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The chemistry of the Group 13 metals is dominated by the +1 and +3 oxidation states, and simple monomeric M(II) species are typically short-lived, highly reactive species. Here we report the first thermally robust monomeric MX2 radicals of gallium, indium and thallium. By making use of sterically demanding boryl substituents, compounds of the type M(II)(boryl)2 (M = Ga, In, Tl) can be synthesized. These decompose above 130 °C and are amenable to structural characterization in the solid state by X-ray crystallography. Electron paramagnetic resonance and computational studies reveal a dominant metal-centred character for all three radicals (>70% spin density at the metal). M(II) species have been invoked as key short-lived intermediates in well-known electron-transfer processes; consistently, the chemical behaviour of these novel isolated species reveals facile one-electron shuttling processes at the metal centre.

The reactivity of cobalt dibromide with the bulky N -heterocyclic carbenes IMes and IPr has been explored [IMes = N , N ′- bis (2,4,6-trimethylphenyl)imidazol-2-ylidene; IPr = N , N ′- bis (2,6-diisopropylphenyl)imidazol-2-ylidene], with the aim of generating synthetically versatile complexes of the types (NHC)CoBr 2 and (NHC) 2 CoBr 2 . Differences between the two carbene systems illustrate the key role of ligand sterics in this synthetic approach, while the importance of donor strength is emphasized by comparative studies with the related sterically encumbered pyridine ligand 2,6-Mes 2 py. Thus, IMes and IPr are differentiated by the inability of the latter, bulkier, donor to form a 2:1 adduct, while 2,6-Mes 2 py gives rise to Co–N bonds only in the presence of a suitable halide abstraction agent.

The interaction of 2,6-dimesitylpyridine with Tl(I) and In(I) cations has been investigated with a view to developing tractable molecular M(I) compounds which are soluble in organic media. In stark contrast to isosteric and isoelectronic terphenyl systems, complexes featuring the [(2,6-Mes(2)py)M](+) fragment feature very weak metal-ligand interactions in the solid state, as revealed by M-N distances of the order of 2.45 Å (M = In) and 2.64 Å (M = Tl). While additional weak π interactions are observed with arene solvate molecules in these systems, the related 2:1 complex [(2,6-Mes(2)py)(2)In][BAr(f)(4)] features an In(I) center wholly encapsulated by the bulky Mes(2)py donors, and even longer In-N distances [2.586(6) and 2.662(5) Å]. These contacts are about 0.5 Å greater than the sum of the respective covalent radii (2.13 Å) and provide evidence for an effectively "naked" In(I) cation stabilized to a minor extent by orbital interactions.

The changes in the steric and electronic properties of N-heterocyclic carbenes (NHCs) as a function of ring size have a profound effect on the reactivity of their late transition metal complexes. Comparison of closely related complexes featuring either a saturated 5- or 6-membered NHC, reveals that the larger ring is associated with an increased propensity towards intramolecular C-H activation, but with markedly lower reactivity towards external substrates. Thus, systems of the type [IrL2(H)2](+) give rise to contrasting chemical behaviour, primarily reflecting the differing possibilities for secondary stabilization of the metal centre by the N-bound aryl substituents: highly labile [Ir(5-Mes)2(H)2](+) can only be studied by trapping experiments, while [Ir(6-Mes)2(H)2](+) is air and moisture stable, and unreactive towards many external reagents. With appropriate substrates, this heightened reactivity can be exploited, and in situ generated [Ir(5-Mes)2(H)2](+) is capable of intermolecular B-H and N-H activation chemistry. In the case of H3B·NMe2H, this affords a rare opportunity to study amine/aminoborane coordination via single crystal neutron diffraction methods.

The reactions of Me(2)NH·BH(3) with cationic Rh(III) and Ir(III) complexes have been shown to generate the 18-electron aminoborane adduct [Ir(IMes)(2)(H)(2){κ(2)-H(2)BNMe(2))](+) and the remarkable 14-electron aminoboryl complex [Rh(IMes)(2)(H)-{B(H)NMe(2))](+). Neutron diffraction studies have been used for the first time to define H-atom locations in metal complexes of this type formed under catalytic conditions.

