaction of 1a with partially fluorinated aromatic substrates C6H(x)F(y) leads to the products of a C-F activation trans-[Ni((i)Pr2Im)2(F)(2-C6FH4)] 7, trans-[Ni((i)Pr2Im)2(F)(3,5-C6F2H3)] 8, trans-[Ni((i)Pr2Im)2(F)(2,3-C6F2H3)] 9a and trans-[Ni((i)Pr2Im)2(F)(2,6-C6F2H3)] 9b, trans-[Ni((i)Pr2Im)2(F)(2,5-C6F2H3)] 10, and trans-[Ni((i)Pr2Im)2(F)(2,3,5,6-C6F4H)] 11. The reaction of 1a with octafluoronaphthalene yields exclusively trans-[Ni((i)Pr2Im)2(F)(1,3,4,5,6,7,8-C10F7)] 6a, the product of an insertion into the C-F bond in the 2-position, whereas for the reaction of 1b with octafluoronaphthalene the two isomers trans-[Ni((i)Pr2Im)2(F)(1,3,4,5,6,7,8-C10F7)] 6a and trans-[Ni((i)Pr2Im)2(F)(2,3,4,5,6,7,8-C10F7)] 6b are formed in a ratio of 11:1. The reaction of 1a or of 1b with pentafluoropyridine at low temperatures affords trans-[Ni((i)Pr2Im)2(F)(4-C5NF4)] 12a as the sole product, whereas the reaction of 1b performed at room temperature leads to the generation of trans-[Ni((i)Pr2Im)2(F)(4-C5NF4)] 12a and trans-[Ni((i)Pr2Im)2(F)(2-C5NF4)] 12b in a ratio of approximately 1:2. The detection of intermediates as well as kinetic studies gives some insight into the mechanistic details for the activation of an aromatic carbon-fluorine bond at the {Ni((i)Pr2Im)2} complex fragment. The intermediates of the reaction of 1b with hexafluorobenzene and octafluoronaphthalene, [Ni((i)Pr2Im)2(eta(2)-C6F6)] 13 and [Ni((i)Pr2Im)2(eta(2)-C10F8)] 14, have been detected in solution. They convert into the C-F activation products. Complex 14 was structurally characterized by X-ray diffraction. The rates for the loss of 14 at different temperatures for the C-F activation of the coordinated naphthalene are first order and the estimated activation enthalpy Delta H(double dagger) for this process was determined to be Delta H(double dagger) = 116 +/- 8 kJ mol(-1) (Delta S(double dagger) = 37 +/- 25 J K(-1) mol(-1)). Furthermore, density functional theory calculations on the reaction of 1a with hexafluorobenzene, octafluoronaphthalene, octafluorotoluene, 1,2,4-trifluorobenzene, and 1,2,3-trifluorobenzene are presented.

Carbon monoxide, CO, is a ubiquitous ligand in organometallic and coordination chemistry. In the present paper we investigate the neutral isoelectronic molecules AB = N2, CO, BF, and SiO and their coordination in the model complexes Fe(CO)4AB and Fe(AB)5, using nonlocal density functional theory and a large, polarized STO basis set of triple-ζ quality (NL-SCF/TZ(2P)). Our aim is to get more insight into the ligating properties of SiO and BF in comparison to CO and N2. The computed 298 K Fe(CO)4−AB bond dissociation enthalpies of C 3 v -symmetric Fe(CO)4AB are 18.1, 42.3, 67.9, and 35.6 kcal/mol for N2, CO, BF, and SiO, respectively; the corresponding values for C 2 v -symmetric Fe(CO)4AB are comparable:  19.0, 42.3, 66.7, and 39.7 kcal/mol. Good, balanced σ donation (through 5σ) and π acceptance (through 2π) are what makes CO a good donor, of course. The gap between these frontier orbitals (5σ and 2π) becomes even smaller in SiO and BF. The analysis of the bonding mechanism of the Fe−AB bond shows that SiO is a better σ donor but a worse π acceptor ligand than CO and that BF should be superior to CO in terms of both σ donor and π acceptor properties. However, these polar ligands are therefore also more reactive; and more sensitive, e.g. to nucleophilic attack, because of a low-energy 2π LUMO. Our results suggest that BF and SiO should, in principle, be excellent ligands. We also find interesting side-on and O-bound local minima, not very unstable, for SiO bound to an Fe(CO)4 fragment.

