Consider the transition metal complex [CoCl3(CO)3]. What is a) The oxidation state of the metal centre. b) The dn configuration of the metal centre. c) The co-ordination number of the metal centre.

a) +3. Cl ligands carry a negative charge while CO ligands are neutral so the all together the ligands have a charge of -3. Since the complex is neutral overall, the metal must cancel out the charge of the ligands, so it is +3.
b) d6. Co is in group 9 so it is d9 in its neutral state. To find the dn configuration, simply subtract the oxidation state (+3) from the neutral state dn configuration.
c) 6. Each ligand forms a single bond to the metal. Since there are 6 of them, the metal co-ordination number is 6.