In the complex [CoCl4]2-, there are four chloride ions Cl- surrounding the central cobalt Co atom, acting as ligands. According to the crystal field theory, the coordination number of a complex is determined by the number of ligand atoms directly bonded to the central metal atom. In this case, the coordination number of the complex [CoCl4]2- is 4.To determine the oxidation state of cobalt in this complex, we can use the following formula:Oxidation state of metal + Charge of ligands Number of ligands = Charge of the complexLet the oxidation state of cobalt be x. The charge of each chloride ligand is -1, and there are four chloride ligands. The overall charge of the complex is -2. So, the equation becomes:x + -1 4 = -2x - 4 = -2x = +2Therefore, the oxidation state of cobalt in the complex [CoCl4]2- is +2.