Chloride abstraction from the group 9 metal bis(N-heterocyclic carbene) complexes M(NHC)(2)(H)(2)Cl [M = Rh, Ir; NHC = IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene or IMes = N,N'-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] leads to the formation of highly reactive cationic species capable of the dehydrogenation of saturated CC and BN linkages. Thus, the reaction of Ir(IPr)(2)(H)(2)Cl (1) with Na[BAr(f)(4)] in fluorobenzene generates [Ir(IPr)(2)(H)(2)](+)[BAr(f)(4)](-) (4) in which the iridium center is stabilized by a pair of agostic interactions utilizing the methyl groups of the isopropyl substituents. After a prolonged reaction period C-H activation occurs, ultimately leading to the dehydrogenation of one of the carbene (i)Pr substituents and the formation of [Ir(IPr)(IPr'')(H)(2)](+)[BAr(f)(4)](-) (5), featuring the mixed NHC/alkene donor IPr'' ligand. By contrast, the related IMes complexes M(IMes)(2)(H)(2)Cl (M = Rh, Ir), which feature carbene substituents lacking beta-hydrogens, react with Na[BAr(f)(4)] in fluorobenzene to give rare examples of NaCl inclusion compounds, viz., [M(IMes)(2)(H)(2)Cl(Na)](+)[BAr(f)(4)](-) (M = Rh, 6; M = Ir, 7). Intercalation of the sodium cation between the mesityl aromatic rings of the two NHC donors has been demonstrated by crystallographic studies of 7. Synthetically, 6 and 7 represent convenient yet highly reactive sources of the putative 14-electron [M(NHC)(2)(H)(2)](+) cations, readily eliminating NaCl in the presence of potential donors. Thus 7 can be employed in the synthesis of the dinitrogen complexes [Ir(IMes)(2)(N(2))(2)](+)[BAr(f)(4)](-) (8a) and [Ir(IMes)(2)(N(2))THF](+)[BAr(f)(4)](-) (8b) (albeit with additional loss of H(2)) by stirring in toluene under a dinitrogen atmosphere and recrystallization from the appropriate solvent system. The interactions of 6 and 7 with primary, secondary, and tertiary amineboranes have also been investigated. Although reaction with the latter class of reagent simply leads to coordination of the amineborane at the metal center via two M-H-B bridges {and formation, for example, of the 18-electron species [M(IMes)(2)(H)(2)(mu-H)(2)B(H).NMe(3)](+)[BAr(f)(4)](-) (M = Rh, 9; M = Ir, 10)}, the corresponding reactions with systems containing N-H bonds proceed via dehydrogenation of the BN moiety to give complexes containing unsaturated aminoborane ligands. Thus, for example, 6 catalyzes the dehydrogenation of R(2)NH x BH(3) (R = (i)Pr, Cy) in fluorobenzene solution (100% conversion over 6 h at 2 mol % loading) to give R(2)NBH(2); the organometallic complex isolated at the end of the catalytic run in each case is shown to be [Rh(IMes)(2)(H)(2)(mu-H)(2)BNR(2)](+)[BAr(f)(4)](-) (R = (i)Pr, 11; R = Cy, 12). In contrast to isoelectronic alkene donors, the aminoborane ligand in these complexes (and in the corresponding iridium compounds 13 and 14) can be shown by crystallographic methods to bind in end-on fashion via a bis(sigma-borane) motif. Similar dehydrogenation chemistry is applicable to the primary amineborane (t)BuNH(2) x BH(3), although in this case the rate of rhodium-catalyzed dehydrogenation is markedly slower. This enables the amineborane complex [Rh(IMes)(2)(H)(2)(mu-H)(2)B(H) x NH(2)(t)Bu](+)[BAr(f)(4)](-) (15) to be isolated at short reaction times (ca. 6 h) and the corresponding (dehydrogenated) aminoborane system [Rh(IMes)(2)(H)(2)(mu-H)(2)BNH(t)Bu](+)[BAr(f)(4)](-) (16) to be isolated after an extended period (ca. 48 h). As far as further reactivity is concerned, aminoborane systems such as 14 show themselves to be amenable to further dehydrogenation chemistry in the presence of tert-butylethylene leading ultimately to the dehydrogenation of the boron-containing ligand and to the formation of a directly Ir-B bonded system described by limiting boryl (Ir-B) and borylene (Ir=B) forms.

