2:1 base adducts

The XeF+ cation forms Lewis acid—base adduct cations containing N—Xe—F linkages with nitrogen bases that are resistant to oxidation by the strongly oxidizing XeF+ cation having an estimated electron affinity of the XeF+ cation of 10.9 eV (12). The thermally unstable colorless salt,... 

Certain base adducts of borane, such as triethylamine borane [1722-26-5] (C2H )2N BH, dimethyl sulfide borane [13292-87-OJ, (CH2)2S BH, and tetrahydrofuran borane [14044-65-6] C HgO BH, are more easily and safely handled than B2H and are commercially available. These compounds find wide use as reducing agents and in hydroboration reactions (57). A wide variety of borane reducing agents and hydroborating agents is available from Aldrich Chemical Co., Milwaukee, Wisconsin. Base displacement reactions can be used to convert one adduct to another. The relative stabiUties of BH adducts as a function of Group 15 and 16 donor atoms are P > N and S > O. This order has sparked controversy because the trend opposes the normal order estabUshed by BF. In the case of anionic nucleophiles, base displacement leads to ionic hydroborate adducts (eqs. 20,21). 

The dimensionless acceptor number, AN, ranked the acidity of a solvent and was defined for an acidic solvent A as the relative P NMR downfield shift (A3) induced in triethyl phosphine when dissolved in pure A. A value of 0 was assigned to the shift produced by the neutral solvent hexane, and a value of 100 to the shift produced by SbClj. Gutmann suggested that the enthalpy of acid-base adduct formation be written as ... 

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

FIGURE 18.27 Pyridoxal-5-phosphate forms stable Schiff base adducts with amino acids and acts as an effective electron sink to stabilize a variety of reaction intermediates. 

The diketonates can form Lewis base adducts such as 5-coordinate Pd[P(o-tolyl)3](CF3COCHCOCF3)2 (Figure 3.25), though with acetylacetone square planar adducts of the type M(acac)2(PR3)2 are usually obtained, where the diketone is monodentate O-bonded [63]. 

No homoleptic tetramethyl has been made, though PtMe - and Lewis base adducts PtMe4L2 (L = PMe2Ph, 1/2 bipy etc.) can be made. 

Trialkyls are only known as Lewis base adducts. Reaction of Au2Br6 with methyllithium at 70°C affords an unstable AuMe3 (which is probably AuMe3Br ), but stable phosphine adducts AuMe3PR3 (R, e.g. Me, Ph) can be made [169]. 

A conventional synthesis may be used for the triaryl obtained as a Lewis base adduct... 

In 1987 a major breakthrough was achieved when two research groups independently succeeded in the synthesis of monomeric silylene complexes in the form of stable base adducts [35-38]. 

For 6, the activation energy for rotation about the MSi bond has been measured as AG = 40.3 (+ 5) kJ/mol [143]. According to MO calculations, a genuine Cr = Si double bond has no rotational barrier worth mentioning. This applies also, with some restrictions, to the discussed base adducts. 

Monomeric base adducts of silylene complexes can be transformed into dimeric compounds at elevated temperatures with loss of the donor. This applies also to reactive donor-free compounds. 

Whether these results will also have an impact on the theory of metallaallenes is difficult to predict at least for the compounds Cp (CO)2Mn = M = Mn(CO)2Cp, (M = Ge, Sn, Pb) a linear structure is established and also linear p-carbido complexes are known [198], Recently, a germanium compound has been synthesized which is directly comparable with 22. In this case, the starting material for the synthesis is not a monomeric base adduct, but a dimeric germylene complex which is cleaved by Na2Fe(CO)4 in pyridine to form 72 [199],... 

TABLE 8. Avoh values for PhOH-base adducts... 

The majority of the work on xanthates of divalent nickel has, in recent years, been centered on the formation of base adducts with [Ni(Rxant)2]. Carlin and Siegel (348) and Daktenieks and Graddon (349) reported the formation of paramagnetic [Ni(Etxant)2B2] or [Ni(Etaxant)2B], where B = pyridine, 4-methylpyridine, bipyridyl, or... 

The first example of a neutral aluminum complex of diazaphosphane, the 1,3,2,4-diazaphosphaluminetidine 50, Eq. (4), has been synthesized by the dehydrogenation reaction between Lewis acid-base adduct H3AI <— NMca and fBuP[N(H)fBu 2 49. The product fBuP(NfBu)2(H)Al [Pg.111]

Besides simple Lewis acid-base adducts, numerous compounds of the type [R2MER x and [RMER ]x (x 2), containing real cr-bonds between the group 13 and 15 elements, have been prepared. Epoch-making studies were performed by famous chemists such as Stock and Poland, who synthesized the... 

Later on, this concept was extended to precursors containing both elements of the desired material already connected by a chemical bond in a single molecule. Such precursors are mainly referred to as single source precursors. Their potential application for the deposition of thin films of the corresponding binary materials by MOCVD processes could be demonstrated. In particular Lewis acid-base adducts R3M—ER3 and four- and six-membered heterocycles [R2MER x (Fig- 1) have been in the focus of research groups both in industry and university. Consequently, the development of powerful synthetic pathways for the preparation of such precursors has been forced. 

A major goal was to investigate the solid state structures of such compounds by single crystal X-ray diffraction. It was found that Lewis acid-base adducts R3M—ER3 show general structural trends, which allow estimations on the relative stability of the adducts. The experimental results were confirmed by computational calculations, giving even deeper insights into the structural parameters and the thermodynamic stability of simple Lewis acid-base adducts. In addition, their thermodynamic stability in solution was investigated by temperature-dependent NMR spectroscopy. 

The reaction between a Lewis acid R3M and a Lewis base ER3, usually resulting in the formation of a Lewis acid-base adduct R3M—ER3, is of fundamental interest in main group chemistry. Numerous experiments, in particular reactions of alane and gallane MH3 with amines and phosphines ER3, have been performed [14]. Several general coordination modes, as summarized in Fig. 2, have been identified by X-ray diffraction. 

AHoiss = 19.3 0.2 kcal moP ) are stronger Lewis acid-base adducts than H3B—NMC3 (AHoiss = 34.8 0.5 kcal mol ) and McsB—NMej = 17.6 ... 

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