This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4

π-Bonding and the Lone Pair Effect in Multiple Bonds Involving Heavier Main Group Elements: Developments in the New Millennium

Small inorganic rings in the 21st century: from fleeting intermediates to novel isolable entities.

Soft, flexible, and stretchable electronics are needed to transmit power and information, and track dynamic poses in next-generation wearables, soft robots, and biocompatible devices. Liquid metal has emerged as a promising material for these applications due to its high conductivity and liquid phase state at room temperature; however, surface oxidation of liquid metal gives it unique behaviors that are often incompatible with scalable manufacturing techniques. This paper reports a rapid and scalable approach to fabricate soft and flexible electronics composed of liquid metal. Compared to other liquid metal patterning approaches, this approach has the advantages of compatibility with a variety of substrates, ease of scalability, and efficiency through automated processes. Nonconductive liquid metal nanoparticle films are sintered into electrically conductive patterns by use of a focused laser beam to rupture and ablate particle oxide shells, and allow their liquid metal cores to escape and coalesce. The l...

Laser Sintering of Liquid Metal Nanoparticles for Scalable Manufacturing of Soft and Flexible Electronics.

We report the results of a comprehensive investigation of the recently discovered stereoselective and controlled polymerization of racemic lactide (d,l-LA) using an initiator prepared in situ from indium(III) chloride (InCl3), benzyl alcohol (BnOH), and triethylamine (NEt3). Linear relationships between number-average molecular weight (Mn) and both monomer to alcohol concentration ratio and monomer conversion are consistent with a well-controlled polymerization. Studies on polymerization kinetics show the process to be first-order in [InCl3]0 and zero-order in both [BnOH]0 and [NEt3]0. The rate of d,l-LA conversion is also dependent on the indium(III) halide (i.e., t1/2(InCl3) ≈ 43 min versus t1/2(InBr3) ≈ 7.5 h, 21 °C, CD2Cl2, [d,l-LA]0/[BnOH]0 ≈ 100, [d,l-LA]0 = 0.84 M, [InX3]0/[BnOH]0 = 1) and lactide stereoisomer (i.e., kobsd,l-LA ≈ kobsmeso-LA > kobsl-LA). A model system that polymerizes d,l-LA with the same high degree of stereoselectivity was developed using 3-diethylamino-1-propanol (deapH) in lie...

Mechanistic study of the stereoselective polymerization of D, L-lactide using indium(III) halides

This critical review focuses on the solution based chemical vapour deposition (CVD) of main group materials with particular emphasis on their current and potential applications. Deposition of thin films of main group materials, such as metal oxides, sulfides and arsenides, have been researched owing to the array of applications which utilise them including solar cells, transparent conducting oxides (TCOs) and window coatings. Solution based CVD processes, such as aerosol-assisted (AA)CVD have been developed due to their scalability and to overcome the requirement of suitably volatile precursors as the technique relies on the solubility rather than volatility of precursors which vastly extends the range of potentially applicable compounds. An introduction into the applications and precursor requirements of main group materials will be presented first followed by a detailed discussion of their deposition reviewed according to this application. The challenges and prospects for further enabling research in terms of emerging main group materials will be discussed.

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Solution based CVD of main group materials

Gallium(III) and indium(III) alkoxides and aryloxides

The donor-functionalised alkoxides [Et2Ga(OR)]2
				(R = CH2CH2NMe2
				(1), CH(CH2NMe2)2
				(2), CH2CH2OMe (3), CH(CH3)CH2NMe2
				(4), C(CH3)2CH2OMe (5)) were synthesised by the 1∶1 reaction of Et3Ga with ROH in hexane or dichloromethane at room temperature. Reaction of Et3Ga with excess ROH in refluxing toluene resulted in the isolation of a 1∶1 mixture of [Et2Ga(OR)]2 and the ethylgallium bisalkoxide [EtGa(OR)2]
				(R = CH2CH2NMe2
				(6) or CH(CH3)CH2NMe2
				(7)). X-ray crystallography showed that compound 6 is monomeric and this complex represents the first structurally characterised monomeric gallium bisalkoxide. Homoleptic gallium trisalkoxides [Ga(OR)3]2 were prepared by the 1∶6 reaction of [Ga(NMe2)3]2 with ROH (R = CH2CH2NMe2
				(8), CH(CH3)CH2NMe2
				(9), C(CH3)2CH2OMe (10)). The decomposition of compounds 1, 4, 5 and 8 were studied by thermal gravimetric analysis. Low pressure CVD of 1 and 5 resulted in the formation of thin films of crystalline Ga2O3.

Molecular precursors to gallium oxide thin films.

Relative partial ionization cross sections and precursor specific relative partial ionization cross sections for fragment ions formed by electron ionization of C2H2 have been measured using time-of-flight mass spectrometry coupled with a 2D ion-ion coincidence technique. We report data for the formation of H+, H+2, C2+, C+/C2+ 2, CH+/C2H+2, CH+2, C+2, and C2H+ relative to the formation of C2H+2, as a function of ionizing electron energy from 30-200 eV. While excellent agreement is found between our data and one set of previously published absolute partial ionization cross sections, some discrepancies exist between the results presented here and two other recent determinations of these absolute partial ionization cross sections. We attribute these differences to the loss of some translationally energetic fragment ions in these earlier studies. Our relative precursor-specific partial ionization cross sections enable us, for the first time, to quantify the contribution to the yield of each fragment ion from single, double, and triple ionization. Analysis shows that at 50 eV double ionization contributes 2% to the total ion yield, increasing to over 10% at an ionizing energy of 100 eV. From our ion-ion coincidence data, we have derived branching ratios for charge separating dissociations of the acetylene dication. Comparison of our data to recent ab initio/RRKM calculations suggest that close to the double ionization potential C2H2+2 dissociates predominantly on the ground triplet potential energy surface (3Sigma*g) with a much smaller contribution from dissociation via the lowest singlet potential energy surface (1Delta g). Measurements of the kinetic energy released in the fragmentation reactions of C2H2+2 have been used to obtain precursor state energies for the formation of product ion pairs, and are shown to be in good agreement with available experimental data and with theory.

Simon J. King

Papers

Gallium(III) and indium(III) alkoxides and aryloxides

Molecular precursors to gallium oxide thin films.

Electron ionization of acetylene.

Electron ionization of CO2

Electron ionization of H2O