Several major developments in the synthetic organometallic and inorganic chemistry of molecules featuring multiple bonds between open-shell heavier main-group elements (period 3 or heavier) that coincide with PPP’s independent career are reviewed herein. An emphasis has been placed on the discovery of new classes of molecules and element–element bonds that were not known prior to 1981. The major theoretical work that describes the bonding in these species is also briefly reviewed along with some of the unusual reactivity that is now accessible. It is concluded that these advances have come about mainly as a result of exploratory inorganic and organometallic synthesis.

One-electron reduction of the diplumbyne AriPr6PbPbAriPr6, 1, with KC8 in the presence of 18-crown-6 affords crystals of the salt of its radical anion, [K(18-crown-6)][AriPr6PbPbAriPr6], 2, AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2. Its X-ray crystal structure shows that the Pb-Pb distance becomes remarkably shortened by ca. 0.25 Å upon reduction, consistent with population of the π-bonding LUMO of 1. The radical nature of 2 is confirmed by its electron paramagnetic resonance spectrum which shows coupling of the unpaired electron to both 207Pb nuclei.

The reaction of three equivalents of LiCH(SiMe3)2 with TiCl3(NMe3)2 afforded the rare homoleptic Ti(III) alkyl Ti{CH(SiMe3)2}3 (1) which crystallized as blue needles in 32 % yield. Single crystal X‐Ray data for 1 showed a trigonal pyramidal coordination geometry around titanium, which could be ascribed to weak interactions between the C–H bonds and the Ti(III) atom based on computational results. X‐band EPR spectroscopy give spectral parameters consistent with the proposed Ti(III) formulation. Solutions of 1 are unstable at room temperature owing to intramolecular C–H activation that gave a dimeric Ti(IV) complex [{(Me3Si)2HC}Ti{μ‐CH HSiMe3}]2 (2).

The terphenyl substituted distannynes and diplumbynes ArEEAr (E = Sn, Pb; Ar = AriPr4, C6H3-2,6-(C6H3-2,6-iPr2)2; Ar = AriPr6, C6H3-2,6-(C6H2-2,4,6-iPr3)2) undergo metathesis and ligand redistribution reactions in solution at ambient temperature to yield mixtures of asymmetrically substituted and heterometallic heavy alkyne analogues. The ambient temperature solution EPR spectrum of AriPr6SnSnAriPr6 shows a weak signal corresponding to the (SnAriPr6)· radical, and the thermolysis of AriPr8PbPbAriPr8 (AriPr8 = C6H-3,5-iPr2-2,6-(C6H2-2,4,6-iPr3)2) in toluene yields the plumbylene Pb-(CH2C6H5)-AriPr8, also suggesting that a transient Pb-(AriPr8)· radical is liberated on heating. These results provide further evidence to the solution equilibria of the heavy group 14 analogues with the respective one-coordinate metal radicals.

Careful analysis of the crystals formed in the reduction of AriPr8AlI2 (AriPr8 = C6H‐2,6‐(C6H2‐2,4,6‐iPr3)2‐3,5‐iPr2) with sodium on sodium chloride showed them to contain the long sought‐after dialuminene AriPr8AlAlAriPr8 (1) that forms alongside the previously characterized alanediyl:AlAriPr8. The single crystal X‐ray structure of 1 revealed a nearly planar, trans‐bent C(ipso)AlAlC(ipso) core with an Al–Al distance of 2.648(2) Å. The molecular and electronic structure of 1 are consistent with a Al–Al double dative interaction augmented with diradical character and stabilized by dispersion interactions. Density functional theory calculations showed that the reactivity of:AlAriPr8 with dihydrogen involves 1, not:AlAriPr8, as the reactive species. In contrast, the reaction of:AlAriPr8 with ethylene gave two products, the 1,4‐dialuminacyclohexane AriPr8Al(C2H4)2AlAriPr8 (2) and the aluminacyclopentane AriPr8Al(C4H8) (3), that can both form from the aluminacyclopropane intermediate AriPr8Al(C2H4). Although the [2+2+2] cycloaddition of 1 with two equivalents of ethylene was also calculated to be exergonic, it is likely to be kinetically blocked by the numerous isopropyl substituents surrounding the Al–Al bond. Attempts to fine‐tune the steric bulk of the terphenyl ligand to allow stronger Al–Al bonding were unsuccessful, leading to the isolation of the sodium salt of a cyclotrialuminene, Na2[AlAriPr6]3 (4), instead of AriPr6AlAlAriPr6.

