Q1(f). Oxidising power and charge transfer

Answer in brief

MnO₄⁻ is reduced more readily and has lower-energy, longer-wavelength LMCT absorption than ReO₄⁻.

Key points

• Both metals are formally +7 and d⁰. Their intense bands arise from O-to-metal ligand-to-metal charge transfer, not d–d transitions.
• An O-based occupied orbital transfers an electron to a predominantly metal-based empty orbital, locally resembling reduction of the metal.
• The +7 state is more strongly stabilised for the heavier 5d metal Re, so reduction of Re(VII) is less favourable than of Mn(VII).
• The LMCT energy increases in the order MnO₄⁻ < TcO₄⁻ < ReO₄⁻; wavelengths follow the reverse order.
• Thus MnO₄⁻ absorbs in the visible and is intensely purple, whereas ReO₄⁻ absorbs at higher energy, chiefly in the ultraviolet.