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The synthesis and the analytical features of group 6 carbonyl complexes of electron-poor mono(imidazolium)- and bis(imidazolium)-substituted Cp ylides are presented. With the neutral ylidic imidazo...

Ru-alkylidenes bearing sterically demanding arylthiolate ligands (SAr) constitute one of only two classes of catalyst that are Z-selective in metathesis of 1-alkenes. Of particular interest are complexes bearing pyridine as a stabilizing donor ligand, [RuCl(SAr)(═CHR)(NHC)(py)] (R = phenyl or 2-thienyl, NHC = N-heterocyclic carbene, py = pyridine), which initiate catalysis rapidly and give appreciable yields combined with moderate to high Z-selectivity within minutes at room temperature. Here, we extend this chemistry by synthesizing and testing the first two such complexes (5a and 5b) bearing 3-phenylindenylidene, a ligand known to promote stability in other ruthenium-based olefin metathesis catalysts. The steric pressure resulting from the three bulky ligands (the NHC, the arylthiolate, and the indenylidene) forces the thiolate ligand to position itself trans to the NHC ligand, a configuration different from that of the corresponding alkylidenes. Surprisingly, although this configuration is incompatible with Z-selectivity and slows down pyridine dissociation, the two new complexes initiate readily at room temperature. Although their thermal stability is lower than that of typical indenylidene-bearing catalysts, 5a and 5b are fairly stable in catalysis (TONs up to 2200) and offer up to ca. 80% of the Z-isomer in prototypical metathesis homocoupling reactions. Density functional theory (DFT) calculations confirm the energetic cost of dissociating pyridine from 5a (= M1-Py) to generate 14-electron complex M1. Whereas the latter isomer does not give a metathesis-potent allylbenzene π-complex, it may isomerize to M1-trans and M2, which both form π-complexes in which the olefin is correctly oriented for cycloaddition. The olefin orientation in these complexes is also indicative of Z-selectivity. publishedVersion

The reactivity of tridentate bis-aryloxy N-heterocyclic carbene (NHC) titanium complexes ([κ3-O,C,O]-NHC)Ti(X1)(X2) (X1 = X2 = Cl (1); X1 = X2 = OiPr (2)) via salt metathesis (with LiOBn, NaOAc), protonolysis (with (tBuO)3SiOH), and σ-bond metathesis (with Me3SiN3) were investigated, leading to a series of NHC titanium complexes bearing various X and XL type coligands, ([κ3-O,C,O]-NHC)Ti(X1)(X2) (X1 = X2 = OBn (3); X1 = Cl, X2 = OSi(OtBu)3 (4); X1 = OiPr, X2 = OSi(OtBu)3 (5); X1 = X2 = OAc (6); X1 = X2 = N3 (7)). The molecular structures of complexes 3 and 5 were identified by X-ray crystallographic studies, disclosing five-coordinate complexes, while complexes 4 and 7 crystallize only in the presence of THF, leading to the six-coordinate Ti–THF adducts ([κ3-O,C,O]-NHC)Ti(X1)(X2)(THF) (4-THF and 7-THF). In contrast, the structure of 6 reveals a rare example of a seven-coordinate Ti complex in which the tridentate NHC and two bidentate OAc ligands are coordinated in a pentagonal-bipyramidal fashion around ...

Pyridine as a stabilizing donor ligand drastically improves the performance of ruthenium monothiolate catalysts for olefin metathesis in comparison with previous versions based on a stabilizing benzylidene ether ligand. The new pyridine-stabilized ruthenium alkylidenes undergo fast initiation and reach appreciable yields combined with moderate to high Z selectivity in self-metathesis of terminal olefins after only a few minutes at room temperature. Moreover, they can be used with a variety of substrates, including acids, and promote self-metathesis of ω-alkenoic acids. The pyridine-stabilized ruthenium monothiolate catalysts are also efficient at the high substrate dilutions of macrocylic ring-closing metathesis and resist temperatures above 100 °C during catalysis. publishedVersion