We report the isolation of a series of cyclohexyl-substituted, homoleptic main group arylthiolates, {Ge(SC6H2-2,4,6-Cy3)2}2 (1), {Sn(SC6H2-2,4,6-Cy3)2}2 (2), and {Pb(SC6H2-2,4,6-Cy3)2}2 (3), as well as an improved one-pot synthesis of the thiol HSC6H2-2,4,6-Cy (4) with increased purity and yield. The solid-state structures of compounds 1–3 show that the group 14 atoms are bridged by two thiolato ligands whose hydrocarbon substituents are in either cis (1 and 2) or trans (3) conformations. In solution, the Ge(II) derivative 1 exists as a mixture of dimeric cis and trans isomers or as the monomer Ge(SC6H2-2,4,6-Cy3)2, as inferred from 1H NMR data. Contrary to a previous report of derivatives of the isopropyl-substituted thiol HSC6H2-2,4,6-Pri3, which led to the formation of a Ge(IV) hydride, no such hydride was observed during the synthesis of 1. Computational studies showed that the dimeric structure of 1 is stabilized by intramolecular dispersion interactions that are higher than those in similar systems employing the isopropyl-substituted ligand, in agreement with the preferred formation of HGe(SC6H2-2,4,6-Pri3)3 over the putative dimer {Ge(SC6H2-2,4,6-Pri3)2}2, although the exact mechanism leading to the hydride remains unclear. The corresponding Sn(II) derivative 2 is the first structurally characterized dimeric tin(II) thiolate. The Pb(II) species 3 is a rare example of a lead(II) arylthiolate that crystallizes in the absence of additional donor molecules.

Salt metathesis routes to five new -N(SiMe3)(2) nickel derivatives were studied to illuminate their mode of formation, structures, and spectroscopy. The reaction between NiI2 and K{N(SiMe3)(2)} afforded the Ni(II) and Ni(I) complexes [K][Ni{N(SiMe3)(2)}(3)] (1) and [K][Ni{N(SiMe3)(2)}(2)] (2). Dissolving 1 in tetrahydrofuran (THF) gave the Ni(II) species [K(THF)(2)][Ni{N(SiMe3)(2)}(3)] (3). The Ni(I) salt [K(DME)][Ni-2{N(SiMe3)(2)}(3)] (4) was obtained by using NiCl2(DME) (DME = 1,2-dimethoxyethane) as the nickel source rather than NiI2. The isolation of the Ni(I) complexes 2 and 4 highlights the tendency for K{N(SiMe3)(2)} to function as a reducing agent. Introduction of adventitious O-2 to solutions of [K][Ni{N(SiMe3)(2)}(2)] (2) gave the nickel inverse crown ether (ICE) species [K-2][O(Ni{N(SiMe3)(2)}(2))(2)] (5). Complex 5 is the first ICE complex of nickel and is one of four known ICE complexes for the 3d metals. The experimental results indicate that the reduced Ni(I) bis(trimethylsilyl)amides are relatively easily generated, whereas Ni(III) derivatives that might be expected from a disproportionation of a Ni(II) derivative are apparently not yet isolable by the above routes. Overall, the new species crystallize readily from the reaction mixtures, but under ambient conditions, they begin to decompose as solids within ca. 24 h, which hinders their characterization.

