Question

what is the tau bond and for which formulae it will be there ?

Answer 1

Tau bond is a banana bond too.

"Banana bond" is not necessarily only for 3c2e bonds. Any bent bond is called a "banana" bond or tau bond. Tau-bonds also provide an alternative valid description of electron density in alkenes and alkynes
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Answer 2

At the same time, the carbon-to-hydrogen bonds gain more s-character, which shortens them.