In the cobalt-carbonyl complex : $$[Co_2(CO)_8]$$, number of Co $$-$$ Co bonds is "X" and terminal CO ligands is "Y". $$X + Y =$$ ______

We need to find the number of Co-Co bonds (X) and terminal CO ligands (Y) in $$[Co_2(CO)_8]$$. The structure of $$[Co_2(CO)_8]$$ (dicobalt octacarbonyl) is well known: in the solid state it has two bridging CO groups and six terminal CO groups, connected by one Co-Co bond, consisting of two Co atoms joined by a single Co-Co bond, two bridging CO ligands between the Co atoms, and three terminal CO ligands on each Co atom (total 6 terminal CO ligands).

Therefore:

$$ X = \text{Number of Co-Co bonds} = 1 $$

$$ Y = \text{Number of terminal CO ligands} = 6 $$

$$ X + Y = 1 + 6 = 7 $$

Therefore, $$X + Y = $$ 7.