The bismuth hydride (2,6-Mes2H3C6)2BiH (1, Mes = 2,4,6-trimethylphenyl), which has a Bi-H 1H NMR spectroscopic signal at δ = 19.64 ppm, was reacted with phenylacetylene at 60 °C in toluene to yield [(2,6-Mes2C6H3)2BiC(Ph)=CH2] (2) after 15 min. Compound 2 was characterized by 1H, 13C NMR, and UV-vis spectroscopy, single crystal X-ray crystallography, and calculations employing density functional theory. Compound 2 is the first example of a hydrobismuthation addition product and displays Markovnikov regioselectivity. Computational methods indicated that it forms via a radical mechanism with an associated Gibbs energy of activation of 91 kJ mol-1 and a reaction energy of -90 kJ mol-1.

Half a century after the photolytic disproportionation of Lappert's dialkyl stannylene SnR2, R = CH(SiMe3)2 (1) gave the persistent trivalent radical [˙SnR3], the characterization of the corresponding Sn(I) product, ˙SnR is now described. It was isolated as the hexastannaprismane Sn6R6 (2), from the reduction of 1 by the Mg(I)-reagent, Mg(BDIDip)2 (BDI = (DipNCMe)2CH, Dip = 2,6-diisopropylphenyl).

London dispersion (LD) interactions, which stem from long-range electron correlations arising from instantaneously induced dipoles can occur between neighboring atoms or molecules, for example, between H atoms within ligand C-H groups. These interactions are currently of interest as a new method of stabilizing long bonds and species with unusual oxidation states. They can also limit reactivity by installing LD enhanced groups into organic frameworks or ligand substituents. Here, we address the most recent advances in the design of LD enhanced ligands, the sterically counterintuitive structures that can be generated and the consequences that these interactions can have on the structures and reactivity of sterically crowded heavy group 14 species.

A series of ferriostannylenes of formula ArMe6SnFeCp(CO)2 (ArMe6 = - C6H3-(C6H2-2,4,6-Me3)2, Cp = eta 5-C5H5) (1), ArMe6SnFeCp*(CO)2 (Cp* = eta 5-C5Me5) (2), and ArMe6SnFeCp(CO)(PMe3) (3) with differing iron and/or tin substituents was synthesized. Their structures and spectroscopic properties were examined by X-ray crystallography, NMR, IR, and UV-vis spectroscopy and compared with data for related species. The structural data showed that, as the size of the terphenyl substituent increases, the C-Sn-Fe angle decreases slightly which is contrary to steric expectations. The 1H and 119Sn NMR chemical shifts of the least crowded species 1 is similar to, but slightly upfield of those of its more hindered ferriostannylene analogs. Unexpectedly, 3 displayed a 119Sn NMR chemical shift that is ca. 800 ppm downfield of 1 as a result of the substitution of one of the iron carbonyl groups with a PMe3 ligand. This unusual finding is probably a reflection of a decreased paramagnetic shielding of the 119Sn nucleus by the more electron releasing character of the PMe3 ligand which decreases the n -> p energy gap. The infrared spectrum of 1 also displayed slightly higher nu CO frequencies, and its electronic spectrum indicated a small hypsochromic shift in the energy of its n -> p transition whereas both 2 and 3 displayed much greater hypsochromic shifts than 1 consistent with the more electron donating character of the iron phosphine substituent. The results were interpreted in terms of the electronic/steric properties of the various substituents and their effects on metal electron density.

