Why hemoglobin and cytochromes are considered as chromoproteins instead of metalloproteins?

A chromoprotein is a conjugated protein that contains a pigmented prosthetic group (or cofactor). A common example is haemoglobin, which contains a heme cofactor, which is the iron...

please don't copy and paste the lump sum of material.. i have read all of that.. my doubt is why hemoglobin is not considered as metalloprotein, it also contain Fe2+ in its core.

A chromoprotein is a conjugated protein that contains a pigmented prosthetic group (or cofactor). A common example is haemoglobin, which contains a heme cofactor, which is the iron...