Actually, according to very recent work, carbon-carbon bond length is 1.12 Armstrong; ln literature, bond length of single bond,double bond & tripple bond are respectively 1.54 - 1.57, 1.3 -1.31 & 1.18 -1.2 Armstrong. So, from the experimental results​, carbon—carbon bond has to be quadruple bond…While , according to M.O.T.& diagram, bond order of C2 is 2 ; Again, another problem is to form quadruple bond, the elements need to posses d-electrons/d-orbitals to produce delta-bond. Now, to explain, the quadruple bond of C2, former MOT is collapsed; now, we need a totally different theory that would able to explain that. Recently, Chemists think that may be ,it is possible via some sort of bridging between two electrons , but it's not clear solely. One thing I'm sure about that it can't be explained by banana bond as it requires 3c-2e bond, but here we aren't talking about 3-centres. Now, the aspect of other sort of bridging then, I wonder,is there any sort of diffusion/delocalization of charge cloud , I don't know!? But there are examples of quintuplet bond (Cr…) & even six bonds man…can u believe !!! Oh yes, it is proved found in case of W and Mo. Surely , now we require new hypertension & new model