In this study, we demonstrate the structural variety of the Cp*SmIy system with respect to the amount of donor molecules (THF) present. The reaction of SmI2(THF)2 with 0.5 equiv of bis(pentamethylcyclopentadienyl)lead(II), PbCp*2, in THF gives the highly solvated monomeric complex Cp*SmI2(THF)3, when crystallized from THF. The same reaction performed in n-hexane followed by crystallization from toluene gave the partially solvated trimetallic complex [Cp*3Sm3I(μ2-I)3(μ3-I)2(THF)2]. Recrystallization of the Sm3 cluster from either C6D6 at ambient temperature or toluene at −40 °C produced the tetrametallic complex [Cp*4Sm4I(μ2-I)4(μ3-I)2(μ4-I)(THF)]. Reaction of SmI2, obtained via treatment of SmI2(THF)2 with excess AlEt3 in n-pentane, with 0.5 equiv of PbCp*2 in toluene led to the donor-free mixed-valent pentametallic complex [Cp*5Sm5(μ2-I)4(μ3-I)4(μ5-I)]. All compounds were characterized by single-crystal X-ray structure analysis.

The divalent bis(trimethylsilyl)amide complexes Ln[N(SiMe3)2]2(THF)2 (Ln = Sm, Yb) react with 0.5 equiv of lead(II) pentamethylcyclopentadienide, Cp*2Pb (Cp* = C5Me5), in n-hexane to form the half-sandwich complexes Cp*Ln[N(SiMe3)2]2 (Ln = Sm, Yb) in almost quantitative yield. The same reaction performed with Eu[N(SiMe3)2]2(THF)2 resulted in the cocrystallization of the sandwich complex Cp*2Eu[N(SiMe3)2] and homoleptic Eu[N(SiMe3)2]3. The divalent bis(dimethylsilyl)amide complexes Ln{[μ-N(SiHMe2)2]2Ln[N(SiHMe2)2](THF)}2 (Ln = Sm, Yb) react with 1.5 equiv of Cp*2Pb in n-hexane/THF to form the half-sandwich complexes Cp*Ln[N(SiHMe2)2]2(THF) (Ln = Sm, Yb). The corresponding europium reaction did not provide any crystalline material. Treatment of divalent Eu[N(SiMe3)2]2(THF)2 with 2 equiv of 3-tert-butyl-5-methylpyrazole (HpztBu,Me) in THF generates [(pztBu,Me)Eu(μ-pztBu,Me)(THF)2]2. Oxidation of the europium(II) pyrazolate complex with 1 equiv of Cp*2Pb in THF afforded Cp*Eu(μ-pztBu,Me)2(THF)2. The tetrameth...

The reactivity of alkylaluminum hydrides HAlMe2, HAl(CH2SiMe3)2, and H2AlMes* (Mes* = C6H2tBu3-2,4,6) toward yttrium methyl complexes was assessed. From the reaction with [Cp2YMe]2 the corresponding methylated alkylaluminum compounds could be isolated and characterized along with [Cp2Y(thf)(μ-H)]2, obtained upon workup in thf (Cp = C5H5 = cyclopentadienyl). Structurally characterized complexes Cp14Y6H3Cl and Cp15Y6H3 represent side-products of the transformation. The reactions of [YMe3]n and [(C5Me5)YMe2]3 indicated occurrence of hydrido transfer for H2AlMes* and led to product mixtures for HAl(CH2SiMe3)2. Following the reaction of HAlMe2 with these compounds as well as the half-sandwich derivatives (C5Me4R)Y(AlMe4)2 (R = Me, SiMe3) indicated reversible hydrido binding, leading ultimately to the formation of insoluble hydride clusters. The organoaluminum compounds [HAl(CH2SiMe3)2]3, [(μ-H)(Me)AlMes*]2, and Me2AlMes* were analyzed by X-ray crystallography.

A series of coinage metal complexes containing the redox-active metalloligands [RuCpX(κ3N-Tpmd)] {κ3N-Tpmd = κ3N-[C(pz)3] with pz = pyrazolyl; [RuCp(Tpmd)] (2a) and [RuCp*(Tpmd)] (2b)} are presented. 2a and 2b are isolable, relatively stable compounds, despite the fact that they feature a “naked” carbanion at the bridgehead position of the κ3N-coordinated tris(pyrazolyl)methanide ligand scaffold. As expected, both complexes act as κ1C ligands toward coinage metal fragments to yield dinuclear complexes of the general formula [RuCpX(μ-Tpmd){MX}] (μ-Tpmd = μ-κ1C:κ3N-[C(pz)3]; M = Au, X = Cl, CpX = C5H5 (3a) or C5Me5 (3b); M = Au, X = CN, CpX = C5H5 (4a) or C5Me5 (4b); M = Cu, X = OC(O)Me, CpX = C5H5 (5a); M = Cu, X = Si(SiMe3)3, CpX = C5H5 (6a) or C5Me5 (6b); M = Ag, X = SC(S)NEt2, CpX = C5H5 (7a), M = Au, X = C≡C–Ar, CpX = C5H5 {Ar = C6H5 (8a), 4-NH2-C6H4 (9a), 3,5-(CF3)2-C6H3 (10a)}). All complexes under study were fully characterized by common spectroscopic techniques; the structural parameters of 2a, 3a,...

