The chemistry of heavier main-group elements such as aluminium, silicon and phosphorus is very different from that of the lighter ones such as boron, carbon and nitrogen, yet discussions of this topic have been dominated by comparisons with the light elements. Philip Power's review focuses on advances in chemistry of the heavier main-group elements that reveal them as having more in common with the transition metals than the lighter members of the main groups. The concept of heavier main-group elements as 'transition metals' is supported by recent work showing that many of the new compounds react with small molecules such as H2, NH3, C2H4 and CO under mild conditions and display potential as catalysts. The last quarter of the twentieth century and the beginning decade of the twenty-first witnessed spectacular discoveries in the chemistry of the heavier main-group elements. The new compounds that were synthesized highlighted the fundamental differences between their electronic properties and those of the lighter elements to a degree that was not previously apparent. This has led to new structural and bonding insights as well as a gradually increasing realization that the chemistry of the heavier main-group elements more resembles that of transition-metal complexes than that of their lighter main-group congeners. The similarity is underlined by recent work, which has shown that many of the new compounds react with small molecules such as H2, NH3, C2H4 or CO under mild conditions and display potential for applications in catalysis.

Main-group elements as transition metals

π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements

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

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Theoretical Studies of Some Transition-Metal-Mediated Reactions of Industrial and Synthetic Importance.

Recent topics in the chemistry of heavier congeners of carbenes

X-Ray structures of the isomorphous centrosymmetric crystalline Ge2R4 and Sn2R4[R = CH(SiMe3)2] reveal a trans-folded C2h M2C4 framework, with a fold angle θ of 32° for M = Ge and 41° for M = Sn, but no twist of the MC2 planes about the M–M axis. The M–M distance [2.347(2)A for Ge and 2.768(1)A for Sn] is slightly shorter (4% for M = Ge, 1.5% for M = Sn) than in the tetrahedral element, M∞. The conformation of each MR2 moiety in M2R4 approximates to planar syn,anti(cf. Ca. syn,syn in gaseous MR2), and the four ligands R– are oriented in a ‘paddle-wheel’ fashion. There is consequently an asymmetry in the M–C bonding in M2R4 as shown (data for M = Sn in square brackets) by variations in M–C and M–C′(A), 1.979(9) and 2.042(8)[2.207(5) and 2.225(6)]; M′MC and M′MC′(°),113.7(3) and 122.3(2)[112.0(1) and 119.4(1)°]; individual MCSi angles (°), 110.0(4)[110.2(3)], 113.9(4)[109.3(3)], 119.1(4)[119.1(2)], and 121.8(4)[118.9(2)]. The average M–C bond lengths are comparable but the CMC angles are wider in M2R4 than those previously found for gaseous MR2. Ab initio molecular orbital calculations with better than double zeta basis on the model compounds M2H4 show that (i)trans-folded equilibrium structures are more stable than planar by 13 kJ mol–1(θ= 40°) for M = Ge and 26 kJ mol–1(θ= 46°) for Sn; (ii) the M–M bond distance is 2.30 A for Ge and 2.71 A for Sn; and (iii) the M–M dissociation energy is 130 kJ mol–1 for Ge and 90 kJ mol–1 for Sn. These energies are about half the experimental M–M single-bond dissociation energies of H3GeGeH3 or Me3MMMe3. The decreasing strength of M–M bonding in the series M2R′4[M = C, Si, Ge, or Sn; R′= R, Ph, C6H2Me3-2,4,6, or C6H3Et2-2,6] or, more generally, M2X4(X = R′ or H) with increasing atomic number of M, as well as the increasing stability of the trans-folded relative to planar structures, is attributed to the increasing inertness of the electron lone pair in the MX2(X = R′ or H) monomer, which in turn is reflected in an increasing singles → triplet excitation energy.

Subvalent Group 4B metal alkyls and amides. Part 9. Germanium and tin alkene analogues, the dimetallenes M2R4[M = Ge or Sn, R = CH(SiMe3)2]: X-ray structures, molecular orbital calculations for M2H4, and trends in the series M2R′4[M = C, Si, Ge, or Sn; R′= R, Ph, C6H2Me3-2,4,6, or C6H3Et2-2,6]

Subvalent Group 4B Metal Alkyls and Amides. Part 9. Germanium and Tin Alkene Analogues, the Dimetallenes M2R4 (M: Ge or Sn, R: CH(SiMe3)2): X-Ray Structures, Molecular Orbital Calculations for M2H4, and Trends in the Series M2R′4 (M:

Aralkylpolyamine complexes-III: Some thiocyanate and selenocyanate complexes of copper(II), nickel(II) and cobalt(II) with n-benzylethylenediamine and N,N′-dibenzylethylenediamine

Effect of β-diketone structure on the synergistic extraction of uranyl ion by tributylphosphate

Further studies of the rate affected synergic solvent extraction of Fe(III)-TTA—I: The effect of tri-n-octylphosphine oxide on the solvent extraction of iron(III) with 2-thenoyltrifluoroacetone

David E. Goldberg

Papers

Subvalent Group 4B metal alkyls and amides. Part 9. Germanium and tin alkene analogues, the dimetallenes M2R4[M = Ge or Sn, R = CH(SiMe3)2]: X-ray structures, molecular orbital calculations for M2H4, and trends in the series M2R′4[M = C, Si, Ge, or Sn; R′= R, Ph, C6H2Me3-2,4,6, or C6H3Et2-2,6]

Subvalent Group 4B Metal Alkyls and Amides. Part 9. Germanium and Tin Alkene Analogues, the Dimetallenes M2R4 (M: Ge or Sn, R: CH(SiMe3)2): X-Ray Structures, Molecular Orbital Calculations for M2H4, and Trends in the Series M2R′4 (M:

Aralkylpolyamine complexes-III: Some thiocyanate and selenocyanate complexes of copper(II), nickel(II) and cobalt(II) with n-benzylethylenediamine and N,N′-dibenzylethylenediamine

Effect of β-diketone structure on the synergistic extraction of uranyl ion by tributylphosphate

Further studies of the rate affected synergic solvent extraction of Fe(III)-TTA—I: The effect of tri-n-octylphosphine oxide on the solvent extraction of iron(III) with 2-thenoyltrifluoroacetone