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.     

How big are atoms in crystals?

Atoms and bonds are central concepts in structural chemistry, but neither are concepts that arise naturally from the physics of condensed phases. It is ironic that the internuclear distances in crystals that are readily measured depend on the sizes of atoms, but since atoms in crystals can be defined in many different ways, all of them arbitrary and often incompatible, there is no natural way to express atomic size. I propose a simple coherent picture of Atoms-in-Crystals which combines properties selected from three different physically sound definitions of atoms and bonds. The charge density of the free atom that is used to construct the procrystal is represented by a sphere of constant charge density having the quantum theory of atoms in molecules (QTAIM) bonded radius. The sum of these radii is equal to the bond length that correlates with the bond flux (bond valence) in the flux theory of the bond. The use of this model is illustrated by answering the question: How big are atoms in crystals? The QTAIM bonded radii are shown to be simple functions of two properties, the number of quantum shells in the atomic core and the flux of the bond that links neighbouring atoms. Various radii can be defined. The univalent bonded radius measures the intrinsic size of the atom and is the same for all cations in a given row of the periodic table, but the observed bonded radius depends also on the bond flux that reflects the chemical environment.     

An improved set of mndo parameters for sulfur

The MNDO parameters for sulfur have been reoptimized. Calculations for a number of sulfur compounds indicate a very significant improvement. Inclusion of d AOs failed to correct the errors for compounds of sulfur in its higher valence states. Since d AOs are not included, the calculations are still confined to compounds of divalent sulfur.     

Recommended Questions on the Road towards a Scientific Explanation of the Periodic System of Chemical Elements with the Help of the Concepts of Quantum Physics

Periodic tables (PTs) are the ‘ultimate paper tools’ of general and inorganic chemistry. There are three fields of open questions concerning the relation between PTs and physics: (i) the relation between the chemical facts and the concept of a periodic system (PS) of chemical elements (CEs) as represented by PTs; (ii) the internal structure of the PS; (iii)␣The relation between the PS and atomistic quantum chemistry. The main open questions refer to (i). The fuzziness of the concepts of chemical properties and of chemical similarities of the CE and their compounds guarantees the autonomy of chemistry. We distinguish between CEs, Elemental Stuffs and Elemental Atoms. We comment on the basic properties of the basic elements. Concerning (ii), two sharp physical numbers (nuclear charge and number of valence electrons) and two coarse fuzzy ranges (ranges of energies and of spatial extensions of the atomic orbitals, AOs) characterize the atoms of the CEs and determine the two-dimensional structure of the PS. Concerning (iii), some relevant ‘facts’ about and from quantum chemistry are reviewed and compared with common ‘textbook facts’. What counts in chemistry is the whole set of nondiffuse orbitals in low-energy average configurations of chemically bonded atoms. Decisive for the periodicity are the energy gaps between the core and valence shells. Diffuse Rydberg orbitals and minute spin–orbit splittings are important in spectroscopy and for philosophers, but less so in chemical science and for the PS.     

Comment on“direct consequences of the bond index statistical interpretation”

It is shown that half the valence of an atom, in MO theory, is equal to the fluctuation of its atomic charge, and that the softness of an atom in a molecule is linearly related to the valence. In consequence the hardness of the atoms in the first row of the periodic table increases symmetrically from carbon to lithium and to fluorine.     

A personal reminiscence about theories used and misused in structural chemistry

In the last half-century, chemists acquired enough information about the structures and force fields of molecules to formulate what appeared to be successful theories (better called models) of molecular properties. For various reasons, chemists relied particularly heavily on studies of small hydrogen-containing compounds to shape their thinking. As discussed in the following text, this led to some inappropriate generalizations. In my first studies of molecular structure by gas-phase electron diffraction (GED), I was fortunate to find contradictions to the theories proposed at the time. What follows are reminiscences of how that came about and what happened subsequently. The main lesson to be earned, at least for molecules with central atoms that are not extremely large, is that the principal forces governing molecular structure are neither hybridization nor even the popular valence shell electron pair repulsions, explanations still taught routinely in chemistry courses but, instead, are the forces between geminal atoms resulting in the close-packing of ligands.     

Thermodynamic Properties of the Isomers of [HNOS], [HNO2S], and [HNOS2] and the Role of the Central Sulfur

Simple hydrides of compounds containing N, S, and O are of significant interest due to the role that they play in atmospheric chemistry and in biological pathways. There is a lack of quantitative thermodynamic data on these compounds. We have used a reliable computational chemistry approach based on valence CCSD(T) calculations extrapolated to the complete basis set limit with additional corrections to predict the heats of formation and bond dissociation energies of such compounds. The results show that compounds with the ability of the central S atom to effectively expand its valency leads to more stable isomers and, as a consequence, that those with the NSO structural motif are thermochemically more stable than those with the SNO motif. In addition, S?O bonds are preferred over N?O bonds.     

