Electron-pair bases

A series of Rh and Ir complexes is also basic enough to add boron halides, although the metal-boron bonds are weak, e.g., dppe and PPh3-Rh complexes serve as electron-pair bases when they react with BF3 and BCI3 . Table 1 summarizes the results. 

The ions [rj -CpMofCO),]-, [rj -CpWCCOj] , [ 0(00)4], and [Mn(CO)5] serve as electron-pair bases toward TIR3 to give complexes of the type [RjTKM)]" (R = Cf,FjH2, C(,Fs M = metal complex anion) . 

Reaction with Co2(CO)g 6.5.2.1 Reaction with electron pair bases 6.2.2.1... 

Reaction with electron pair bases CI5P 12 C215H)H(... 

This is a common method for preparing transition-metal hydrides (see Table I) " . Borohydride reduction is complex, e.g., RujfCOjj with NaBH in THF gives over a dozen products, but a few useful mechanistic generalizations can be offered. Syntheses of metal hydrides proceed through intermediate borohydride complexes. Electron-pair bases (an ethereal solvent may be sufficient) are then necessary to complex and remove BHj. In a few examples the borohydride complexes are observable ... 

In I, the electron pair base character of CO can arise from donation of the lone pair on carbon, while its acid character can arise from utilization of an empty p-orbital on carbon. In MO terms the ir-acidity of CO originates from the interaction of filled orbitals of appropriate symmetry with empty, antibonding orbitals, (iTy ) or (.if ), of CO. Despite lone pairs located formally at both the carbon and the oxygen ends, carbon monoxide behaves as a base only at carbon in metal carbonyls . However, spectroscopic evidence points to the existence of linkage isomerism for carbon monoxide in Au(CO)(OC) obtained by cocondensation of gold vapors and CO at approximately lOK. ... 

Electron pair bases have been used extensively to induce a-hydride elimination, probably by bringing about the steric crowding necessary to initiate the reaction Equation (m) exemplifies this process ... 

Specify the hybrid orbitals needed to accommodate the electron pairs based on their geometric arrangement ( TABLE 9.4). 

An alternative approach to QM, for the elucidation of the electronic basis of MM, is to reexamine the traditional model of covalent interaction mediated by electron pairs, based on the notion of chemical affinity. The assumption is that the interaction between a pair of atoms in a molecule only involves their valence electrons. If the two atoms are sufficiently alike, equally shared electron density between the atoms binds them together if they are of different chemical nature, the sharing is unequal and the covalency reduced. 

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