central cation

Meaning: In a coordination compound, the central cation is the metal ion at the center of the complex, usually a transition metal such as Fe³⁺, Cu²⁺ or Ni²⁺. It acts as a Lewis acid, accepting lone pairs from ligands (Lewis bases such as NH₃, H₂O, CN⁻ or Cl⁻) to form coordinate covalent bonds. The number of donor atoms bonded to it is the coordination number, commonly 2, 4 or 6.

Equation: charge of complex = charge on central cation + sum of ligand charges, with all charges in units of e (1.60 × 10⁻¹⁹ C). For [Cu(NH₃)₄]²⁺: +2 + 4(0) = +2.

On the MCAT: Passages give a formula and ask for the metal's oxidation state, the coordination number or the net charge. A common reasoning task is solubility: AgCl dissolves in aqueous NH₃ because Ag⁺ forms [Ag(NH₃)₂]⁺, which has a large formation constant. This removes free Ag⁺ and shifts the solubility equilibrium forward (Le Châtelier).

Common confusion: Counterions written outside the brackets are not ligands and do not count toward the coordination number or the complex's charge. In K₃[Fe(CN)₆], six CN⁻ contribute −6 and iron is +3, so the complex is 3−; the K⁺ ions sit outside. Also, the central cation is the electron-pair acceptor, not the donor.