Protonolysis of [YMe3]n with 2-{(N-2,6-dialkylphenyl)iminomethyl)}pyrroles (alkyl = iPr (L1), Me (L2)) gave homoleptic iminopyrrolyl complexes YL13 and YL23 as well as the complex [L2YL2,Me]2 containing a dianionic pyrrolaldiminato ligand, formed via methylation of the imino backbone. Treatment of the half-sandwich complex [(C5Me5)YMe2]3 and yttrocene (C5Me5)2YMe(THF) with either 2 or 1 equiv of HL afforded the monomeric complexes (C5Me5)YL2 and (C5Me5)2YL,respectively. The complex (C5Me5)YL22 readily underwent Ln→Al iminopyrrolyl ligand transfer in the presence of trimethylaluminum, producing the known (C5Me5)Y(AlMe4)2. Salt metatheses of homoleptic Ln(AlMe4)3 (Ln = Y, La) with KL gave complicated reaction mixtures from which the η5/η1:κ1 pyrrolaldiminato-bridged complex [L1,MeLa(AlMe4)]2 and bis(tetramethylaluminate) complex L2Y(AlMe4)2 could be isolated and crystallographically characterized. Moreover, the solid-state structures of YL23, [L2YL2,Me]2, (C5Me5)YL12, (C5Me5)2YL1, and L2AlMe2 are presented.

The bimetallic formamidinate complexes Ln(Form)(AlMe4)2 (Ln = Y, Form (ArNCHNAr) = EtForm (Ar = 2,6-Et2C6H3), MesForm (Ar = 2,4,6-Me3C6H2), DippForm (Ar = 2,6-iPr2C6H3), tBuForm (Ar = 2-tBuC6H4); Ln = La, Form = DippForm, tBuForm) were obtained in high yield by protonolysis reactions between formamidines (FormH) and homolepticrare-earth-metal tetramethylaluminates Ln(AlMe4)3. Y(Form)(AlMe4)2 (Form = EtForm, DippForm) were also prepared by treatment of Y(Form)[N(SiHMe2)2]2(thf) with trimethylaluminum after the former were prepared by the protonolysis of Y[N(SiHMe2)2]3(thf)2 complexes with EtFormH or DippFormH. The monomeric six-coordinate complexes Ln(Form)(AlMe4)2 (Ln = Y, Form = EtForm, MesForm, DippForm, tBuForm; Ln = La, Form = DippForm, tBuForm) show similar molecular structures with distorted-octahedral geometry and bidentate (N,N′) Form and AlMe4 ligands. The complex [La(EtFormAlMe3)(AlMe4)2](C7H8)1.5 from a protonolysis reaction between La(AlMe4)3 and EtFormH has the EtForm ligand adopting a config...

Our previously reported microwave synthesis of (N–N)AuCl2+ complexes (where N–N = 2,2′-bipyridine (bpy) and sterically unencumbered bpy derivatives) was used to prepare derivatives where the bpy moiety was substituted in the 6,6′-positions. Instead of the square-planar complexes, these reactions produced neutral (N–N)AuCl3 complexes. In these, the tethered N–N ligand is bonded such that one N occupies a regular position in the square coordination plane of the Au(III) center and the other N occupies a pseudoaxial position, interacting with Au through an elongated Au–N bond, as determined by X-ray crystallography of two complexes. Variable-temperature 1H NMR spectroscopy reveals that the two sites of the N–N ligand undergo exchange on the NMR time scale. For N–N = 6,6′-Me2bpy the activation parameters were determined to be ΔH⧧ = 8.5 ± 0.4 kcal mol–1 and ΔS⧧ = 0.7 ± 2.0 cal K–1 mol–1. The dynamic behavior of (6,6′-Me2bpy)AuCl3 was investigated by a DFT computational study, which detailed the in-plane rocking...

