Carbonyl compound binding strength

In other aromatic carbonyl compounds, the relative binding strength is based mainly on steric considerations. For benzaldehydes (Entries 7 and 8),127,130 equilibrium constants are on the order of 104 for adduct formation and the borane binds syn to the sterically insignificant aldehyde proton. In the acetophenone adduct, the borane binds syn to the methyl group, but the equilibrium constant is an order of magnitude lower due to... 

The equilibrium is further affected by steric factors. The bond strength of BF3 adducts has been measured by calorimetry (in the absence of diethyl ether, Table 2). AH appears to be very sensitive to steric hindrance, with a difference of nearly 10 kcal/mol between di-terf-butyl ketone and acetone (entries 1 and 7). Sterically unhindered aldehydes, ketones, and esters bind BF3 with AH = -13.6 to -18.2 kcal/mol (entries 3-8). Compared to these carbonyl compounds, diethyl ether forms a slightly stronger bond (AH = -18.8 kcal/mol, entry 9), and tetrahydrofiiran forms an even stronger bond (-21.6 kcal/mol). This explains why ether and tetrahydrofiiran are rarely used as solvents BF3 binds to ethereal solvents more effectively than to the intended aldehyde. 

The work on cobalt compounds raised two questions which could best be answered by studying a smaller model compound. One was the role of dispersion forces in metal ligand binding. The other was the elusive role of pi back bonding which is supposed to account for a large fraction of the metal carbonyl bond strength. 

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