Sentence:  A 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 A , but aside from the possible example of <WSD>nickel</WSD> dimethylglyoxime there have been no convincing reports of symmetric **f bonds . 
Meaning of the word marked with <WSD> tags:  a hard malleable ductile silvery metallic element that is resistant to corrosion; used in alloys; occurs in pentlandite and smaltite and garnierite and millerite