From the Metal to the Molecule-Ternary Bismuth Subhalides

Subvalent compounds, that is, metal-rich substances in which the average oxidation state of the cation is smaller than would be expected from the (8-N) rule, have proved to be a rich source of unexpected structural and physical features. The extraordinary structural chemistry generally observed in subvalent compounds is a consequence of the low and often non-integer oxidation states of the metal atoms coupled with the low concentration of valence electrons. Both factors can lead to a wide-range of bonding types within the same compound. A characteristic of these compounds is the interplay between "metallic" regions, with delocalized electrons and mainly nonpolar bonds between the metal atoms, and "saltlike" regions, which are characterized by strong localization of the electrons and heteropolar exchange between the metal and nonmetal atoms. The volumes of the different structural regions as well as the extent to which they interpenetrate can vary from compound to compound. The ternary subhalides of bismuth belong to a new class of substances which cover the whole spectrum from partially oxidized "porous" metals, through one- and two-dimensional metals, up to semiconducting ionic or molecular cluster compounds. These subvalent compounds with their unusually high chemical stabilities provide excellent vehicles for further research and their potential is described in the following article.     

The RhX n (X = S,Se, Te) Coordination Polyhedra in Crystal Structures

The Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres were applied to crystal-chemical analysis of all known compounds whose structures contain rhodium atoms surrounded by chalcogen atoms. The influence of the rhodium valence state and the nature of the chalcogen on the main features of Rh stereochemistry are discussed. Rhodium atoms exhibit coordination numbers of 6, 5, or 4 with respect to S, Se, or Te atoms; in addition to the bonds with chalcogens, rhodium can form 1 to 4 bonds with metal atoms. The VDP volume for Rh(III), Rh(2.67), and Rh(II) atoms in selenides and tellurides very weakly depends on the valence state, whereas in the case of sulfides, the volume increases rather regularly with a decrease in the metal oxidation number from Rh(III) to Rh(I).     

Cyclic and linear analogues of N2O. Theoretical evaluation of bonding properties

Various ABC (A, B=N, P; C=O, S) structures with 16 valence clectrons in a linear and cyclic geometry, together with the corresponding transition states for interconversion, are evaluated by means of quantum chemical calculations at an ab initio double-ζ level and with inclusion of electron correlation treatment. The stability of cyclic structures comes to the fore with increasing substitution of the ABC systems by third row elements. Hence in contrast to N₂O its higher element congener P₂S is more stable in a cyclic structure rather than a linear arrangement of atoms. In the linear ABC systems where either A or B is constituted from the third row of the periodic table of elements, the heavier element is always terminal rather than than central. This effect and the preference for cyclic rather than linear structures is the consequence oforbital nonhybridization as outlined in a previous publication.     

过渡金属重卡拜的合成、结构与反应性

过渡金属硅卡拜、锗卡拜、锡卡拜和铅卡拜是过渡金属分别与硅、锗、锡和铅之间形成三键的化合物。自从1996年首次发现钼锗卡拜以来,相继得到了一系列过渡金属锗卡拜、锡卡拜和铅卡拜化合物,而首例过渡金属硅卡拜则于2010年得以成功制备,标志着过渡金属与第14族元素之间形成三键的空白被全部填满。这些化合物的出现大大丰富和完善了过渡金属主族元素化学在成键、结构和反应方面的内容,引起化学界的重视。本文结合这方面最新的实验研究进展,系统全面总结了这些化合物的合成、结构和反应性,并对该领域的发展前景作了讨论。     

Intermetallic compounds of the heaviest elements and their homologs: the electronic structure and bonding of MM', where M=Ge, Sn, Pb, and element 114, and M'=Ni, Pd, Pt, Cu, Ag, Au, Sn, Pb, and element 114

Fully relativistic (four-component) density-functional theory calculations were performed for intermetallic dimers MM', where M=Ge, Sn, Pb, and element 114, and MM'=group 10 elements (Ni, Pd, and Pt) and group 11 elements (Cu, Ag, and Au). PbM and 114M, where M are group 14 elements, were also considered. The results have shown that trends in spectroscopic properties-atomization energies D(e), vibrational frequencies omega(e), and bond lengths R(e), as a function of MM', are similar for compounds of Ge, Sn, Pb, and element 114, except for D(e) of PbNi and 114Ni. They were shown to be determined by trends in the energies and space distribution of the valence ns(MM')atomic orbitals (AOs). According to the results, element 114 should form the weakest bonding with Ni and Ag, while the strongest with Pt due to the largest involvement of the 5d(Pt) AOs. In turn, trends in the spectroscopic properties of MM' as a function of M were shown to be determined by the behavior of the np(1/2)(M) AOs. Overall, D(e) of the element 114 dimers are about 1 eV smaller and R(e) are about 0.2 a.u. larger than those of the corresponding Pb compounds. Such a decrease in bonding of the element 114 dimers is caused by the large SO splitting of the 7p orbitals and a decreasing contribution of the relativistically stabilized 7p(1/2)(114) AO. On the basis of the calculated D(e) for the dimers, adsorption enthalpies of element 114 on the corresponding metal surfaces were estimated: They were shown to be about 100-150 kJ/mol smaller than those of Pb.     