Binuclear complexes of copper, silver, and gold bearing a 2,2′-bipyridine analogue, the pyridazine annelated bis(N-heterocyclic carbene) ligand (vegi) 1, were prepared and structurally characterized. They all feature the shortest metal–metal distances that have been measured so far in complexes with this structural motif bearing neutral bidentate ligands, indicative of d10–d10 interactions. While in the silver complex the linear coordination of each silver atom with two carbene ligands results in a planar complex, the ligand planes are twisted by 70° in the Cu complex 4 and by 31° in the gold complex 3. The copper complex shows a solvent-dependent equilibrium between the [Cu2L2]2+ complex and a [Cu2L3]2+ complex along with solvated CuPF6.

The μ2-imide complexes [Ln(AlMe4)(μ2-Nmes*)]x (mes* = C6H2tBu3-2,4,6) (Ln = Y, La, Nd, Lu) are synthesized from homoleptic heterobimetallic complexes Ln(AlMe4)3 utilizing two distinct protocols: reaction with 2,4,6-tri-tert-butylaniline in hexane via methane elimination or with potassium (2,4,6-tri-tert-butylphenyl)amide in toluene according to a salt metathesis–protonolysis tandem reaction. Complexes [Ln(AlMe4)(μ2-Nmes*)]2 (Ln = Y, Nd, Lu) revealed isomorphous solid-state structures, featuring an asymmetrically bridged Ln2N2 core with very short Ln–N distances (2.071(1)–2.155(3) A). In the solid-state structure of the lanthanum derivative similar dimeric subunits assemble as La4Al4 oligomers through μ2-η1:η2-bridging tetramethylaluminato moieties. [La(AlMe4)(μ2-Nmes*)]4 displays La---arene interactions (2.702(6) A) that are considerably shorter than the La(μ-CH3)aluminato bond lengths.

A series of 3,5-substituted tris(pyrazolyl)borate (TpR,Me; R = Me, Ph, tBu) complexes of the alkaline-earth metals (Mg, Ca, Ba) was synthesized by salt metathesis reactions. The influence of different organometallic precursors on Schlenk-type rearrangement reactions was studied, putting emphasis on the metal size and the steric encumbrance of the Tp ligands. Magnesium alkyls MgR2 (R = AlMe4, CH3) react with KTpR,Me to form the heteroleptic complexes (TpMe,Me)Mg(CH3), (TptBu,Me)Mg(CH3), and (TpMe,Me)Mg(AlMe4). The latter tetramethylaluminate complex can also be obtained by treatment of TpMe,MeMg(CH3) with an excess of trimethylaluminum. The formally six-coordinate cyclopentadienyl derivative (C5Me5)Mg(Me)(thf)2 is synthesized from MeMgBr and 1 equiv of K(C5Me5). Equimolar reactions of the tetraethylaluminates [M(AlEt4)2]n of the heavier alkaline-earth metals calcium and barium with KTpR,Me give the homoleptic complexes of Ca(TpR,Ph)2 and Ba(TpR,Me)2. Heterotrimetallic [BaK(AlEt4)3]n is identified as a liga...

Two synthetic protocols, a salt metathesis reaction and a direct metalation, were developed for preparing 1,4-diaza-1,3-butadiene complexes of barium, strontium, and calcium, in which 1,4-diaza-1,3-butadiene serves as a dianionic or monoanionic ligand. A salt metathesis reaction of BaI2 with the dipotassium salt of N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (1; abbreviated (Dip)2DAD) afforded the iodide-bridged dinuclear complex [[K((Dip)2DAD)(THF)2][Ba(μ-I)(THF)2]]2 (2) bearing a dianionic ene-diamide ligand, while the reaction of MI2 (M = Sr, Ca) with the dipotassium salt of 1 gave the mononuclear complexes [M((Dip)2DAD)[THF]4] (4, M = Sr; 5, M = Ca). A direct metalation reaction of barium powder with (Dip)2DAD in the presence of iodine (10 mol%) afforded an iodide-bridged dinuclear complex, [Ba((Dip)2DAD)(μ-I)(THF)2]2 (3), in which (Dip)2DAD coordinates as a monoanionic ligand to the barium center, as was evident from the X-ray analysis and the EPR spectral data. The products from the dire...

Stable and isolable cyclometalated monomethyl Au(III) complexes are readily available from the precursor (tpy)Au(Me)Cl, prepared from (tpy)AuCl2 and excess SnMe4. Reaction of (tpy)Au(Me)Cl with suitable silver salts generates (tpy)Au(Me)(X) (X = OAc, OTf). Ligand exchange reactions with (tpy)Au(Me)(OTf) provide the cationic monomethyl Au(III) adducts [(tpy)Au(Me)(L)][OTf] (L = PPh3, THT).

