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Coordination number refers to the central transition metal. The ligand (en) has the formula H2NCH2CH2NH2, and each nitrogen atom has a lone pair to donate, so it can attach to a transition metal at two binding sites 90º apart. Therefore, we classify ethylenediamine or the en ligand as bidentate.
Since it's a bidentate, it has two binding sites, and therefore only contributes 2 bonds to the coordination number. Remember that the coordination number is the number of bonds of the ligands directly attached to the central metal atom or ion.
Do molecules/anions outside the coordination sphere contribute to the compound's coordination number? For example, for [Cu(NH3)4]SO4.H2O, is the coordination number 4 (since each NH3 is monodentate and there are four of them), or is the coordination number 6 (including the SO4 and H2O)?
The molecules outside the coordination sphere do not contribute to the compound's coordination number because they can only interact with the compound to help neutralize the charge. These molecules are not directly bound to the central atom so they do not count. Remember the coordination number is the number of bonds to the central atom. In that case, the coordination number for [Cu(NH3)4]SO4.H2O is just 4.
Caitlin Dillon 4G wrote:So since en is bidentate? Is dien tridentate because it has 3 nitrogens?
dien is a tridente because when you draw the lewis structure, the nitrogens have a lone pair and can therefore donate both of those electrons to from a coordinating bond.
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