Metathesis reactions constitute a key component of modern synthetic chemistry; olefin metathesis, for example, provides a versatile and widely exploited carbon-carbon bond-forming methodology.[1] Such reactions are typically catalyzed by organometallic complexes that contain M=C bonds.[2] The synthesis of analogous complexes that contain M=Si bonds, for example, has led to an in-depth investigation of their reactivity towards unsaturated substrates. [3] Synthetic approaches that lead to the isolation of related systems with M = B bonds have been developed only recently: [4][5][6][7] for example, halide-abstraction chemistry gives access to cationic terminal borylene complexes, [L n M= BX][7] Consequently, reports of the fundamental chemistry of M=B bonds are somewhat limited (predominantly to metal-metal transfer reactions and addition/substitution reactivity towards nucleophiles). [4,6,7] Thus, the chemistry of [Cp*Fe(CO) 2 (BMes)] + (Mes = mesityl = 2,4,6-Me 3 C 6 H 2 , Cp* = pentamethylcyclopentadienyl), for example, is dominated by the electrophilic character at both the Fe and B centers. [7] In an attempt to tune the reactivity of these highly unsaturated complexes we investigated the synthesis of cationic aminoborylene systems, [L n M = BNR 2 ]+ .[5] Chloride abstraction from [CpFe(CO) 2 {B(NiPr 2 )Cl}] (2) (Cp = cyclopentadienyl) by Na(BAr À . The steric bulk of the amino substituents is a key point: the use of the smaller NMe 2 group results in the formation of a thermally fragile borylene product in the subsequent halide-abstraction step, [7b, 8] whereas [CpFe(CO) 2 {B(tmp)Cl}] (tmp = tetramethylpiperamino) was inaccessible from tmpBCl 2 . Compound 2 is a pale yellow sublimable crystalline solid, which was characterized by multinuclear NMR and IR spectroscopy, mass spectrometry, and X-ray crystallographic analysis (Figure 1).The reaction of 2 with Na(BAr f 4 ) in dichloromethane results in a quantitative conversion (determined by 1 H and 11 B NMR spectroscopy) into 3. The latter product (along with the C 5 H 4 Me analogue) is a colourless oil at (or close to) room temperature, but its formulation can be definitively established from spectroscopic and reactivity data. The measured 11 B chemical shift for 3 (d B = 93.5 ppm) is very close to that reported by Braunschweig et al. for neutral terminal-aminoborylene systems of the type L n M=BN(SiMe 3 ) 2 (d B = 86.6-98.3 ppm).[5] The downfield shift upon chloride abstraction (Dd B = 38.1 between 2 and 3) mirrors that found for and 88.0 ppm, respectively). [7,8] The 1 H and 13 C NMR data are