Oxidation States and Group-Number Anomalies in d-Block Ions

Titanium sits in group 4, so +4 (its full group oxidation state, using all four valence electrons, [Ar]3d2 4s2) is its most common and most stable state, seen in compounds like TiCl4 and TiO2. But titanium also readily shows the lower +3 state (a d1 configuration, as in TiCl3 and the violet [Ti(H2O)6]3+ ion) and even +2 (a d2 configuration, in compounds like TiCl2), both accessible because removing fewer than all four valence electrons is still energetically reasonable for an early transition metal. Do not assume an early transition metal is restricted to just its maximum group oxidation state — titanium genuinely accesses +2, +3 AND +4.

For many transition-metal oxoanions, the central atom oxidation state DOES equal its periodic table group number: in MnO4- (permanganate), Mn is +7, matching group 7; in Cr2O7²- (dichromate), Cr is +6, matching group 6; in VO4³- (vanadate), V is +5, matching group 5. But FeO4²- (ferrate) is the exception in this set: iron sits in group 8, yet ferrate has Fe at +6, not +8 — iron cannot actually reach the full +8 oxidation state under normal conditions (unlike ruthenium and osmium, its heavier group 8 congeners, which do reach +8 in RuO4 and OsO4). Whenever a question asks which oxoanion breaks the oxidation-state-equals-group-number pattern, look specifically for iron (or another first-row transition metal whose heavier congeners reach higher states than it does).

For a vanadium chloride (V, atomic number 23, ground-state configuration [Ar]3d3 4s2) to have exactly one unpaired d-electron, vanadium must be in the +4 oxidation state (removing all 2 4s electrons plus 2 of the 3d electrons leaves a single 3d1 electron) — so the formula must be VCl4. Work magnetic-moment questions like this backwards: count unpaired electrons from the given moment, then find the oxidation state whose d-electron count matches that number of unpaired electrons.

Forming a trihalide (an MF3-type compound) requires a metal that can stably reach the +3 oxidation state. Cobalt does this readily (Co3+ is a common, well-characterised oxidation state, especially stabilised in many coordination complexes), so CoF3 is a real, known compound. By contrast, zinc (which has a very stable, filled d10 configuration and is essentially locked at +2), copper (whose most stable common states are +1 and +2, with +3 being unusual and hard to access with a halide), and mercury (dominated by +1 and +2) do not readily form a trihalide under ordinary conditions — recognise +3 accessibility, not just electronegativity or halide chemistry, as the deciding factor for which metal in a series can form an MF3-type compound.