Coordination compounds consist of a central metal atom or ion bonded to surrounding molecules or ions called ligands. The bonds between the central metal and the ligands are typically coordinate covalent bonds, where both electrons in the bond originate from the ligand. These compounds exhibit a variety of geometries, such as octahedral, tetrahedral, and square planar, depending on the number and arrangement of ligands. Coordination compounds are significant in many biological systems and industrial applications. For example, hemoglobin, the oxygen-carrying protein in red blood cells, is a coordination compound with iron as the central metal and oxygen as one of the ligands. The color, magnetic properties, and reactivity of coordination compounds are influenced by factors such as the oxidation state of the metal and the nature of the ligands. Which of the following is a characteristic of coordination compounds?
A. They always have a tetrahedral geometry.
B. They consist of a central metal atom or ion bonded to ligands.
C. They do not exhibit any color.
D. They only form with transition metals.