New types of products obtained by combining two dinuclear multiply metal-metal bonded species (doublets) to obtain cyclic tetranuclear products (quartets) are reported. One new type consists of pairs of quadruply bonded Mo 2 4+ units united by two µ-X links (X ) H, OH, O) although in the last case oxidation has also occurred so that these are Mo 2 5+ moieties. In these cases, the subunits are Mo 2 (DArF) 3 . The second new type consists of Mo 2 (DArF) 2 subunits linked by four µ-X bridges, where X ) Cl, Br, I. The previously known compound Mo 4 I 8 (PBu n 3 ) 4 has been shown to differ from all previously known Mo 4 X 8 (PR 3 ) 4 molecules (X ) Cl, Br) by retaining two quadruple bonds instead of being a metallacyclobutadiyne, as are the others.

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1,7-(SMe2)2B12H10, 1,12-(SMe2)2B12H10, and [SMe3][B12H11(SMe2)] were prepared and isolated from the self-condensation reaction of BH3·SMe2 in the absence of a solvent. At 150 °C the reaction yields, primarily, two isomers:  1,7-(SMe2)2B12H10, the major component, and 1,12-(SMe2)2B12H10 the minor component, that are separated by thin layer chromatography. Single-crystal X-ray structure determinations were performed for both isomers, confirming the structures inferred from 1H, 11B, and 11B{1H}-11B{1H} (COSY) NMR spectra. Crystal data for 1,7-(SMe2)2B12H10:  trigonal P3221, a = 12.901(10) Å, b = 12.901(10) Å, c = 23.73(2) Å, γ = 120°, Z = 9. Crystal data for 1,12-(SMe2)2B12H10:  orthorhombic Pbca, a = 10.101(2) Å, b = 11.220(2) Å, c = 13.464(2) Å, Z = 4. At 60 °C, the self-condensation of BH3·SMe2 is very slow, but yields [SMe3][B12H11(SMe2)] as the major product. Multinuclear and 2-dimensional NMR spectra are in full accord with the structure determined by X-ray methods. Crystal data for [SMe3][B12H11(SMe2)]·MeCN:  monoclinic P21, a = 8.904(10) Å, b = 9.08(2) Å, c = 12.39(3)Å, β = 93.82°, Z = 2.

The anion [H2B2(μ-H)(μ-C4H8)2]- reacts with zirconocene dichloride or hafnocene dichloride to form (η5-C5H5)2ZrCl{(μ-H)2BC4H8} or (η5-C5H5)2HfCl{(μ-H)2BC4H8}, complexes which contain a borocyclopentane ring that is bound to the metal through hydrogen bridges. In these reactions the organodiborate anion disproportionates to form 1/2 mol of B2(μ-H)2(μ-C4H8)2 as well as the 5-membered borocyclopentane ring.

Two tetranuclear complexes of the type [Mo2(DArF)3]2(μ-H)2, DArF = N,N‘-diarylformamidinate, Ar = p-tolyl (1) and p-anisyl (2), have been prepared by reaction of Mo2(DArF)3Cl2 and NaHBEt3. The cores are similar and show two quadruply bonded dimolybdenum units joined by two hydrogen atoms. These are the first structurally characterized tetranuclear molecules with localized quadruple bonds and also the first molecules with two independent M−(μ-H)−M units, in which each metal atom is also bonded to another M atom. It is remarkable that in each case the compound resists hydrolysis by water but will react with HCl. The δ → δ* transitions, which are normally the lowest energy bands in the visible region of the electronic absorption spectra, appear as shoulders at 449 and 444 nm for 1 and 2. However, each compound also shows two more transitions at lower energy (ca. 505 and 728 nm). These bands have been assigned to transitions involving the three center−two electron Mo−H−Mo bridges.

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The anion [H2B2(μ-H)(μ-C4H8)2]-, II, reacts with zirconocene dichloride and with hafnocene dichloride to form (η5-C5H5)2ZrCl{(μ-H)2BC4H8}, III, and (η5-C5H5)2HfCl{(μ-H)2BC4H8}, IV, complexes, which contain a boracyclopentane ring that is bound to the metal through hydrogen bridges. In these reactions, 0.5 mol of the transannular hydrogen-bridged 10-membered ring compound B2(μ-H)2(μ-C4H8)2 is also formed. A possible reaction pathway for the formation of these complexes is proposed on the basis of time-lapse 11B NMR spectroscopy. The methyl-substituted complex (η5-C5H5)2Zr(CH3){(μ-H)2BC4H8}, V, is prepared from the reaction of III with CH3Li. The molecular structure of III was reported earlier. The structures of IV and V are reported here. Crystal data for IV:  space group P21/n (No. 14), a = 8.565(1) Å, b = 14.786(3) Å, c = 11.120(3) Å, β = 96.17(2)°, Z = 4. Crystal data for V:  space group Pbca (No. 61), a = 8.436(1) Å, b = 16.343(2) Å, c = 20.827(2) Å, Z = 8.

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