This account is a review on the synthesis and transition-metal coordination chemistry of N-heterocyclic silylenes (NHSi's) over the last 20 years till the present time (2012). Recently, fascinating and novel synthetic methods have been developed to access transition-metal-NHSi complexes as an emerging class of compounds with a wealth of intriguing reactivity patterns. The striking influence of coordinating NHSi's to transition-metal complex fragments affording different reactivities to the "free" NHSi is a connecting theme ("leitmotif") throughout the review, and highlights the potential of these compounds which lie at the interface of contemporary main-group and classical organometallic chemistry towards new molecular catalysts for small-molecule activation.

Twice, and faster: Reaction of the zwitterionic silylene 1 with AsH3 occurs stepwise at ambient temperature to give the first crystalline, donor‐stabilized arsasilene 3 via its 1,1‐addition product (silylarsane 2). In contrast, the activation of PH3 by 1 merely leads to the phosphorus analogue of 2. The strikingly different metal‐free activation of the series of Group 15 hydrides EH3 (E=N, P, As) by 1 was rationalized with DFT calculations.

Not copy and paste: Although β-diketiminato ligands have been employed for the stabilization of Ge(II) and Sn(II) hydrides, the corresponding Si(II) hydride is not accessible. However, coordination of silicon(II) to a {Ni(CO)(3)} fragment allowed the isolation of the first Si(II) hydride metal complex 1. This complex was used for the first silicon(II)-based and Ni(0)-mediated, stereoselective hydrosilylation of alkynes. R = phenyl, tolyl.

Reaction of the zwitterionic N-heterocyclic silylene (NHSi) 1 L′Si: (L′ = [HC(CMeNAr)(C(CH2)NAr)], Ar = 2,6-iPr2C6H3) with HCl at low temperatures affords the kinetically stable 1,4-addition product of 1, LSiCl (L = [HC(CMeNAr)2], Ar = 2,6-iPr2C6H3) (9a), which upon reaction with [Rh(Cl)cod]2 and [Ir(Cl)cod]2 (cod = 1,5-cyclooctadiene) selectively affords the NHSi complexes [L(Cl)Si:→Rh(Cl)cod] (10a) and [L(Cl)Si:→Ir(Cl)cod] (10b), respectively. The latter were employed as pre-catalysts in the catalytic reduction of amides in the presence of silanes. Remarkably, they show strikingly different activities and selectivities. While complex 10a yields selectively the C–O cleavage product, 10b affords both cleavage products (C–O and C–N). Moreover, the total conversion of the catalytic amide reduction with 10b is significantly higher than the conversion with a benchmark system [Ir(Cl)cod]2 highlighting the enhanced catalytic activity afforded by the coordination of the NHSi ligand. Introducing the hydride source Li[HBEt3] into the catalytic reactions retards the catalyst performance due to a competitive decomposition pathway. This appears to occur via a H-shift onto the cod ligand with concomitant liberation of cyclooctene, which is also presented. The different reactivity of 10a and 10b towards nucleophiles such as MeLi is also discussed. The reaction of 10a with MeLi affords an intractable array of products, while the reaction of 10b with one equivalent of MeLi selectively affords [L(Cl)Si:→Ir(CH3)cod] (14) with selective methylation at the Ir centre. The analogous reaction with two equivalents of 10b affords the double methylated product [L(CH3)Si:→Ir(CH3)cod] (15).

Doppelt und schneller: Die Reaktion des zwitterionischen Silylens 1 mit AsH3 verläuft bei Raumtemperatur schrittweise und ergibt das erste kristalline, donorstabilisierte Arsasilen 3 über sein 1,1‐Additionsprodukt (Silylarsan 2). Dagegen führt die Aktivierung von PH3 durch 1 lediglich zum Phosphoranalogon von 2. Die auffallend unterschiedliche metallfreie Aktivierung der Hydride EH3 (E=N, P, As) der 15. Gruppe durch 1 kann mithilfe von Dichtefunktionalrechnungen erklärt werden.