Homoatomic Bonding of Main Group Elements

Clusters of main group elements are not rare. On the contrary, it is becoming difficult to avoid the discovery of new substances of this type. Clusters are the natural intermediate stages between an element and its isolated atoms or ions. In the form of polycations and polyanions they offer models for the stepwise oxidation and reduction of an element and represent a bridge between the elements. The great majority of homonuclear bonded structures are already present in the solid phases of simple systems. Mobilization of these clusters as molecules represents a great challenge.     

Spectroscopic Investigation of Various Phosphorus-Sulfur Bonds

The results of spectroscopic investigations on phosphorus-sulfur compounds, and particularly the force constants, reveal several factors that decisively influence the bonding. The effects of hybridization, the other bonding partners and their electronegativities, the π-bonding components, and the ionic charge are discussed. It is shown that the P? S bond, unlike the P? O bond, never attains full double bond character, despite its great variability.     

The Chemistry of Simple Phosphorus-Carbon Compounds

With the aid of selected examples an overview is given of the development trends in phosphoruscarbon chemistry over the past few years. An attempt is made to demonstrate the relationships between various parameters and properties such as constitution, basicity, substitution by functional groups, reaction behavior etc. of the compounds. In the case of basis compounds containing methylphosphorus groups the state of development of industrially interesting processes is also outlined. In addition, the synthesis of a few bifunctional phosphorus-carbon compounds which can be employed as comonomers in the production of polymers is described.     

Tertiary Phosphane/Tetrachloromethane,a Versatile Reagent for Chlorination,Dehydration, and P?N Linkage

In this review the processes occurring on the treatment of triphenylphosphane with tetrachloromethane are first reported; a key role is played here by (trichloromethyl)phosphonium chloride which reacts with a further amount of the phosphane to give the stable salt [chloro(triphenylphos-phoranediyl)methyl]triphenylphosphonium chloride by way of dichlorophosphorane and (dichloromethylene)phosphorane that is detectable only as an intermediate. The preparative application of the two-component system as a chlorinating, dehydrating, and P—N-linking reagent is ascribable to “phosphorylation”

1 Here and below, “phosphorylation” is used lo mean in general the formation of a phosphorus derivative. In the narrower sense this term denotes the introduction of phosphoryl groups P(O).

of the substrate in question through several reactive species. In these applications the reactions with the substrate compete with reactions of the two-component system with itself, so that the overall course of the reaction is in nearly all cases much more complex than was previously assumed. Nevertheless. very good results can be achieved by the use of this reagent, high yields and mild reaction conditions being characteristic.     

Phospha-alkenes and Phospha-alkynes,Genesis and Properties of the (p-p)π-Multiple Bond

Just as the octet rule had prevented the discovery of the inert-gas compounds, the systematic search for phosphorus-carbon- and phosphorus-nitrogen-compounds having (p-p)π-multiple bonds was hindered by the double bond rule. The first successful synthesis of mesomeric-stabilized phosphorus-carbon compounds with coordination number 2, seventeen years ago, was followed nine years later by the preparation of the imino-phosphanes whose intensive investigation has continued to the present day. A dramatic development has now begun in the phospha-alkene and -alkyne fields, and a large variety of preparative methods have been found. Several compounds of this type are amazingly stable, however they do not participate in typical ylide reactions such as the Wittig reaction. In contrast, the PC double bond is more comparable to that of olefins, which has been confirmed by the occurrence of E,Z-isomers and pericyclic reactions of phospha-l,5-hexadienes.     

Boron-Phosphorus Compounds and Multiple Bonding

Boron-phosphorus compounds have not been as thoroughly studied as their boron-nitrogen counterparts. Until recently many classes of B-P compounds that had been well established in B-N chemistry were either unknown or poorly characterized. This statement is particularly true for compounds involving possible multiple bonding between boron and phosphorus. For example, detailed structural information on simple monomeric phosphino-boranes, R₂BPR′₂, did not become available until 1986 even though the isoelectronic SiC double bonded species, the silenes, had already been reported. However, new work has shown that it is possible to prepare and characterize several novel types of boron-phosphorus compounds with varying degrees of multiple B—P bonding. These include not only monomeric phosphinoboranes but also phosphanediylborates (borylphosphides), three- and four-membered rings (diphosphadi-boretanes), boron phosphorus analogues of borazine, B-P skeletal analogues of allyl cations and anions, butadiene and cage compounds. Structural, spectroscopic (mainly NMR) and theoretical studies reveal some important differences between B-P and B-N compounds which in many cases can be traced to the presence of a high inversion barrier at phosphorus that reduces the π interaction. This usually causes compounds such as R₂BPR′₂ to associate through σ bonding between B and P. Supporting evidence for this view comes from species that involve phosphorus and nitrogen in competitive π bonding with a boron p orbital in which the dative interaction between B and N is dominant and the phosphorus center remains pyramidal. Recently published work has shown that steric and electronic factors can be used to favor π bonding and give an approximately planar system. Furthermore, theoretical studies reveal that p? p π overlap in a planar B-P system is of similar efficiency to its B-N analogue. Good examples are seen in the phosphanediyl borates, the boron-phosphorus analogues of borazine and the π-allyl cations, whose molecular configurations and B—P bond lengths support strong boron—phosphorus π bonding.     

