The oxidation number is usually numerically equal to the oxidation state. However, in a few cases the ligand atom can be less electronegative than the central atom (e.g., in iridium phosphine complexes), resulting in a formal oxidation state that is different from the oxidation number.
Although formal oxidation numbers can be helpful for classifying compounds, they are unmeasureable and their physical meaning can be ambiguous. Formal oxidation numbers require particular caution for molecules where the bonding is covalent, since the formal oxidation numbers require the heterolytic removal of ligands, which essentially denies covalency. Spectroscopic oxidation states, as defined by Jorgenson and reiterated by Wieghardt, are measureables that are bench-marked using spectroscopic and crystallographic data.