Anthony J. Downs

Structures and Aggregation of the Methylamine−Borane Molecules, MenH3−nN·BH3 (n = 1−3), Studied by X-ray Diffraction, Gas-Phase Electron Diffraction, and Quantum Chemical Calculations

The structures of the molecules methylamine-borane, MeH(2)N.BH(3), and dimethylamine-borane, Me(2)HN.BH(3), 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, Me(n)H(3-n)N.BH(3) (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 Me(n)H(3-n)N.BH(3) (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 Me(n)H(3-n)N 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 Group 13 Metals Aluminium, Gallium, Indium and Thallium: Chemical Patterns and Peculiarities: Aldridge/The Group 13 Metals Aluminium, Gallium, Indium and Thallium: Chemical Patterns and Peculiarities

The last two decades have seen a renaissance in interest in the chemistry of the main group elements. In particular research on the metals of group 13 (aluminium, gallium, indium and thallium) has led to the synthesis and isolation of some very novel and unusual molecules, with implications for organometallic synthesis, new materials development, and with biological, medical and, environmental relevance. The Group 13 Metals Aluminium, Gallium, Indium and Thallium aims to cover new facts, developments and applications in the context of more general patterns of physical and chemical behaviour. Particular attention is paid to the main growth areas, including the chemistry of lower formal oxidation states, cluster chemistry, the investigation of solid oxides and hydroxides, advances in the formation of III-V and related compounds, the biological significance of Group 13 metal complexes, and the growing importance of the metals and their compounds in the mediation of organic reactions. Chapters cover:

The hydrides of aluminium, gallium, indium, and thallium: a re-evaluation

boron gives little hint of the comparative wasteland making up much of the hydride estate of the heavier Group 13 elements., At a recent count2 about 100 binary boranes are now known, typically as discrete molecules remarkable for their stoicheiometries and structures which have done much to challenge and reshape our understanding of chemical bonding at large. By contrast, aluminium forms only one binary hydride stable under normal conditions as a polymeric solid, [AIH,],, the a-form of which is isostructural with AlF,, featuring 6-coordinate aluminium atom^.^^^ Attempts to prepare the analogous gallium compound have a chequered hist~ry,~ and it has taken nearly 50 years from the first reported sighting to establish the true credentials of gallane, [GaH,],,6 which now emerges as showing obvious affinities to diborane in the vapour state (i.e. n = 2) while being relatively short-lived under normal conditions. Despite some claims, however, it is unlikely that the hydrides [JnH,], and [TlH,], have yet materialized. In this account we review the current status of the hydrides formed by the Group 13 metals aluminium, gallium, indium, and thallium. Coordinatively saturated derivatives like MH, (M = Al, Ga, In, or T1) and Me,N. MH, (M = A1 or Ga) having been known for some year^,^?^ we are concerned primarily with the parent hydrides, [MH,], (m = 1, 2, or 3; n = 1, 2...), and related unsaturated derivatives. The last category includes species with more than one Group 13 element, for example tetrahydroborate derivatives like Al(BH,), and H,Ga(BH,),-, (m = 1 or 2) and tetraborane(l0) derivatives like 2-R2MB,H, (M = Al, R = Me; M = Ga, R = H or Me). It is appropriate first to consider the physical properties of the binary hydrides [MH,],. Hence it is possible to identify not only feasible methods of synthesizing compounds with M-H bonds, but also the origins of the thermal lability and reactivity besetting such compounds. 1.1 Theoretical Modelling Exploration of the Group 13 metal hydrides has been spurred by the greatly enhanced sophistication of modern computational methods which now admit the use of relatively elaborate basis sets, as well as making due allowance for factors like configuration interaction and relativistic correction^.^ Where comparisons can be made, such calculations typically yield dimensions and energetics which reproduce closely the experimental findings, and in some cases improve upon those findings. Such is the case, for example, with the monohydride molecules MH (M = B, Al, Ga, In, or Tl), which are short-lived under normal condition^.^,^ Accordingly we can place some trust in such results to anticipate the likely equilibrium molecular structures, vibrational properties, and binding energies of Group 13 hydrides, including numerous species whose existence has yet to be authenticated. Just what inferences are to be drawn will be discussed in Section 2.

