Boron halides adducts with

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

The organic analogues of the reactions to be discussed here are the borane reductions of aldehydes and ketones and the addition of metal alkyls across ketonic carbonyls, equation 15. In contrast to the ease of these organic reactions, qualitative data which has accumulated in our laboratory over the last decade demonstrates that the carbonyl group in organometallies is fairly resistant to addition across CO. For example, many stable adducts of organometallie carbonyls with aluminum alkyls are known, eq. lc, but under similar conditions a ketone will quickly react by addition of the aluminum alkyl across the CO bond. A similar reactivity pattern is seen with boron halides. 

Boron halides, such as BF3 or BC13, are electron-deficient molecules because they do not have an octet of electrons surrounding the boron atom. In accord with this property, they tend to act as strong Lewis acids by accepting electron pairs from bases to form stable acid-base adducts. Such electron donors as pyridine or ether can be used ... 

Salts containing these ions can be isolated as solids with large cations such as R4P+. Because they have an unshared pair of electrons on the Sn atom, these ions are Lewis bases that form adducts with Lewis acids such as boron halides ... 

In a study of the B2Cl4-trichloroethylene system, no isolable adduct was obtainable and all the haloolefin could be recovered upon treatment of the reaction mixture with excess ammonia to remove the boron halide 35). It was reported that peaks attributable to an addition product and to an intermediate compound retaining the double bond were observed in the proton NMR spectrum, but later workers 28, 84, 117) were unable to verify this finding. 

Boron Halides.—N.m.r. data have been recorded for the adducts formed by (XC6H4)3 PC1 with boron trihalides." A correlation was found between the shielding of the orfho-protons and the stability of the adducts. 

F N.m.r. spectra of the adducts formed by p-F- and m-F-phenyl-phosphines and -arsines with boron halides reveal a correlation between the chemical shift, J(F,H) and J(P,F), and the stabilities of the adducts formed. ... 

The greater basicity of bridging CO than terminal CO leads to a strikingly richer Z —CO— chemistry for polynuclear carbonyls than for their mononuclear counterparts. For example, simple neutral carbonyls such as Fe(CO)5 and Cr(CO)6 do not form stable adducts with boron or aluminum halides, whereas neutral polynuclear carbonyls containing bridging CO ligands readily form such adducts. 

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