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I know that there are 18 electrons available for bonding in the entire molecule, and that 6 of these are used for sigma bonds. That leaves 12 left over. Which molecular orbitals best describe the locations of these electrons?

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Here is the structure of cyanogen

enter image description here

Each carbon has 4 valence electrons and each nitrogen has 5, for a total of ((4x2)+(5x2)) 18 electrons as you said. Again, as you noted, 6 electrons are involved in sigma bonds, the 2 C-N sigma bonds and the one C-C sigma bond. Each nitrogen also has a lone pair of electrons and that accounts for (2x2) 4 more electrons. That leaves 8 electrons to fill the 4 pi bonds. Just like acetylene with 2 pi bonds, the two carbon-nitrogen triple bonds in cyanogen also have two pi bonds, with each pi bond holding 2 electrons.

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    $\begingroup$ It's also worth noting that the internal C-C bond is significantly shorter than a normal single bond, indicating significant delocalization across it. $\endgroup$ – Geoff Hutchison Dec 21 '14 at 17:15
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To visualize ron's answer, here is the valence orbital scheme of cyanogen. Note, that this is the canonical molecular orbital picture, which does not use hybridisation. From this depiction, the stated fact from Geoff is also clearly visible. Due to delocalisation the carbon-carbon bond is significantly shorter than a normal single bond. The right hand side orbitals are taken from a DF-BP85/def2-SVP calculation. (Since I forgot to add that to the picture itself, the horizontal dotted line separates unoccupied from occupied orbitals.)
mo scheme of cyanogen

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