Recent advances in the chemistry of the Group 13 metals: hydride derivatives and compounds involving multiply bonded Group 13 metal atoms

Abstract Two aspects of Group 13 metal chemistry are surveyed in the light of recent research, particularly that reported in the period 1992–1998. The first concerns compounds containing M–H bonds (M=Al, Ga or In) and including binary as well as mixed derivatives in either the base-free or complexed conditions. The second concerns compounds offering the opportunity for the metal atom to engage in multiple bonding. Seemingly unrelated, the two types of molecular compound have several features in common. Both typically contain highly reactive functional groups that are unusually susceptible to oxidation, hydrolysis or aggregation; as a result, developments in both areas have depended on similar strategies. The account focuses (i) on the reactions giving access to the relevant compounds; (ii) on the methodologies of trapping or shielding needed for the preservation of reactive M–H fragments or putative multiple bonds to M atoms; (iii) on the characteristics of the compounds, with particular reference to the structures they assume, and to their response to thermal, photolytic and chemical stimuli; and (iv) on the interplay between experimental and theoretical methods. Some of the hydrides show promise as synthetic reagents or as sources of the metal or metal compounds with extended structures of predefined morphology. On the evidence to hand, π-type interactions play only a minor role in the chemistry of the Group 13 metals, their contribution being obscured by the effects of low coordination number and non-covalent forces.

The Hunting of the Gallium Hydrides

Publisher Summary This chapter provides an overview of gallane and its derivatives. Monochlorogallane has indeed been the turning point in the hunt for gallane and its derivatives. It reacts in vacuo with lithium tetrahydrogallate at 243–250 K to give not only substantial quantities of elemental gallium and hydrogen but also gallane in yields up to 50%. The identity of the volatile, thermally perishable product has been established unequivocally by chemical analysis, by its vibrational and 1 H NMR spectra, and by chemical trapping with trimethylamine to give the known molecular adduct (Me 3 N) 2 GaH 3 as the sole product at 178 K. Efforts to improve the yield of gallane by using a solvent to give better control over the metathesis reaction have so far met with only limited success. The choice of medium is restricted by the need to minimize both the basic properties and the susceptibility to reduction of the would-be solvent. Thus, the mildly basic properties characterizing ether may enhance its ability to dissolve the reagents, but only at the expense of producing a sample of the gallane that can be freed from the solvent with difficulty.

Molecular aluminium trihydride, AlH3: generation in a solid noble gas matrix and characterisation by its infrared spectrum and Ab initio calculations

Broad-band photolysis of a solid noble gas matrix containing Al atoms and H2 gives rise to the planar, monomericAlH3 molecule, isotopically natural and deuteriated forms of which have been characterised by their IR spectra; confirmation is afforded by the results of MP2 ab initio and normal coordinate analysis calculations.

Characterization and Photochemistry of the Gallium and Indium Subhydrides Ga2H2 and In2H2

Herein we describe the cocondensation of Ga or In vapor with H2 in an excess of Ar at 10 ± 12 K, and the subsequent irradiation of the resulting matrix with light of different wavelengths. We show that the reaction of Ga2 or In2 with H2 directly or indirectly gives rise to three isomers of Ga2H2 and two isomers of In2H2, namely the bis( -hydrido) species Ga( -H)2Ga (1a) and In( -H)2In (1b), the trans-bent species HGaGaH (3a) and HInInH (3b), and GaGaH2 (2a) with two terminal Ga H bonds (Scheme 1). All these molecules have

The length, strength and polarity of metal-carbon bonds: dialkylzinc compounds studied by density functional theory calculations, gas electron diffraction and photoelectron spectroscopy