The new Ge(II) cluster [Ge6(μ3-O)4(μ2-OC6H2-2,4,6-Cy3)4](NH3)0.5 (1) and three divalent Group 14 aryloxide derivatives [Ge(OC6H2-2,4,6-Cy3)2]2 (2), [Sn(OC6H2-2,4,6-Cy3)2]2 (3), and [Pb(OC6H2-2,4,6-Cy3)2]2 (4) of the new tricyclohexylphenyloxo ligand, [(-OC6H2-2,4,6-Cy3)2]2 (Cy = cyclohexyl), were synthesized and characterized. Complexes 1-4 were obtained by reaction of the metal bissilylamides M(N(SiMe3)2)2 (M = Ge, Sn, Pb) with 2,4,6-tricyclohexylphenol in hexane at room temperature. If the freshly generated reaction mixture for the synthesis of 2 is stirred in solution for 12 h at room temperature, the cluster [Ge6(μ3-O)4(μ2-OC6H2-2,4,6-Cy3)4](NH3)0.5 (1), which features a rare Ge6O8 core that includes ammonia molecules in non-coordinating positions, is formed. Complexes 3 and 4 were also characterized via119Sn{1H} NMR and 207Pb NMR spectroscopy and feature signals at -280.3 ppm (119Sn{1H}, 25 °C) and 1541.0 ppm (207Pb, 37 °C), respectively. The spectroscopic characterization of 3 and 4 extends known 119Sn parameters for dimeric Sn(II) aryloxides, but data for 207Pb NMR spectra for Pb(II) aryloxides are rare. We present also a rare VT-NMR study of a homoleptic 3-coordinate Pb(II) aryloxide. The crystal structures of 2, 3, and 4 feature interligand H⋯H contacts that are similar in number to those of related transition metal derivatives despite the larger size of the group 14 elements.

Thermal Sn-C cleavage in the diarylstannylene Sn(AriPr4)2 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) was used to generate ˙Sn(AriPr4) and ˙AriPr4 radicals for alkyne arylstannylation. The radical pair and RCCR' (R = H, R' = Ph; R = Ph, R' = Ph; R = H, R' = C4H9; R = H, R' = SiMe3) in refluxing benzene generate the aryl vinyl stannylene complexes, AriPr4Sn{C(C6H5)-C(H)(AriPr4)} (1), AriPr4Sn{C(C6H5)-C(H)(C6H5)} (2) and AriPr4Sn{C(C4H9)-C(H)(AriPr4)} (3) respectively. For HCCSiMe3, the known distannene {Sn(CCSiMe3)AriPr4}2 (4) was also generated from this new method.

[This corrects the article DOI: 10.1021/acs.organomet.3c00190.].

A series of Mn(II), Fe(II), and Co(II) bisaryloxide dimers ([M(OC6H2-2,4,6-Cy3)2]2 {M = Mn (1), Fe (2), and Co (3)} were synthesized by the addition of 2,4,6-tricyclohexylphenol (HOC6H2-2,4,6-Cy3) to the silyl amido dimers [M(N(SiMe3)2)2]2 (M = Mn, Fe, Co; Cy = cyclohexyl). An unexpected and unique Co(II) phenoxide derivative (4), [Co(OC6H2-2,4,6-Cy3)(O2C6H-3,5,6-Cy3)]2, was obtained via ligand rearrangement of 3 at ca. 180 °C. This yielded 4 in which there are two unchanged, bridging phenoxide ligands as well as a terminal bidentate semiquinone ligand bound to each cobalt. Complexes 1 and 2 did not undergo such a rearrangement under the same conditions; both are thermally stable to temperatures exceeding 250 °C and feature numerous short-contact (<2.5 Å) H···H interactions consistent with the presence of dispersion stabilization. Use of the aryloxide ligand -OC6H3-2,6-Pri2 (Pri = isopropyl), which is sterically similar to -OC6H2-2,4,6-Cy3 but produces fewer close H···H interactions, gave the trimeric species [M(OC6H3-2,6-Pri2)2]3 {M = Fe (5) or Co (6)} which feature a linear array of three metal atoms bridged by aryloxides. The higher association number in 5 and 6 in comparison to that of 1-3 is due to the lower dispersion energy donor properties of the -OC6H3-2,6-Pri2 ligand and the lower stabilization it produces.