Phosphaalkynes—Syntheses,Reactions, Coordination Behavior [New Synthetic Methods (73)]

Organophosphorus compounds have been applied in two ways in chemical synthesis. They can either be used as a reagent in a step of the synthesis (for example, in the Wittig reaction) or they can be incorporated directly into the target molecule. This second application, in particular, has expanded greatly in the last few years with the preparation of low-coordination phosphorus compounds. These include the phosphaalkynes, which are of great interest to organic and inorganic chemists. Phosphaalkynes have been employed in the synthesis of heterocyclic compounds, phosphaarenes and their valence isomers, and polycyclic compounds. Further applications have been the use of phosphaalkynes as new ligand systems in complex chemistry and their cyclooligomerization with organometallic reagents. While the chemical properties of phosphaalkynes have little in common with those of nitriles, they are in many ways very similar to those of the isoelectronic acetylenes.     

Covalent versus Dative Bonds to Main Group Metals,a Useful Distinction

Textbooks of inorganic chemistry describe the formation of adducts by coordination of an electron donor to an electron acceptor, often using the amine-boranes, X₃N → BY₃, as examples. In the Lewis (electron dot) formulas of the compounds, the dative bond in H 3 N → BH₃ and the covalent bond in H₃C?CH₃ are both represented by a shared electron pair. In the simple molecular orbital or valence bond models the wave functions of both electron pairs would be constructed in the same manner from the appropriate sp³ type atomic orbitals on the bonded atoms; the difference between the covalent and the dative bond becomes apparent only after the orbital coefficients have been analyzed. This may be the reason why many structural chemists seem reluctant to distinguish between the two types of bonds. The object of this article is to remind the reader that the physiocochemical properties of covalent and dative bonds may be – and often are – quite different, and to show that a distinction between the two provides a basis for understanding the structures of a wide range of main group metal compounds.     

Solid-State Chemistry with Nonmetal Nitrides

Among the nonmetal nitrides, the polymeric binary compounds BN and Si₃N₄are of particular interest for the development of materials for high-performance applications. The outstanding features of both substances are their thermal, mechanical, and chemical stability, coupled with their low density. Because of their extremely low reactivity, boron and silicon nitride are hardly ever used as starting materials for the preparation of ternary nitrides, but are used primarily in the manufacture of crucibles or other vessels or as insulation materials. The chemistry of ternary and higher nonmetal nitrides that contain electropositive elements and are thus analogous with the oxo compounds such as borates, silicates, phosphates, or sulfates was neglected for many years. Starting from the recent successful preparation of pure P₃N₅, a further binary nonmetal nitride which shows similarities with Si₃N₄ with regard to both its structure and properties, this review deals systematically with the solid-state chemistry of ternary and higher phosphorus(V ) nitrides and the relationship between the various types of structure found in this class of substance and the resulting properties and possible applications. From the point of view of preparative solid-state chemistry the syntheses, structures, and properties of the binary nonmetal nitrides BN, Si₃N₄, and P₃N₅ will be compared and contrasted. The chemistry of the phosphorus(V ) nitrides leads us to expect that other nonmetals such as boron, silicon, sulfur, and carbon will also participate in a rich nitride chemistry, as initial reports indeed indicate.     

Monovalent Group 13 Organometallic Compounds: Weak Association to Monomeric,Versatile Two-Electron Donors

A weakly associated hexamer is formed for [GaCp*] (Cp*=C₅Me₅) in the solid state (see picture). The recent X-ray crystal structure analyses of [GaCp*] as well as the monomeric In^I and Tl^I compounds [M(2,4,6-Trip₃C₆H₂)] (Trip=2,4,6-iPr₃C₆H₂) throw new light on the association and aggregation of monovalent Group 13 elements in the solid state. The synthesis of [Ni⁰{In[C(SiMe₃)₃]}₄], a complex with terminally bonded In^IR ligands, offers alternative σ-donor/π-acceptor ligands to organometallic chemists. The newest results in this area are likely to open up new and intriguing possibilities in the preparation of main group–transition metal clusters.     

The Ionization (Heterolysis) of Covalent Compounds as a Coordination-Chemical Phenomenon

The ionization of a covalent compound into a cation and an anion is a coordination-chemical phenomenon. The ions are formed with coordination either by nucleophilic attack of an electron pair donor or by electrophilic attack of an electron pair acceptor. The dielectrically controlled component of the ionization affects only the degree to which it occurs. It is shown that the ionization generally increases with increasing donor strength of the donor or with increasing acceptor strength of the acceptor. For a given M, the ease of ionization of the M? X bond increases in the order X = fluoride < chloride < bromide < iodide. These general aspects are discussed with the aid of examples.     