Side-on or end-on? Rhodium and iridium complexes featuring aminoboranes (R2N= BH2) as ligands have been synthesized and structurally characterized. Crystallographic measurements show that the ligands-in contrast to isoelectronic alkene donors-bind in an end-on fashion through a bis(o-borane) binding motif (see structure). (Figure Presented) © 2010 Wiley-VCH Verlag GmbH and Co. KGaA.

Lithium borohydride absorbs anhydrous ammonia to form four stable ammoniates; Li(NH(3))(n)BH(4), mono-, di-, tri-, and tertraammoniate. This paper focuses on the monoammoniate, Li(NH(3))BH(4), which is readily formed on exposure of LiBH(4) to ammonia at room temperature and pressure. Ammonia loss from Li(NH(3))BH(4) commences around 40 degrees C and the compound transforms directly to LiBH(4). The crystal structure of Li(NH(3))BH(4) is reported here for the first time. Its close structural relationship with LiBH(4) provides a clear insight into the facile nature and mechanism of ammonia uptake and loss. These materials not only represent an excellent high weight-percent ammonia system but are also potentially important hydrogen stores.

The reaction of [Ir(coe)2Cl]2 (coe = cyclooctene) with the N-heterocyclic carbene N,N′-bis(2,6diisopropylphenyl)imidazol-2- ylidene (IPr) in tetrahydrofuran under anaerobic conditions leads to the formation of two main products, the planar four-coordinate Ir(I) complex Ir(IPr)(IPr″)Cl (1) formed by dehydrogenation of one of the IPr isopropyl substituents (to give the mixed NHC/alkene donor IPr″) and the trigonal bipyramidal Ir(III) system Ir(IPr)(aIPr)(H)2Cl (6), which features both "normal" and "abnormal" C-bound isomers of the NHC ligand. Formation of 1 presumably proceeds via initial C-H activation at iridium(I); moreover, subsequent reactivity for 1 initiated by chloride abstraction suggests that C-H oxidative addition chemistry is facile for the methyl C-H bonds of the carbene isopropyl substituent. Thus, the square pyramidal Ir(III) alkene alkyl hydride [Ir(IPr′)(IPr′)H]+[BAr 4f]- (2) is formed on reaction with Na[BAr 4f] in fluorobenzene. In contrast to the Ir(I) system 1, the alignment of the alkene ligand in solid 2 is such that it lies coplanar with the IrC4 basal plane. Quantum chemical investigations imply that the energetic difference between this alkene orientation and an alternative perpendicular conformation is small (ca. 5 kcal mol-1), with steric factors (notably at the alkene 2-position) being important. Hydrogénation of 1 proceeds via an intermediate identified as Ir(IPr)(IPr″)(H) 2Cl to give the Ir(III) dihydride Ir(IPr)2(H) 2Cl (4), the structure of which can be compared with those of the tautomeric isomer 6 and the mixed IPr/IMes carbene complex Ir(IPr)(IMes)(H) 2Cl [IMes = N,N′-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]. Crystallographically determined bond lengths for the Ir-C linkages trans to the respective carbenes imply relatively similar σ-donor properties for the IPr, aIPr, and IMes ligands. © 2009 American Chemical Society.

10 páginas, 7 figuras, 4 tablas., The measured Raman and IR spectra of solid, polycrystalline bis(pentamethylcyclopentadienyl)dizinc, (η5-C5Me5)2Zn2, 1, and bis(pentamethylcyclopentadienyl)monozinc, (η5-C5Me5)(η1-C5Me5)Zn, 8, are reported in some detail. The IR spectra of the vapors of 1 and 8 each trapped in a solid Ar matrix at 12 K confirm the essentially molecular character of the solids. The experimental results have been interpreted with particular reference (i) to the corresponding spectra of 68Zn-enriched samples of the compounds, and (ii) to the spectra simulated by density functional theory (DFT) calculations at the B3LYP level. The marked differences of structure of 1 and 8 contrast with the relatively close similarity of their vibrational spectra, disparities being revealed only on detailed scrutiny, including the effects of 68Zn enrichment, and primarily at wavenumbers below 1000 cm−1. The Zn−Zn stretching motion of 1 features not as a single, well-defined mode identifiable with intense Raman scattering but in several normal modes which respond in varying degrees to 68Zn substitution. A stretching force constant of 1.42 mdyne Å−1 has been estimated for the Zn−Zn bond of 1., Financial support from the Spanish Ministerio de Educación y Ciencia (MEC) (project CTQ2007-62814 and Consolider-Ingenio 2010 CSD2007-00006) and the Junta de Andalucía (project FQM672).

