Give Examples of steric effects
These are both examples of steric effects. Placing three highly branched groups around a central boron results in van der Waals repulsions. These repulsions are severe enough that only two groups can be bound to boron in the case of disiamylborane, and only one group in the case of thexylborane. (Notice the greater number of alkyl substitutions in the "thexyl" group.) Now, alkyl branches stabilize sp2-hybridized boron (see the solution to Problem 4.48 on p. 62 of the Study Guide and Solutions Manual) just as they stabilize sp2-hybridized carbon; but if the alkyl branches are themselves branched, they form a thicket of methyl groups that interact repulsively with each other as more of these branches are accumulated. These repulsive interactions reduce the relative stability of the trialkylboranes-evidently, so much so that they cannot form.