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The major theme of this review concerns the reaction pathways of heavier main group 13 dimetallenes with olefins, cyclic olefins, polyolefins, other unsaturated molecules, and hydrogen. For gallium derivatives, these reactions proceed readily in high yield under ambient conditions. Early investigations showed that the group 13 dimetallenes dissociated to one-coordinate metallanediyl monomers, which suggested that they reacted as monomeric species due to their more open, less hindered structures. However, recent DFT calculations by Tuononen and co-workers show that the reaction of the monomers with olefins or hydrogen involves very large energy barriers and that the dimetallenes react instead as undissociated dimers. In contrast to the corresponding all-carbon systems, where [2 + 2] cycloadditions are forbidden because of π–π* symmetry mismatch, the dimetallenes react with olefins via two stepwise, symmetry-allowed [2 + 2] cycloadditions to give 1,4-digallacyclohexanes. The addition of the H2 proceeds by a...

A wide variety of monodentate, sterically encumbering ligands have been used to stabilize low coordination numbers in both main group and transition element compounds [1]. In recent years, the very crowded m-terphenyls have emerged as a particularly effective class of bulky ligand for the stabilization of numerous main group element compounds with unusual bonding and electron configurations [2, 3]. These include, for example, the first compounds with formal triple bonds between group 13 [4, 5] and group 14 metals [6, 7], as well as a stable crystalline heavier main group radical species [8]. In contrast, transition metal terphenyl derivatives have received less attention and most of the work has been performed within the last 5 years. This account focuses on these recently discovered transition metal species.

The synthesis and characterization of several previously unknown ring systems that involve heavier main grop elements are described. Four new types of potentially delocalized six-membered ring compounds that comprise the cyclic arrays (RBPR′)3, (RAINR′)3, (RZnSR′)3 and (GenR)3, are considered. The synthetic routes to these rings are straightforward and require only one or two steps to give the respective products in good yield. All the rings have been structurally characterized. The structures, in conjunction with spectroscopic data, indicate considerable variation with ring type. Quasi-aromaticity in the case of the (RBPR′)3 ring is well supported by the experimental data and, indirectly, by the non-planar structure that was observed for the `anti-aromatic' four-membered (RBPR′)2 rings. Further support comes from recent calculations at the SCF level using the basis set 6-31G* which indicate that the (HBHP)3 system has a similar stabilization to that seen in borazine, whereas the (HAINH)3 ring exhibits only a minor degree of delocalization of the π electrons.

The 1,2-dihalogenodiborane(4) compound Br(Me2N)BB(NMe2)Br 1 reacts with Li(mes)(mes = C6H2Me3-2,4,6) or LiPh to form the derivatives Br(Me2N)BB(NMe2)(mes)2, (mes)(Me2N)BB(NMe2)(mes)3, and the previously reported (synthesised by a different route) species Ph(Me2N)BB(NMe2)Ph 4 in moderate yields. Compounds 2–4 are thermally stable, colourless, crystalline solids. All compounds were characterized by X-ray crystallography and NMR spectroscopy. Their structures have B–B bond lengths that vary from 1.682(16)A in 1 to 1.717(15)A in 3. In addition, all compounds display high angles (from 58.9 to 88.7 °) between the planes at the boron atoms and short B–N distances that are near 1.39 A. The structures of 1–4 represent, apparently, the first such acyclic examples for diborane(4) derivatives of the type B2X2(NR2)2 where X = halogen or aryl group. Crystal data: 130 K: 1, Br(Me2N)BB(NMe2)Br, monoclinic, space group P21, a= 6.789(2), b= 7.720(2), c= 10.012(4), β= 103.06(3)°, Z= 2, R= 0.048 for 1109 [I > 2.0σ(I)] data; 2, Br(Me2N)BB(NMe2)(mes), monoclinic, space group C2/c, a= 24.890(14), b= 7.662(4), c= 18.500(12), β= 116.36(4)°, Z= 8, R= 0.063 for 1910 [I > 2.0σ(I)] data; 3(mes)(Me2N)BB(NMe2)(mes), orthorhombic, space group Pbcn, a= 37.983(14), b= 14.167(5), c= 12.282(4)A, Z= 12, R= 0.095 for 1829 [I > 3.0σ(I)] data; 4, Ph(Me2N)BB(NMe2)Ph, orthorhombic, space group P212121, a= 8.579(2), b= 12.920(4), c= 13.925(3)A, Z= 4, R= 0.045 for 1776 [I > 4.0σ(I)] data.