The reaction of LiGaH4 with an excess of the hydrochloride of a primary amine RNH2 in ether solution yields in [(RH2N)2GaH2]Cl (R = Me, 1, or iPr, 2) additional examples of cationic gallane derivatives. The cation [(iPrH2N)2GaH2]+ reacts further with LiGaH4, also in ether solution, with H2 elimination and formation of the trigallium cationic derivative [{(iPrH2N)GaH2NHiPr}2GaH2]Cl, 3. Prolonged standing of [(iPrH2N)2GaH2]Cl in the presence of LiCl results in partial exchange of Ga–H by Ga–Cl bonds, with the isolation in low yield of crystals, 4, of [(iPrH2N)2GaH2]Cl in which about 7 % of the H atoms attached to Ga were replaced by Cl. All of the compounds 1–4 having been characterized crystallographically, the results are discussed in relation to the different reaction pathways open to [RNH3]Cl and LiMH4 (M = Al or Ga). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Whereas methylammonium chloride, [MeNH(3)]Cl, reacts with LiGaH(4) in an ether solution to give, according to the conditions, either the adduct MeH(2)N x GaH(3) or the cationic derivative [(MeH(2)N)(2)GaH(2)](+)Cl(-), the corresponding reaction of [MeNH(3)]Cl or [(t)BuNH(3)]Cl with LiAlH(4) proceeds mainly, with H(2) elimination, to the imidoalane Li(2)[(RN)(4)(AlH(2))(6)] (R = Me, 1, or (t)Bu, 2). The crystal structure of 1 x 2Et(2)O reveals, for the first time, anionic units with an adamantane-like Al(6)N(4) skeleton. The Li cations exist at two distinct sites, each linked via Li(mu-H)Al bridges to two [(MeN)(4)(AlH(2))(6)](2-) cages. Despite disordering of the tBu groups, the crystal structure of 2 evidently includes analogous anionic units. By contrast, the main product of the reaction between [(i)PrNH(3)]Cl and LiAlH(4) under similar conditions is the known neutral, hexameric imidoalane [(i)PrNAlH](6), 3, the crystal structure of which has been redetermined.

The crystal structures of piperidino-alane and -gallane at 150 K have each been shown to consist of dimeric molecules [CH2(CH2)4NMH2]2 centred on a planar, nearly square M(μ-N)2M core (M = Al or Ga). The molecular structures thus contrast with the hydrogen-bridged units favoured by the sterically encumbered piperidino derivatives {[CMe2(CH2)3CMe2N]AlH2}3 and {[CMe2(CH2)3CMe2N]2AlH}2 and are also compared with those of other amido derivatives of the Group 13 hydrides.

The structure of the gallane adduct Me3P·GaH3 in the vapour and crystalline states has been investigated. The gas-phase electron-diffraction (GED) pattern has been analysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rh1 structure) were found to be: r(Ga–H) 159.0(11), r(Ga–P) 244.3(6), r(P–C) 184.0(2), r(C–H) 108.3(7) pm; H–Ga–P 98.4(12) and Ga–P–C 117.7(3)°. The structure of a single crystal at 150 K shows that the adduct retains the same monomeric unit in the crystalline phase, with dimensions generally close to those of the gaseous molecule and an eclipsed conformation of the C3PGaH3 skeleton. The results are discussed and analysed in the light of quantum chemical calculations and of the properties of related adducts of Group 13 metal hydrides.

