The coordination number of cobalt in [Co(en)2Cl2]Cl is (en = ethane-1,2-diamine, a bidentate ligand)

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The coordination number of cobalt in $[Co (en) X_{2} Cl X_{2}] Cl$ is (en = ethane-1,2-diamine, a bidentate ligand)

Coordination number is the number of donor atoms bonded to the metal. Two bidentate en contribute

2 \times 2 = 4

and two

Cl X^{-}

contribute 2, giving 6. The outer chloride does not count.