Cage Compounds of Phosphorus and the Heavier Group 14 Elements

The Sn/P cage compounds [Sn₁₀(tBuP)₄] ( 1 ) and [Sn₁₀(tBuP)₇Cl₄] ( 2 ) were obtained by the reaction of LitBuPH with SnBr₂ and SnCl₂, respectively. Theoretical calculations confirm that the tin atoms in 2 feature different formal oxidation states. The reaction of PbCl₂ with LitBuPH and LiPHSiiPr₃ yields the Pb/P cage compounds [Pb₇(tBuP)₇] ( 3 ) and [Pb₆(PSiiPr₃)₆] ( 4 ), respectively, that represent new derivatives of known polyhedrons of this class of compounds. The new products were characterized by X‐ray diffraction, elemental analysis and partially by MS and MAS‐NMR spectroscopy.     

Multiple Bonds between Transition Metals and “Bare” Main Group Elements: Links between Inorganic Solid State Chemistry and Organometallic Chemistry

The last two decades have seen a dramatic development in the study of metal-metal multiple bonds, particular successes being recorded in the field of organometallic chemistry. Syntheses designed to produce novel transition metal complexes with single, double, triple and quadruple metal-metal bonds occupy a most important place in such research, as also do reactivity studies. A striving to establish general principles has provided much of the motivation for such work, but one less obvious goal—the commercial application of the catalytic properties of metal-metal multiple bonding systems, in the medium and long term—should not be overlooked. All aspects of the investigations of metal-metal multiple bonds also apply to a particular class of compound that has, however, enjoyed little lime-light and thus deserves the present review: complexes with multiple bonds between transition metals and substituent-free (“bare”) main group elements. Although based mostly on accidental discoveries, the few noteworthy examples are now beginning to unfold general concepts of synthesis that are capable of being extended and thus are deserving of exploitation in preparative chemistry. The availability of further structural patterns exhibiting multiple bonds between transition metals and ligand-free main group elements might enable preparative organometallic chemistry to expand in a completely new direction (for instance by the stabilizing or activation of small molecules at the metal complex). This essay discusses the chemistry of complexes of bare carbon, nitrogen, and oxygen ligands (carbido-, nitrido-, and oxo-complexes) and their relationships to higher homologues from both a synthetic and a structural point of view.     

Structurally Defined Allyl Compounds of Main Group Metals: Coordination and Reactivity

Organometallic allyl compounds are important as allylation reagents in organic synthesis, as polymerization catalysts, and as volatile metal precursors in material science. Whereas the allyl chemistry of synthetically relevant transition metals such as palladium and of the lanthanoids is well‐established, that of main group metals has been lagging behind. Recent progress on allyl complexes of Groups 1, 2, and 12–16 now provides a more complete picture. This is based on a fundamental understanding of metal–allyl bonding interactions in solution and in the solid state. Furthermore, reactivity trends have been rationalized and new types of allyl‐specific reactivity patterns have been uncovered. Key features include 1) the exploitation of the different types of metal–allyl bonding (highly ionic to predominantly covalent), 2) the use of synergistic effects in heterobimetallic compounds, and 3) the adjustment of Lewis acidity by variation of the charge of allyl compounds.     

Disubstituted Azidotetrazoles as Energetic Compounds

Base‐catalyzed activation of the C? F bond in the trifluoromethylazo‐substituted cyclic and acyclic alkanes provides a route to disubstituted azidotetrazoles. For example, the reaction of 1,2‐bis(trifluoromethylazo)ethane with four equivalents of NaN₃ gave the alkyl‐bridged bis(5‐azido‐1H‐tetrazol‐1‐yl)‐1,2‐dimine, N,N′‐bis(5‐azido‐1H‐tetrazol‐1‐yl)‐1,2‐diiminoethane, in 75 % yield (see scheme).

     

Synthesis of Five-Membered N,N,N- and N,N,N,N-Heterocyclic Compounds: Applications of Microwaves

The development of new strategies for the synthesis of small-sized heterocycles has remained a highly attractive but challenging proposition. An overview of the application of microwave irradiation to the synthesis of five-membered heterocyclic compounds containing three and four nitrogen atoms is presented, focusing on the developments in the past 5–10 years. This contribution covers the literature concerning the total synthesis of N,N,N- and N,N,N,N-heterocycles. The literature data are summarized based on the size and type of cycles.     

Saturated Heavier Group 14 Compounds as ‐Electron‐Acceptor (Z‐Type) Ligands

This review article describes the chemistry of transition‐metal complexes containing heavier group 14 elements (Si, Ge, and Sn) as the σ‐electron‐acceptor (Z‐type) ligands and discusses the characteristics of bonds between the transition metal and Z‐type ligand. Moreover, we review the iridium hydride mediated cleavage of E–X bonds (E=Si, Ge; X=F, Cl), where the key intermediates are pentacoordinate silicon or germanium compounds bearing a dative M→E bond.