The molecular structures of Zr[(mu-H)(3)BH](4) and U[(mu-H)(3)BH](4) have been investigated by density functional theory (DFT) calculations and gas electron diffraction (GED). The triply bridged bonding mode of the tetrahydroborate groups in the former is confirmed, but both DFT calculations and GED structure refinements indicate that the BH(4) groups are rotated some 12 degrees away from the orientation in which the three bridging B-H bonds are staggered with respect to the opposing ZrB(3) fragment. As a result the symmetry of the equilibrium conformation is reduced from T(d) to T. Bond distances and valence angles are as follows (DFT/GED): Zr-B = 232.2/232.4(5) pm; Zr-H(b) = 214.8/214.4(6) pm; B-H(b) = 125.3/127.8(8) pm; B-H(t) = 119.4/118.8(17) pm; angle ZrBH(b) = 66.2/65.6(3) degrees; the smallest dihedral angle of type tau(BZrBH(b)) = 48/45(2) degrees. DFT calculations on Hf(BH(4))(4) indicate that the structure of this molecule is very similar to that of the Zr analogue. Matrix-isolation IR spectroscopy and DFT calculations on U(BH(4))(4) show that while the polymeric solid-state structure is characterized by terminal triply bridging and metal-metal bridging bidentate BH(4) groups, all BH(4) groups are triply bridging in the gaseous monomer. Calculations with one of the two nonbonding 5f electrons on U occupying an a(1) and the other distributed equally among the three t(2) orbitals indicate that the equilibrium conformation has T(d) symmetry, i.e. that the three B-H(b) bonds of each tetrahydroborate group are exactly staggered with respect to the opposing UB(3) fragment with tau(BUBH(b)) = 60 degrees. Calculations including spin-orbit interactions indicate that Jahn-Teller distortions from T(d) symmetry are either absent or very small. The best agreement between observed and calculated GED intensity data was obtained for a model of T(d) symmetry, but models of T symmetry with dihedral angles tau(BUBH(b)) > 42 degrees cannot be ruled out. Bond distances and valence angles are as follows (DFT/GED): U-B = 248.8/251.2(4) pm; U-H(b) = 227.7/231.5(6) pm; B-H(b) = 126.0/131.6(5) pm, B-H(t) = 119.5/117.8(11) pm; angle UBH(b) = 65.6/63.1(3) degrees. It is suggested that the different equilibrium conformations of the three molecules are determined primarily by repulsion between bridging H atoms in different tetrahydroborate groups.

Heavy metal boryl complexes: the first boryl complexes of cadmium and mercury are reportedviatransmetallation from boryllithium; the syntheses of related tin and lead systems highlight the additional factors associated with extension to more redox-active post-transition elements.

The adduct Me2(H)N·AlH3 (1) has been isolated for the first time at low temperatures from the reaction of [Me2NH2]Cl with LiAlH4 in Et2O solution at ca. 250 K. Single crystals of this and of the corresponding gallane Me2(H)N·GaH3 (2) have been grown, also at low temperature; the structures of these at 150 K determined by X-ray diffraction reveal dimeric units Me2(H)N·H2M(μ-H)2MH2·N(H)Me2 with highly unsymmetrical M–H⋯M hydrogen bridges, as well as evidence of significant intermolecular N–H⋯H–M interactions, particularly for M = Al. The M–N distances are quite similar [1 2.028(1), 2 2.079(3) A], but the M⋯M distance in 2 [3.7519(15) A] is much longer than in 1 [2.8743(8) A]. The adducts decompose at ambient temperatures, rapidly in the case of 1, slowly in the case of 2, with the elimination of H2 and the formation of the corresponding dimethylamido derivative. Unlike the vapour at low pressure in which the dimeric molecule predominates, crystals of the gallane (3) at 150 K are composed of trimers [Me2NGaH2]3 structurally analogous to the corresponding alane, with a chair-like conformation and a mean Ga–N distance of 1.981(3) A.

The structure of the gallane adduct Me3N·GaH3 has been investigated by ab initio quantum chemical calculations. The results of gas-phase electron-diffraction (GED) measurements, together with earlier microwave measurements, have been reanalysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rαo structure) were found to be: r(Ga–H) 151.1(13), r(Ga–N) 213.4(4), r(N–C) 147.6(3), r(C–H) 108.4(4) pm; H–Ga–N 99.3(8) and Ga–N–C 108.8(2)°. Unlike the corresponding alane derivative, the adduct is monomeric in the crystalline phase with dimensions very close to those of the gaseous molecule, as confirmed by a redetermination of the structure of a single crystal at 150 K.