The peralkylated magnesium complex Mg(AlMe4)2 reacts with bis-N,N′-diimine PhCH═NCH2CH2N═CHPh to yield the compound [PhCH(Me)NCH2CH2N═CHPh]Mg(AlMe4)(AlMe3), via an alkyl migration. Such a 1,2-addition of the Mg–CH3 moiety to the unsaturated N═C imino group is also observed for [MgMe2], affording dimagnesium complex [{PhCH(Me)NCH2CH2N═CHPh}Mg(Me)]2. In contrast, treatment of PhCH═NCH2CH2N═CHPh with AlMe3 gives the donor-adduct [PhCH═NCH2CH2N═CHPh][(AlMe3)2]. Based on these distinct reactivities a plausible reaction mechanism for the formation of [PhCH(Me)NCH2CH2N═CHPh]Mg(AlMe4)(AlMe3) is proposed. Donor-adduct formation is also observed when the perethylated barium aluminate [Ba(AlEt4)2]n is reacted with a β-diimine, affording the complex [Ba(AlEt4)2(PhC═NCH2CH2N═CPh)]n. The scope of this reaction behavior was further investigated by reacting Mg(AlMe4)2, AlMe3, and [MgMe2] with β-diimines bearing substituted phenyl rings. Neither the ortho-methyl- nor the meta-tert-butyl-disubstituted proligands show any r...

Quinolyl-substituted half-sandwich complexes (CpQ)Ln(AlMe4)2 (CpQ = 2,3,4,5-tetramethyl-1-(8-quinolyl)cyclopentadienyl; Ln = Y, La) were obtained in quantitative yield via protonolysis reactions utilizing HCpQ and homoleptic tetramethylaluminates Ln(AlMe4)3. X-ray structure analyses revealed that the quinolyl-substituted cyclopentadienyl ligand coordinates to the rare-earth metal centers in an η5:η1 fashion through the Cp ring carbon atoms and the N atom of the quinolyl substituent. The complexes (CpQ)Ln(AlMe4)2 show good activity and high trans-1,4-stereoselectivity (maximum 93%) in the polymerization of isoprene upon activation with the organoborates [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4]. The effects of metal size, cocatalyst, temperature, and solvent were assessed in these polymerization reactions, and the performance of such N-donor-functionalized complexes was compared to that of (CpR)Ln(AlMe4)2 containing different types of nondonor-functionalized cyclopentadienyl ligands.

The reaction between [Me2AuCl]2 and the lithium salt of a β-diketimine, [HC{C(Me)N(C6H3-2,6-Me2)}2Li], yields the neutral Au(III) compound [HC{C(Me)N(C6H3-2,6-Me2)}2AuMe2] (1). Compound 1 reacts with 1 equiv of triflic acid (HOTf) at −78 °C to provide the cationic Au(III) compound [H2C{C(Me)N(C6H3-2,6-Me2)}2AuMe2][OTf] (2). Treatment of 1 with 2 equiv of HOTf results in the displacement of Me2AuOTf and formation of the β-diketiminato triflate salt [HC{C(Me)NH(C6H3-2,6-Me2)}2][OTf] (3). Compound 1 reacts with I2 to yield the cation [HC(I){C(Me)N(C6H3-2,6-Me2)}2AuMe2]+, which is isolated as the triflate salt [HC(I){C(Me)N(C6H3-2,6-Me2)}2AuMe2][OTf] (4). The reaction of 1 with (PPh3)AuCl in the presence of AgOTf provides [HC(AuPPh3){C(Me)N(C6H3-2,6-Me2)}2AuMe2][OTf] (5). Molecular structures of 1, 4, and 5 were elucidated by single-crystal X-ray diffraction experiments; that of 4 was additionally confirmed by quantum-chemical calculations. The reactivity experiments indicate that the methine carbon in the β-...

Homoleptic tetramethylgallate Lu(GaMe4)3 reacts selectively with superbulky (TptBu,Me)H according to a methane elimination reaction, affording the quantitative formation of monomeric base-free low-coordinate (TptBu,Me)LuMe2. Weak interaction of the strongly basic lanthanide methyl groups with the comparatively weak Lewis acid trimethylgallium in Lu(GaMe4)3 is indicated by their separation into [LuMe3]n and GaMe3 even at −35 °C.