The molecular structures and thermodynamic functions of seven dialkyl zinc compounds, R2Zn, R = Me, Et, i-Pr, t-Bu, n-Pr, neopentyl and the silaneopentyl Me3SiCH2, of the parent hydrocarbons RH and of the radicals R have been determined by density functional theory calculations at the B3LYP/SDD level. The molecular structures of the i-Pr, t-Bu, neo-Pe and Me3SiCH2 derivatives have been determined by gas electron diffraction. Me2Zn, Et2Zn, t-Bu2Zn and neo-Pe2Zn have been studied by photoelectron spectroscopy and the ionisation energies calculated. Both experimental and calculated Zn-C bond distances were found to increase in the order Me2Zn ≈ (Me3SiCH2)2Zn < Et2Zn ≈ n-Pr2Zn ≈ neo-Pe2Zn < i-Pr2Zn < t-Bu2Zn.Calculated mean bond rupture enthalpies indicate that the strength of the Zn–C bonds decrease in the same order, viz. Me2Zn ≈ (Me3SiCH2)2Zn > Et2Zn ≈ n-Pr2Zn ≈ neo-Pe2Zn > i-Pr2Zn > t-Bu2Zn.Both bond lengths and bond strengths were found to be strongly correlated with the inductive Taft constant I, indicating that the bond strength increases and the bond length decreases with increasing electron withdrawing power of the alkyl group. Evidence from the literature indicates that bond strengths and bond lengths in homoleptic alkyl derivatives of the main group metals in Groups 12, 13 and 14 and of the transition elements in Group 4 vary in the same manner.

Low valent and would-be multiply bonded derivatives of the Group 13 metals Al, Ga and In revealed through matrix isolation

Abstract Two aspects of Group 13 metal chemistry are surveyed in the light of recent matrix-isolation and quantum chemical studies. The first concerns derivatives of the metals Al, Ga and In (M) in formal oxidation states less than +3: these include the M0 derivatives MLn, where L=CO, N2, NH3 or PH3, and n=1 or 2; the MI derivatives MX, where X=H, CH3 or NH2, and M2H2; and the MII derivatives HMX, where X=H, CH3, NH2 or PH2. The second concerns derivatives in which there is the opportunity for the metal to engage in multiple bonding, as exemplified by the cases of M2H2 and H2MNH2. Experimental studies have involved thermally or photolytically excited reactions between M atoms, or sometimes M2 molecules, and an appropriate substrate, e.g. H2, CO, N2, CH4, NH3 or PH3. The reagents have been trapped in solid noble gas matrices, the IR spectra of which have been exploited to deduce the course of events and identities of the products. Confirmation of the identity and geometry of each of the products have been achieved primarily on the basis (i) of isotopic effects and (ii) of comparison of the measured spectrum with that anticipated by quantum chemical calculations. This liaison of experiments with theory has led to the characterisation not only of numerous simple molecules that are short-lived under normal conditions, but also of some of the reaction channels open to them, typically through photolytic activation. The structure, energies and reactivities of the compounds are reviewed.

The spectroscopic properties and X-ray crystal structure of a discrete molecular alane, dimethylamidoalane, [Me2NAlH2]3

Abstract The vibrational, 1H and 13C NMR, and mass spectra of dimethylamidoalane (1) have been determined and analysed. The implications of these and other results have been confirmed by the X-ray crystal structure determination of 1, which reveals a discrete molecular species, [Me2NAlH2]3, containing a six-membered [AlN]3 ring in a chair conformation complying with C3v, symmetry (1876 reflections). The mean values for salient interatomic distances and angles are: r(AlN) 1.936(3), r(CN) 1.505(5), r(AlH) 1.55 A, ∠ NAlN 108.8(1), ∠ AlNAl 114.9(1) and ∠ CNC 106.2(3)°. The properties of the alane are compared with those of related compounds.