Reaction of {LiC6H2-2,4,6-Cyp(3)center dot Et2O}(2) (Cyp= cyclopentyl) (1) of the new dispersion energy donor (DED) ligand, 2,4,6-triscyclopentylphenyl with SnCl2 afforded a mixture of the distannene {Sn(C(6)H2 -2,4,6-Cyp3)2} 2 (2), and the cyclotristannane {Sn(C6H2-2,4,6-Cyp(3))(2)} 3 (3). 2 is favored in solution at higher temperature (345 K or above) whereas 3 is preferred near 298 K. Van't Hoff analysis revealed the 3 to 2 conversion has a Delta H= 33.36 kcalmol(-1) and Delta S = 0.102 kcalmol(-1)K(-1), which gives a Delta G(300) (K) = + 2.86 kcalmol(-1), showing that the conversion of 3 to 2 is an endergonic process. Computational studies show that DED stabilization in 3 is -28.5 kcalmol(-1) per {Sn-(C6H2 2,4,6-Cyp(3))(2) unit, which exceeds the DED energy in 2 of 16.3 kcalmol(-1) per unit. The data clearly show that dispersion interactions are the main arbiter of the 3 to 2 equilibrium. Both 2 and 3 possess large dispersion stabilization energies which suppress monomer dissociation (supported by EDA results).

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Interactions between sterically crowded hydrocarbon-substituted ligands are widely considered to be repulsive because of the intrusion of the electron clouds of the ligand atoms into each other's space, which results in Pauli repulsion. Nonetheless, there is another interaction between the ligands which is less widely publicized but is always present. This is the London dispersion (LD) interaction which can occur between atoms or molecules in which dipoles can be induced instantaneously, for example, between the H atoms from the ligand C-H groups. These LD interactions are always attractive, but their effects are not as widely recognized as those of the Pauli repulsion despite their central role in the formation of condensed matter. Their relatively poor recognition is probably due to the relative weakness (ca. 1 kcal mol(-1)) of individual H center dot center dot center dot H interactions owing to their especially strong distance dependence. In contrast, where there are numerous H center dot center dot center dot H interactions, a collective LD energy equaling several tens of kcal mol(-1) may ensue. As a result, in some molecules the latent importance of the LD attraction energies emerges and assumes a prominence that can overshadow the Pauli effects (e.g., in the stabilization of high-oxidation-state transition-metal alkyls, inducing disproportionation reactions, or in the stabilization of otherwise unstable bonds). Despite being known for over a century, the accurate quantification of individual H center dot center dot center dot H LD effects in molecular species is a relatively recent phenomenon and at present is based mainly on modified DFT calculations. A few leading reviews summarized these earlier studies of the C-H center dot center dot center dot H-C LD interactions in organic molecules, and their effects on the structures and stabilities were described. LD effects in sterically crowded inorganic and organometallic molecules have been recognized. The author's interest in these LD effects arose fortuitously over a decade ago during research on sterically crowded heavier main-group element carbene analogues and two-coordinate, open-shell (d(1)-d(9)) transition-metal complexes where counterintuitive steric effects were observed. More detailed explanations of these effects were provided by dispersion-corrected DFT calculations in collaboration with the groups of Tuononen and Nagase (see below). This Account describes our development of these initial results for other inorganic molecular classes. More recently, the work has led us to move to the planned inclusion of dispersion effects in ligands to stabilize new molecular types with theoretical input from the groups of Vasko and Grimme (see below). Our approach sought to use what Grimme has described as dispersion effect donor (DED) groups (i.e., spatially close-lying, densely packed substituents either as ligands (e.g., -C6H2-2,4,6-Cy-3, Cy = cyclohexyl) or as parts of ligands (e.g., a Cy substituent) that produce relatively large dispersion energies to stabilize these new compounds. We predict that the future design of sterically crowding hydrocarbon ligands will include the consideration and incorporation of LD effects as a standard methodology for directed use in the attainment of new synthetic targets.