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The reaction behavior of [Cp*Fe(η 5 -As 5 )] (I) (Cp*=C 5 Me 5 ) towards carbenes and their heavier analogs was investigated. The reaction of I with NHCs (NHCs=N-heterocyclic carbenes) results in the first substitution products of polyarsenic ligand complexes by NHCs [Cp*Fe(η 4 -As 5 NHC)] (1 a: NHC=IMe=1,3,4,5-tetramethylimidazolin-2-ylidene, 1 b: NHC=IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene). In contrast, the reaction of I with Et CAAC ( Et CAAC=2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene) leads to a fragmentation and the formation of an unprecedented As 6 -sawhorse-type compound [As 2 (As Et CAAC) 4 ] (2). The reaction of (LE) 2 (L=PhC(N t Bu) 2 ; E=Si, Ge) with I resulted in a rearrangement and an insertion of LE fragments, forming unique silicon- (4: [Cp*Fe(η 4 -As 4 SiL)], 5 a: [Cp*Fe(η 4 -As 6 SiL)) and germanium-containing (5 b: [Cp*Fe(η 4 -As 6 GeL)) cyclic polyarsenic ligand complexes., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

By the reaction of [Cp*Fe(η 5 -As 5 )] (I) (Cp*=C 5 Me 5 ) with main group nucleophiles, unique functionalized products with η 4 -coordinated polyarsenide (As n ) units (n=5, 6, 20) are obtained. With carbon-based nucleophiles such as MeLi or KBn (Bn=CH 2 Ph), the anionic organo-substituted polyarsenide complexes, [Li(2.2.2-cryptand)][Cp*Fe(η 4 -As 5 Me)] (1 a) and [K(2.2.2-cryptand)][Cp*Fe{η 4 -As 5 (CH 2 Ph)}] (1 b), are accessible. The use of KAsPh 2 leads to a selective and controlled extension of the As 5 unit and the formation of the monoanionic compound [K(2.2.2-cryptand][Cp*Fe(η 4 -As 6 Ph 2 )] (2). When I is reacted with [M]As(SiMe 3 ) 2 (M=Li ⋅ THF; K), the formation of the largest known anionic polyarsenide unit in [M'(2.2.2-cryptand)] 2 [(Cp*Fe) 4 {μ 5 -η 4 :η 4 :η 3 :η 3 :η 1 :η 1 -As 20 }] (3) occurred (M'=Li (3 a), K (3 b))., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

A cyclic alkyl(amino)carbene-stabilized 1,4-diborabenzene (DBB) ligand enables the isolation of 18-electron two-legged parent piano-stool Fe 0 and Ru 0 complexes, [(η 6 -DBB)M(CO) 2 ], the ruthenium complex being the first of its kind to be structurally characterized. [(η 6 -DBB)Fe(CO) 2 ] reacts with E 4 (E=P, As) to yield mixed DBB-cyclo-E 4 sandwich complexes with planar E 4 2- ligands. Computational analyses confirm the strong electron-donating capacity of the DBB ligand and show that the E 4 ligand is bound by four equivalent Fe-P σ bonds., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

A synthetic pathway for the synthesis of novel anionic sandwich complexes with a cyclo-E 3 (E=P, As) ligand as an end deck was developed giving [Cp'''Co(η 3 -E 3 )] - (Cp'''=1,2,4-tri-tert-butylcyclopentadienyl, E=P ([5]), As ([6])) in good yields suitable for further reactivity studies. In the reaction with the chlorophosphanes R 2 PCl (R=Ph, Cy, t Bu), neutral complexes with a disubstituted cyclo-E 3 P (E=P, As) ligand in [Cp'''Co(η 3 -E 3 PR 2 )] (E=P (7 a-c), As (9 a-c)) were obtained. These compounds can be partially or completely converted into complexes with a cyclo-E 3 (E=P, As) ligand with an exocyclic {PR 2 } unit in [Cp'''Co(η 2 :η 1 -E 3 PR 2 )] (E=P (8 a-c), As (10 a-c)). Additionally, the insertion of the chlorosilylene [LSiCl] (L=( t BuN) 2 CPh) into the cyclo-E 3 ligand of [5] and [6] was achieved and the novel heteroatomic complexes [Cp'''Co(η 3 -E 3 SiL)] (E=P (11), As (12)) could be isolated. The reaction pathway was elucidated by DFT calculations., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The reaction of [Cp'''Co(η 4 -P 4 )] (1) (Cp'''=1,2,4-tBu 3 C 5 H 2 ) with Me NHC ( Me NHC=1,3,4,5-tetramethylimidazol-2-ylidene) leads through NHC-induced phosphorus cation abstraction to the ring contraction product [( Me NHC) 2 P][Cp'''Co(η 3 -P 3 )] (2), which represents the first example of an anionic CoP 3 complex. Such NHC-induced ring contraction reactions are also applicable for triple-decker sandwich complexes. The complexes [(Cp*Mo) 2 (μ,η 6:6 -E 6 )] (3 a, 3 b) (Cp*=C 5 Me 5 ; E=P, As) can be transformed to the complexes [( Me NHC) 2 E][(Cp*M) 2 (μ,η 3:3 -E 3 )(μ,η 2:2 -E 2 )] (4 a, 4 b), with 4 b representing the first structurally characterized example of an NHC-substituted As I cation. Further, the reaction of the vanadium complex [(Cp*V) 2 (μ,η 6:6 -P 6 )] (5) with Me NHC results in the formation of the unprecedented complexes [( Me NHC) 2 P][(Cp*V) 2 (μ,η 6:6 -P 6 )] (6), [( Me NHC) 2 P][(Cp*V) 2 (μ,η 5:5 -P 5 )] (7) and [(Cp*V) 2 (μ,η 3:3 -P 3 )(μ,η 1:1 -P{ Me NHC})] (8)., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)