Plants appear green because chlorophyll a absorbs red and purple light; the reflected light consequently appears green. The geometry around the Mg ion is square planar.
Heme is the iron-containing complex in hemoglobin. In hemoglobin, the red heme complex is bonded to a large protein molecule (globin) by coordination of the protein to a position above the plane of the heme molecule. The geometry around the Fe ion is square planar.
vitamin B12 Vitamin B12 is one of the essential vitamins required for living organisms. Note the octahedral structure around the Co atom.
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