Metal-only Lewis pairs

Double Salt Elimination as Access to the -Hi Oxidation State 

A range of compounds has been synthesized by double-salt metathesis in which a two-coordinate, substituent-free gallium atom sits in an essentially linear environment between two transition metals. Compounds of the form (q -CjRjjjdppejFeGaMjCO) (M = Fe, n = 4j M = Cr, W, = 5 R = H, Me) may be accessed via reactions of (q -CjRjjjdppejFeGaCl with carbonylmetallate dianions K2[M(C0) ] [186-188, 244]. In such complexes, the bridging gallium atom is formally sp-hybridized and hence has two vacant p-orbitals, which may accept electron density from metal d-orbitals 

Halide Abstraction as a Route to Cationic DiyI Systems 

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Scheme 13.6 Neutral and zwitterionic forms of the metal-only Lewis pair 54.

In the first two of these reactions, BC13 behaves as a Lewis acid because the boron atom has only three pairs of electrons surrounding it in the BC13 molecule so it functions as an electron pair acceptor. Typical of most ions of transition metals, Cu2+ readily accepts electron pairs from NH3 molecules. 

Kathmann et al. did determine reorientational correlation times for several amine bases in organic solvents based on both NMR relaxation and MD simulations as a test ability MD simulations and DFT calculations to explain the mechanism in complex reactions catalyzed by frustrated Lewis pairs (FLP) in metal free scission of H2. As the Debye-Stokes-Einstein (DSE) model gives only qualitative predictions, MD simulations are found valuable to validate the spectroscopic studies. 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic add appears to be an adduct between a base and the add (Luder, 1940 Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis add in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic adds secondary acids (Bell, 1973). This is an unfortunate term for the traditional adds. 

A measure of the Lewis acidity of a metal ion is determined by its affinity for a pair of electrons, and the greater this affinity, the more stable the complexes formed by the metal ion will be. However, removing electrons from a metal to produce an ion is also related to the attraction the metal atom has for electrons. Therefore, it seems reasonable to seek a correlation between the stability constants for complexes of several metals with a given ligand and the total energy necessary for ionization to produce the metal ions. The first-row transition metal ions react in solution with ethylenediamine, en, to form stable complexes. We will consider only the first two steps in complex formation, which can be shown as follows ... 

Recall that such Lewis-like diagrams are intended to convey only the localized electron-pair assignments about the central hexavalent metal atom, not the molecular shape.) Here Os(CH2)2 typifies allene-like bonding, while HW(CH2)(CH), W(CH)2, and W(CH2)3 represent cases of higher central-atom bond order that are unachievable with main-group elements. 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that main group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pair. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the /ifr-product were seen but the use of chiral porphyrins gave only low ees. 

Our studies have shown that almost all metal atoms will form an adduct with water and in many instances undergo further reaction. Theory and experiment (1 -7 ) support the concept of water acting as a Lewis base which donates electron density to the metal. Theory has also shown for Be and Mg that donation occurs extensively from the 3ai (a lone pair) orbital of water to the metal. This is of interest since this orbital is largely responsible for the nonlinearity of the water molecule. It is known, for instance, that ionization from this orbital causes water to become linear and results in a decrease in its bending frequency by approximately 700 cm . This is consistent with our finding that only the V2 bending mode of water decreases measurably upon adduct formation. The observed Av2 changes for metal-water adducts are compared and discussed below. 

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