Several new arsinoboranes, wherein boron and arsenic are three coordinate, have been prepared and characterized. They are analogous to B-N and B-P species in which p-p π-bonding, involving the pnictide lone pair and the empty boron p-orbital, may occur. The main conclusion or these studies is that B-As π-bonding, although inherently strong, is difficult to achieve because or the larger inversion barrier at arsenic. Nontheless, very electropositive or bulky substituents at arsenic reduce this barrier surfficiently to observe substantial B-As π-interactions

The synthesis of (1-fluorovinyl)tributyltin ( 1b ) is reported, and the utility of 1b as a synthetic equivalent for the 1-fluoroethene anion 2 through palladium-catalyzed couplings is demonstrated.

The synthesis of diastereomeric substituted proline peptidomimetics as conformationally restricted tyrosine derivatives 1a,b has been accomplished utilizing the intramolecular hydroboration-cycloalkylation of azido-olefins 7a,b as the key step.

Publisher Summary The use of sterically crowding ligands to manipulate reactivity and to effect the stabilization of previously unknown bonding types, geometries, or electron configurations is one of the major themes of inorganic and organometallic chemistry. This chapter reviews the element derivatives of sterically encumbering terphenyl ligands. Bulky ligands use large organic substituents at a first-row element through which they are bound to a desired site. The precursors to the bulky terphenyl ligands and their derivatives are discussed. Terphenyl derivatives of s -block elements comprise group 1 and group 2 terphenyl derivatives. After the halide starting materials themselves, the alkali metal salts are the most important terphenyl compounds. This is a result of their widespread and almost exclusive use as transfer agents for terphenyl ligands. Main group 2 terphenyl compounds are of potential importance. As their lithium analogues, magnesium derivatives, such as the Grignard reagents have applications as ligand transfer agents. The use of terphenyl ligands may affect the stabilization of multiply bonded or catenated derivatives of the lightest element, beryllium, about which very little is currently known. Terphenyl derivatives of p -block elements include group 13, 14, 15, 16, and 17 terphenyl derivatives. Terphenyl derivatives of d -block and group 12 elements are also described, which comprises transition-metal derivatives.

This Perspective discusses the synthesis and reactivity of low valent Group 14 hydrides of germanium and tin. The use of hindered terphenyl ligands has facilitated the isolation of a number of unusual new main group hydride structural types (each predicted by computational work), and has culminated in the use of these species as precursors to new cluster archetypes.

Imidazolinium salts derived from (1S,2S)-diphenylethylenediamine were used to prepare chiral N-heterocyclic carbene (NHC) complexes of rhodium of the form (cod)Rh(NHC)Cl via silver transmetalation....

A neutral arene-tethered ruthenium complex was found to be a catalyst precursor for enyne cycloisomerizations and hydroxycyclizations. The observed products were the result of a skeletal rearrangement process, and include an unusual cyclization to form a six-membered ring. Labeling studies on the six-membered ring product are in accord with an electrophilic activation mechanism that proceeds via cationic cyclopropyl carbene intermediates.

Recent experimental results which have described the characterization of the first, stable heavier group 14 element analogues of acetylenes are outlined. It is shown that the use of large terphenyl substituents allows the isolation of transition metal-heavier group 14 element complexes that can achieve essential triple bonding by a three-fold orbital interaction between the transition metal and group 14 moiety. On the other hand the alkyne analogues RMMR (R = Ge, Sn or Pb) display increasing distortions from linearity to trans-bent geometry due to the accumulation of non-bonding electron density at the group 14 element. The non-bonding electron density comes at the expense of electron density in the bonding region between the group 14 elements. Accordingly the bond orders are decreased to values that are near double for the germanium and tin derivatives and single for the lead compound.

Primary alanes [RAlH2]2 can be stabilized kinetically by employment of very bulky alkyl or aryl substituents. They are easily prepared by reacting the appropriate lithium trihydroalanate precursor with organic halides, anhydrous hydrogen halides or halogens. Reactions of these aluminum hydrides with a wide variety of substrates ranging from protic reagents such as water, alcohols, amines, phosphines or thiols over ketones, nitriles or isonitriles to silylated species such as siloxanes or silyl halides are presented. Primary alanes were shown to be unique starting materials for a number of interesting compounds that were otherwise not accessible, or only available in low yields. In addition, postulated intermediates of aluminum hydride reductions of organic substrates such as ketones or nitriles can be isolated when using bulky primary alanes.