Solvent dependent double C-H activation in an Ir(NHC)(2) system generates an agostically stabilized 14-electron complex featuring a face-capping bis(alkyl) tethered NHC ligand [NHC = N-heterocyclic carbene]. These activation processes are reversible, and the resulting ligand-derived hydrogen shuttle can be applied to the dehydrogenation of BN-containing substrates.

Simple source: An intramolecular iridium-mediated C-H borylation proceeds through transfer of a BH3 fragment from borohydride to a benzylic carbon center (see scheme; coe=cyclooctene). Simple and inexpensive LiBH 4 is thus utilized as the boron source in C-H to C-B conversion. © 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Lithium borohydride absorbs anhydrous ammonia to form four stable ammoniates; Li(NH(3))(n)BH(4), mono-, di-, tri-, and tertraammoniate. This paper focuses on the monoammoniate, Li(NH(3))BH(4), which is readily formed on exposure of LiBH(4) to ammonia at room temperature and pressure. Ammonia loss from Li(NH(3))BH(4) commences around 40 degrees C and the compound transforms directly to LiBH(4). The crystal structure of Li(NH(3))BH(4) is reported here for the first time. Its close structural relationship with LiBH(4) provides a clear insight into the facile nature and mechanism of ammonia uptake and loss. These materials not only represent an excellent high weight-percent ammonia system but are also potentially important hydrogen stores.

The structures of the molecules methylamine-borane, MeH2N·BH3, and dimethylamine-borane, Me2HN·BH3, have been investigated by gas-phase electron diffraction (GED) and quantum chemical calculations. The crystal structures have also been determined for methylamine-, dimethylamine-, and trimethylamine-borane, MenH3-nN·BH3 (n = 1-3); these are noteworthy for what they reveal about the intermolecular interactions and, particularly, the N-H···H-B dihydrogen bonding in the cases where n = 1 or 2. Hence, structures are now known for all the members of the ammonia- and amine-borane series MenH3-nN·BH3 (n = 0-3) in both the gas and solid phases. The structural variations and energetics of formation of the gaseous adducts are discussed in relation to the basicity of the MenH3-nN fragment. The relative importance of secondary interactions in the solid adducts with n = 0-3 has been assessed by the semi-classical density sums (SCDS-PIXEL) approach.

The structure of quinuclidine, HC(CH2CH2)3N, has been re-investigated by quantum chemical calculations and by gas-phase electron diffraction (GED). The GED data, together with published rotational constants, have been analysed using the SARACEN method to determine the most reliable structure (rh1) for the gaseous molecule. The structures of two adducts of quinuclidine with group 13 trihydride molecules, MH3 (M = B, Al), have also been determined by GED and quantum chemical calculations. The effect of the coordination of these hydrides to the quinuclidine nitrogen atom has been investigated, and the structural changes and energetics of adduct formation are discussed. We also present the crystal structure of quinuclidine borane.

Herein we report on the synthesis and characterization of the first representative of a new class of gallium hydrides, namely [{GaCl(hpp)H}2] (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidate). Our X-ray diffraction data suggest the presence of weak H--H contacts between adjacent [{GaCl(hpp)H}2] molecules in the crystal. With the aid of quantum-chemical calculations, the pathway of its formation starting with the adduct H2ClGaNMe3 and the pyrimidine hppH were analyzed. [{GaCl(hpp)H}2] slowly decomposes at 25 degrees C, a process which presumably leads under H2 elimination to [{GaCl(hpp)}2]. Although we were not yet able to characterize [{GaCl(hpp)}2] experimentally, DFT calculations provide information about its likely structure. According to these calculations, the molecule features a Ga--Ga single bond.

