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 H(2), NH(3), C(2)H(4) or CO under mild conditions and display potential for applications in catalysis.

ContentsI. Introduction 3463 II. Multiple Bonding Models 3466 III. Lone Pair Character versus Multiple Bonding 3468 IV. Doubly Bonded Compounds 3469 A. Compounds of Formula RE)ER (E ) Group 13 Element) 3469 B. Compounds of Formula RE)E′R 2 (E ) Group 13, E′ ) Group 14 Element) 3470 C. Dianions of Formula [R 2 E)ER 2 ] 2-(E ) Group 13 Element) 3470 D. Monoanions of Formula [R 2 E)E′R 2 ] -(E ) Group 13; E′ ) Group 14 Element) 3472 E. Compounds of Formula R 2 E−E ¨′R 2 (E ) Group 13; E′ ) Group 15 Element) 3472 F. Compounds of Formula R 2 E−E′R and [R 2 E−E′] -(E ) Group 13, E′ ) Group 16 Element) 3475 G. Compounds of Formula R 2 E)ER 2 and [RE ¨)E ¨R] 2-(E ) Group 14 Element) 3477 H. Compounds of Formula R 2 E)E ¨′R (E ) Group 14, E′ ) Group 15 Element) 3481 I. Compounds of Formula [R 2 E)E′R 2 ] + (E ) Group 14; E′ ) Group 15 Element) 3484 J. Compounds of Formula R 2 E)E′: (E ) Group 14; E′ ) Group 16 Element) 3485 K. Compounds of Formula RE ¨)E ¨R (E or E′ ) N, P, As, Sb, or Bi) 3486 L. Compounds of Formula RE)E′ (E ) Group 15, E′ ) Group 16 Element) 3489 V. Compounds with Formal Bond Order >2 and Triply Bonded Compounds 3490 A. Range of Compounds 3490 B. Compounds with Potential Triple Bonding to a Group 13 Element 3491 C. Compounds with Potential Triple Bonding to a Group 14 Element 3493 D. Compounds with Triple Bonding between Group 14 and 15 Elements 3494 E. Compounds with Triple Bonding between Group 15 Elements 3495 VI. Conclusions 3495 VII. Acknowledgments 3496 VIII. References 3496Philip Power received his B.A. degree from Trinity College Dublin in 1974 and his D.Phil. degree, under the supervision of M. F. Lappert, from the University of Sussex in 1977. After postdoctoral studies with R. H. Holm at Stanford, he joined the faculty at the University of California, Davis, where he is currently Professor of Chemistry. His main research interests involve the structural chemistry of organoalkali metal and organocopper compounds, low-coordinate transition metal chemistry, multiple bonding in main group chemistry, and the development of new ligands for the stabilization of low coordination numbers, unusual oxidation states, and multiple bonding in both transition metal and heavier main group compounds. He is a recipient of fellowships from the A. P. Sloan and Alexander von Humboldt foundations. In addition he has been Reilly Lecturer at the University of Notre Dame (1995), Faculty Research Lecturer at the University of Iowa (1993), and the

Contents 1. Introduction 3877 2. Bonding 3877 3. Doubly Bonded Compounds 3882 3.1. Compounds of Formula REdER (E ) Group 13 Element) 3882 3.2. Compounds of Formula REdE′R 2 (E Group 13, E′ ) Group 14 Element) and Related Species 3884 3.3. Dianions of Formula [R 2 EdER 2 ] 2-(E ) Group 13 Element) 3884 3.4. Monoanions of Formula [R 2 EdE′R′ 2 ] -(E ) Group 13, E′ ) Group 14 Element) 3885 3.5. Compounds of Formula R 2 E-E ¨′R 2 (E ) Group 13, E′ ) Group 15 Element) and Related Species 3886 3.6. Compounds of Formula R 2 E-E′R and [R 2 E-E′] -(E ) Group 13, E′ ) Group 16 Element) 3888 3.7. Compounds of Formula R 2 EdER 2 , [L:E ¨dE ¨:L] and [RE ¨dE ¨R] 2-(E ) Group 14 Element) 3888 3.8. Compounds of Formula R 2 EdE ¨′R (E ) Group 14, E′ ) Group 15 Element) 3896 3.9. Compounds of Formula [R 2 EdE ¨′R 2 ] + (E ) Group 14, E′ ) Group 15 Element) 3900 3.10. Compounds of Formula R 2 EdE ¨′ (E ) Group 14, E′ ) Group 16 Element) 3901 3.11. Compounds of Formula RE ¨)E ¨R, (E or E′ ) N, P, As, Sb, or Bi) 3902 3.12. Compounds of Formula RE ¨dE′ (E ) Group 15, E′ ) Group 16 Element) 3908 4. Triply Bonded Compounds 3908 4.1. Group 13 Derivatives 3908 4.2. Group 14 Derivatives 3909 4.3. Compounds with Potential Triple Bonding between Group 14 and Group 15 Elements 3912 4.4. Compounds with Potential Triple Bonding between Group 14 and Group 16 Elements 3913 4.5. Compounds with Triple Bonding between Group 15 Elements 3913 5. Conclusions 3914 6. List of Abbreviations 3914 7. Acknowledgments 3915 8. References 3915

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Although in principle transition metals can form bonds with six shared electron pairs, only quadruply bonded compounds can be isolated as stable species at room temperature. Here we show that the reduction of {Cr(mu-Cl)Ar'}2 [where Ar' indicates C6H3-2,6(C6H3-2,6-Pri2)2 and Pr indicates isopropyl] with a slight excess of potassium graphite has produced a stable compound with fivefold chromium-chromium (Cr-Cr) bonding. The very air- and moisture-sensitive dark red crystals of Ar'CrCrAr' were isolated with greater than 40% yield. X-ray diffraction revealed a Cr-Cr bond length of 1.8351(4) angstroms (where the number in parentheses indicates the standard deviation) and a planar transbent core geometry. These data, the structure's temperature-independent paramagnetism, and computational studies support the sharing of five electron pairs in five bonding molecular orbitals between two 3d5 chromium(I) ions.

The germanium alkyne analogue Ar'GeGeAr' (1, Ar' = C6H3-2,6(C6H3-2,6-Pri2)2) reacts with 1, 2, or 3 equiv of dihydrogen at room temperature, and at 1 atm pressure, to afford a mixture of the products Ar'HGeGeHAr' (2), Ar'H2GeGeH2Ar' (3), or Ar'GeH3 (4). The relative amounts of each product are governed by the number of equivalents of hydrogen used. A mechanism for the initial step in the reaction is proposed. The appearance of 4 among the reaction products was accounted for in terms of either its dissociation to monomers or isomerization to the bridged Ar'Ge(mu-H)2GeAr'. The reactions were monitored by 1H NMR spectroscopy. The products 2, 3, and 4 were characterized by X-ray crystallography, and 4 was synthesized independently by the reduction of Ar'Ge(OMe)3. These reactions represent the first direct addition of hydrogen to a closed shell unsaturated main group compound under ambient conditions.

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