Sentence:  A number of semiempirical estimates by various workers lead to the conclusion that the **f bond becomes <WSD>symmetric</WSD> when the **f bond length is about 2.4 to 2.5 A , but aside from the possible example of nickel dimethylglyoxime there have been no convincing reports of symmetric **f bonds . 
Meaning of the word marked with <WSD> tags:  having similarity in size, shape, and relative position of corresponding parts