The synthesis of half-sandwich tetramethylaluminate complexes Cp*Y(AlMe4)2 (Cp* = C5Me5) was achieved by applying two synthesis protocols: (a) protonolysis of Y(AlMe4)3 with excess HCp* at ambient temperature; (b) alkylamide elimination using Cp*Y(NiPr2)2(THF) and trimethylaluminum at ambient temperature. The tetramethylgallate derivative Cp*Y(GaMe4)2 was obtained from the equimolar reaction of Y(GaMe4)3 with HCp* or by addition of 2 equiv of trimethylgallium to [Cp*YMe2]3. The half-sandwich complexes Cp*Y(MMe4)2 (M = Al, Ga) display similar structural chemistry featuring highly fluxional methyl groups in solution and one planar and one atypically bent Y(μ-Me)2M moiety in the solid state. Complexes [Cp*2Ln(MMe4)] (Ln = Y, La; M = Al, Ga) were synthesized from Ln(MMe4)3 with excess HCp* at elevated temperature. Such metallocene complexes show the known monomer−dimer equilibria in solution, with the entropically disfavored dimer prevailing at low temperature. Dimeric [Cp*2La(AlMe4)]2 and [Cp*2Y(GaMe4)]2 as ...

Heterobimetallic peralkylated calcium complexes [Ca(AlR4)2]n (R = Me, Et) were obtained from {Ca[N(SiMe3)2]2}2 and an excess AlR3 in almost quantitative yield. A similar silylamide elimination prot...

Treatment of rare-earth-metal dialkyl complexes with group 13 cocatalysts is a prominent approach to generate homogeneous catalysts active in olefin polymerization. Reaction of Ln(CH2SiMe3)3(THF)2 with monovalent imino-amido-pyridine [2-{(2,6-iPr2C6H3)N═CMe}-6-{(2,6-iPr2C6H3)NHCMe2}C5H3N] (HL2) gives donor solvent-free discrete dialkyl compounds [L2]Ln(CH2SiMe3)2 (Ln = Sc, Y, Lu). In the solid state the scandium derivative is isostructural to the previously reported lutetium complex (Gordon et al.). Activation by borate cocatalysts [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4] produces ion pairs that polymerize ethylene in moderate yields (activity: Sc > Lu). Cationization with N-[tris(pentafluorophenyl)borane]-3H-indole gives inactive species. 1H/13C/11B/19F NMR spectroscopy is applied to examine the interaction of the rare-earth-metal dialkyl complexes with the boron cocatalysts. In contrast to the monoalkyl diamido-pyridine compounds [L1]Ln(CH2SiMe3)(THF)x (HL1 = [2,6-{(2,6-iPr2C6H3)NHCH2}C5H3N], the dialky...

Transition metal precatalyst−organoaluminum cocatalyst interactions are of fundamental importance in Ziegler−Natta polymerization catalysis. Rare-earth metal tetramethylaluminate complexes (BDPPpyr...

The fundamental reactivity of amorphous [YMe3]n was representatively examined: toward GaMe3 as a Lewis acid, 9-fluorenone as a carbonyl substrate, and tetramethyldisilazane (HN(SiHMe2)2) as a Bron...

A ruthenium complex coordinated by a bidentate aryloxy-NHC ligand has offered the first example of an imidazolium-substituted metal alkylidene, an analogue of existing phosphonium alkylidenes and metal-substituted ketenes. Investigation of the molecular structure and the quantum chemically obtained electronic structure sheds light on the nature of this new compound and the bonds central to its formation.

The diarylplumbylene R2Pb: (3), R = 2-tBu-4,5,6-Me3C6H, and the rearranged alkylarylplumbylene RR‘Pb:, R = 2,4,6-tBu3C6H2, R‘ = CH2C(CH3)2-3,5-tBu2C6H3, were synthesized and characterized by NMR and UV/vis spectroscopy, as well as by X-ray crystallography. Treatment of 3 with the disilylplumbylene R‘‘2Pb:, R‘‘ = Si(SiMe3)3, furnished the heteroleptic plumbylene RR‘‘Pb: (8), which, in the solid state, forms the plumbylene dimer RR‘‘PbPbRR‘‘ (9). The X-ray structure analysis of 9 reveals a trans-bent angle of 46.5° and a Pb...Pb separation of 3.37 A, the shortest observed so far between the lead atoms of two plumbylenes.