Dimethylamine-gallane is relatively slow to decompose in a closed system and vaporises at low temperature primarily as Me2(H)N.GaH3 molecules which can be trapped in a solid Ar matrix and characterised by their IR spectrum. Under the conditions needed to secure a useful gas electron diffraction (GED) pattern, however, the vapour was found to consist of dimeric dimethylamidogallane molecules, [Me2NGaH2]2, formed from the secondary amine adduct by elimination of H2, and the most reliable structure for which has been determined. Salient structural parameters (r(hl) structure) were found to be: r(Ga-N) 202.6(2), r(Ga-H) 155.6(8), r(N-C) 148.0(3), r(C-H) 111.2(6) pm; Ga-N-Ga 90.7(1), C-N-C 109.3(5), N-C-H 109.9(10) and H-Ga-H 119.4(42) degrees.

Primary amine-gallane adducts, RH(2)N.GaH(3)[R = Me (1) and (t)Bu (2)], have been isolated for the first time from the reaction of [RNH(3)]Cl with LiGaH(4) in Et(2)O solution at ca. 273 K and characterised by their vibrational and (1)H NMR spectra. The structures of single crystals, grown at low temperatures and determined by X-ray diffraction, reveal pseudo-polymeric arrays of RH(2)N.GaH(3) molecules with Ga-N distances of 2.0424(18) and 2.058(5)A for 1 and 2, respectively. The adducts decompose at or near ambient temperatures with the elimination of H(2) and formation of the corresponding monoalkylamido derivative [RHNGaH(2)](n). The crystal structures of these at 150 K consist of either [MeHNGaH(2)](3) molecules with a "skew-boat" conformation for the cyclic Ga(3)N(3) skeleton (3) or [(t)BuHNGaH(2)](2) molecules with a four-membered Ga(2)N(2) core (4), with Ga-N distances averaging 1.986(8) and 1.991(3)A for 3 and 4, respectively. The crystalline solids 1, 3 and 4 feature intermolecular N-H...H-Ga interactions with H...H distances estimated to fall in the range 2.1-2.3 A. The significance of these is discussed.

Thermal decomposition of monochlorogallane, [H2GaCl]n, at ambient temperatures results in the formation of subvalent gallium species. To Ga[HGaCl3], previously reported, has now been added a second mixed-valence solid, Ga4[HGaCl3]2[Ga2Cl6] (1), the crystal structure of which at 150 K shows a number of unusual features. Adducts of monochlorogallane, most readily prepared from the hydrochloride of the base and LiGaH4 in appropriate proportions, include not only the 1:1 molecular complex Me3P·GaH2Cl (2), but also 2:1 amine complexes which prove to be cationic gallane derivatives, [H2Ga(NH2R)2]+Cl-, where R = tBu (3a) or sBu (3b). All three of these complexes have been characterized crystallographically at 150 K.

Highly unsaturated mono(N-heterocyclic carbene) Ir(iii) systems have been targeted via ligand abstraction protocols. Hydrogenation of Ir(IPr)(cod)Cl (1a) leads to the formation of the highly reactive (fluxional) trimer [Ir(IPr)(H)(2)Cl](3), while the related IMes system undergoes further C-H bond activation. Chloride abstraction from 1a prior to hydrogenation allows access to sources of the 12-electron [Ir(IPr)(H)(2)](+) fragment, which, in the absence of a suitable donor, dimerizes to give [{Ir(IPr)(H)(μ-H)}(2)](2+).

The structure of the gaseous hafnium tetrakis(tetrahydroborate) molecule, Hf(BH4)4, has been investigated by detailed quantum mechanical calculations and by analysis of its gas electron-diffraction (GED) pattern. The ground-state geometry possesses T symmetry with all of the triply-bridged BH4 groups twisted equally about the Hf...B-H axes. Salient structural parameters (ra distances, r angles) deduced from the GED pattern by the SARACEN method were: r(Hf...B) 231.4(2), r(Hf-Hb) 221.5(7), r(B-Hb) 127.6(5), r(B-Ht) 121(1) pm, Hf...B-Hb 69.4(3), Hb-B-Hb 108.4(4), Hb-B-Ht 110.6(3), B...Hf...B-Hb 166(1) degrees. A notable feature is the large magnitude of the Hf...B and Hf-Hb anharmonicity parameters, attributed to the fluxional hydrogen atom exchange process. The properties are compared with those of related tetrahydroborates..