Metalloid group 14 cluster compounds: an introduction and perspectives to this novel group of cluster compounds

Metalloid cluster compounds of group 14 of the general formulae E(n)R(m) with n > m (E = Si, Ge, Sn and Pb, tetrel elements; R = ligand), where "naked" tetrel atoms are present as well as ligand-bound tetrel atoms, represent a novel class of cluster compounds in group 14 chemistry. Since the "naked" tetrel atoms in these clusters exhibit an oxidation state of 0, the average oxidation state of the tetrel atoms in such metalloid group 14 cluster compounds is between 0 and 1. Thus, these cluster compounds may be seen as intermediates on the way to the elemental state. Therefore, interesting properties maybe expected for these compounds which might complement results from nanotechnology. During the last years many different syntheses of such novel cluster compounds have been introduced, leading to several metalloid group 14 cluster compounds which exhibit new and unusual structure and bonding properties. In this tutorial review an account is given of the first steps in this novel field of group 14 chemistry. Special attention is focused on structural features and bonding properties.     

Atoms and bonds in molecules and chemical explanations

The concepts of atoms and bonds in molecules which appeared in chemistry during the nineteenth century are unavoidable to explain the structure and the reactivity of the matter at a chemical level of understanding. Although they can be criticized from a strict reductionist point of view, because neither atoms nor bonds are observable in the sense of quantum mechanics, the topological and statistical interpretative approaches of quantum chemistry (quantum theory of atoms in molecules, electron localization function and maximum probability domain) provide consistent definitions which accommodate chemistry and quantum mechanics.     

Substitution effect on the geometry and electronic structure of the ferrocene

The substitution effects on the geometry and the electronic structure of the ferrocene are systematically and comparatively studied using the density functional theory. It is found that -NH(2) and -OH substituents exert different influence on the geometry from -CH(3), -SiH(3), -PH(2), and -SH substituents. The topological analysis shows that all the C-C bonds in a-g are typical opened-shell interactions while the Fe-C bonds are typical closed-shell interactions. NBO analysis indicates that the cooperated interaction of d --> pi* and feedback pi --> d + 4s enhances the Fe-ligand interaction. The energy partitioning analysis demonstrates that the substituents with the second row elements lead to stronger iron-ligand interactions than those with the third row elements. The molecular electrostatic potential predicts that the electrophiles are expected to attack preferably the N, O, P, or S atoms in Fer-NH(2), Fer-OH, Fer-PH(2), and Fer-SH, and attack the ring C atoms in Fer-SiH(3) and Fer-CH(3). In turn, the nucleophiles are supposed to interact predominantly by attacking the hydrogen atoms. The simulated theoretical excitation spectra show that the maximum absorption peaks are red-shifted when the substituents going from second row elements to the third row elements.     

Heterodinuclear transition-metal complexes with multiple metal-metal bonds

Interactions between a pair of transition-metals can range from weak antiferromagnetic coupling to bonds of the highest multiplicity known in chemistry, for example, quadruple in isolatable compounds. Tremendous effort has been invested in studying homodinuclear transition-metal-metal bonds. In contrast, relatively little attention has been devoted to heterodinuclear analogues, as it is substantially more challenging to prepare and handle such entities. Yet, in this largely unexplored area of transition-metal chemistry, novel chemical interactions with unprecedented reactivities are likely to be found. Heterodinuclear analogues of diatomic transition-metal dimers being yet inaccessible, dinuclear complexes with Werner-type ligands provide examples of high-multiplicity bonds between different d elements in their least-perturbed form. Such compounds provide an opportunity to probe fundamental issues of chemical bonding between transition-metals, by revealing how and to what extent such bonds are affected by differences in the two metals. Complexes wherein electronically unsaturated heterodinuclear cores are stabilized by pi-acidic ligands (such as CO) hold the potential of new chemical reactions (including catalytic) that capitalize on the synergetic effect of two transition-metal centers.     

Density functional theoretical study of a series of binary azides M(N3)n (n = 3, 4)

Geometrical structures of a series of binary azides M(N3)n (M = elements in groups 3 and 13 (n = 3) and in groups 4 and 14 (n = 4)) were investigated at the B3LYP/6-311+G level of theory. Our calculations found that binary group 3 triazides M(N3)3 (M = Sc, Y, La) and binary group 4 tetraazides M(N3)4 (M = Ti, Zr, Hf) turn out to be stable with all frequencies real having a similar linear M-N-NN structural feature, as previously reported for M(N3)4 (M = Ti, Zr, Hf). However, binary azides of group 13 M(N3)3 (M = B, Al, Ga, In, Tl) and group 14 elements M(N3)4 (C, Si, Ge, Sn, Pb) with bent M-N-NN bond angles differ obviously from binary group 3 and 4 azides in geometrical structure. These facts are mainly explained by the difference in electronic density overlap between the central atom and the alpha-N atoms of the azido groups. Two lone-pair electrons on the sp hybridization alpha-N atoms in the binary group 3 and 4 azides donate electron density into two empty d orbitals of the central transition metal atom and a pair of valence bonding electrons, resulting in the alpha-N atoms acting as a tridentate ligand. The sp2 hybridization alpha-N atoms of the binary group 13 and 14 azides only give one valence electron to form one valence bonding electron pair acting virtually as monodentate donors.