premise: <mask> number of semiempirical estimates by various workers lead to the conclusion that the **f bond becomes symmetric when the **f bond length is about 2.4 to 2.5 <mask> , but aside from the possible example of nickel dimethylglyoxime there have been no convincing reports of symmetric **f bonds .
hypothesis: the proper context for <mask> here should be a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation