Sentence:  A <WSD>number</WSD> 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 nickel dimethylglyoxime there have been no convincing reports of symmetric **f bonds . 
Meaning of the word marked with <WSD> tags:  the property possessed by a sum or total or indefinite quantity of units or individuals