The NHC‐stabilized complex [Ni2(iPr2Im)4(cod)] (1) was isolated in good yield from the reaction of [Ni(cod)2] with 1,3‐diisopropylimidazole‐2‐ylidene (iPr2Im). Compound 1 is a source of the [Ni(iPr2Im)2] complex fragment in stoichiometric and catalytic transformations. The reactions of 1 with ethylene and CO under atmospheric pressure or with equimolar amounts of diphenylacetylene lead to the compounds [Ni(iPr2Im)2(η2‐C2H4)] (2), [Ni(iPr2Im)2(η2‐C2Ph2)] (3), and [Ni(iPr2Im)2(CO)2] (4) in good yields. In all cases the [Ni(iPr2Im)2] complex fragment is readily transferred without decomposition or fragmentation. In the infrared spectrum of carbonyl complex 4, the CO stretching frequencies are observed at 1847 and 1921 cm−1, and are significantly shifted to lower wavenumbers compared with other nickel(0) carbonyl complexes of the type [NiL2(CO)2]. Complex 1 activates the CF bond of hexafluorobenzene very efficiently to give [Ni(iPr2Im)2(F)(C6F5)] (5). Furthermore, [Ni2(iPr2Im)4(cod)] (1) is also an excellent catalyst for the catalytic insertion of diphenylacetylene into the 2,2′ bond of biphenylene. The reaction of 1 with equimolar amounts of biphenylene at low temperature leads to [Ni(iPr2Im)2(2,2′‐biphenyl)] (6), which is formed by insertion into the strained 2,2′ bond. The reaction of diphenylacetylene and biphenylene at 80 °C in the presence of 2 mol % of 1 as catalyst yields diphenylphenanthrene quantitatively and is complete within 30 minutes.

Lewis base adducts of tetra-alkoxy diboron compounds, in particular bis(pinacolato)diboron (B2 pin2 ), have been proposed as the active source of nucleophilic boryl species in metal-free borylation reactions. We report the isolation and detailed structural characterization (by solid-state and solution NMR spectroscopy and X-ray crystallography) of a series of anionic adducts of B2 pin2 with hard Lewis bases, such as alkoxides and fluoride. The study was extended to alternative Lewis bases, such as acetate, and other diboron reagents. The B(sp(2) )-B(sp(3) ) adducts exhibit two distinct boron environments in the solid-state and solution NMR spectra, except for [(4-tBuC6 H4 O)B2 pin2 ](-) , which shows rapid site exchange in solution. DFT calculations were performed to analyze the stability of the adducts with respect to dissociation. Stoichiometric reaction of the isolated adducts with two representative series of organic electrophiles-namely, aryl halides and diazonium salts-demonstrate the relative reactivities of the anionic diboron compounds as nucleophilic boryl anion sources.

The reaction of [Ni(COD)2] with stable N-heterocyclic carbenes R2Im (R2Im = 1,3-di(R)imidazole-2-ylidene; R2 = Me2, n Pr2, Me i Pr, i Pr2) affords homoleptic [Ni(Me2Im)3] (1) or dinuclear, COD-bridged complexes of the type [Ni2(R2Im)4(COD)] (R2 = n Pr2, 2; Me i Pr, 3; i Pr2, 4). Compounds 1−4 are suitable precursors for the synthesis of [Ni(R2Im)2]-containing complexes in solution, exemplified by the reaction with CO under atmospheric pressure, with equimolar amounts of diphenyl acetylene or with biphenylene to give carbonyl complexes [Ni(R2Im)2(CO)2] (R2 = Me2, 5; n Pr2, 6; Me i Pr, 7; i Pr2, 8), diphenyl acetylene complexes [Ni(R2Im)2(η2-C2Ph2)] (R2 = Me2, 9; n Pr2, 10; Me i Pr, 11; i Pr2, 12), and biphenylene complexes [Ni(R2Im)2(2,2‘-biphenyl)] (R2 = Me2, 13; n Pr2, 14; Me i Pr, 15; i Pr2, 16). Furthermore, the reaction of 4 with 3-hexyne or 2-butyne afforded [Ni( i Pr2Im)2(η2-C2R2)] (R = Me, 18; Et, 19) in good yields. The compounds 1, 11, 13, 17, and 18 have been structurally characterized. Complexes 13−16 are the products of a stoichiometric carbon−carbon activation of biphenylene, and compounds 1−4 (as well as 9−12) are efficient catalysts for the insertion of diphenyl acetylene into the C−C bond of biphenylene, a process in which the C−C activation of biphenylene is incorporated into a catalytic cycle. The reaction rate of the formation of 9,10-di(phenyl)phenanthrene depends on the nature of the carbene ligand of the catalyst; the highest was observed for [Ni2( i Pr2Im)4(COD)] (4).