General Organic Chemistry question

Apparently the C2 molecule has been studied. It has been made, and its

fleeting existence has been investigated, probably spectroscopically.  

It has a bond energy of 620 kJ/mol and a bond length of 1.31A, These

values are close to 614 kJ/mol and 1.34A, the average values found for a

C=C double bond in organic compounds.

 

In the C2 molecule each carbon 

makes only two bonds.  This is probably the reason that C2 is 

not "stable"; the other forms of carbon: diamond, graphite, charcoal, 

C60, nanotubes, etc. in which carbon makes four bonds are more stable. C2 

molecules spontaneously hooks up with other C2 molecules to form one of 

these more stabe forms of carbon.