Question

A) Iron(II) forms what is termed a “charge transfer complex” with the 1,10-phenanthroline. What is meant by this term and why are such complexes highly colored (i.e. how/why is the absorption of light so favored at their characteristic wavelength)? (Note: your textbook specifically discusses the iron-1,10-phenanthroline complex; be careful – it is an exception to some generalities about charge transfer complexes.) (0.5) 3

B) Iron(III) also forms a complex with the phenanthroline ligand; explain why it cannot form a charge transfer complex like iron(II)? (Hint: redox chemistry.) (0.5)

C) Beer’s Law predicts a linear relationship between Abs. and conc., but for very high concentration solutions, Beer’s Law can fail for real and theoretically predictable reasons (negative deviation). State at least two instrumental and at least two chemical causes of negative deviation from ideal Beer’s Law behavior. (Note: see your textbook re Beer’s Law.)

Answer 1

A.) A charge-transfer complex (CT complex) or electron-donor-acceptorcomplex is an association of two or more molecules, or of different parts of one large molecule, in which a fraction of electronic charge istransferred between the molecular entities.

These complexes are so coloured because the color is reflective of the relative energy balance resulting from the transfer of electronic charge from donor to acceptor which is high in these complexes.

B) Iron(III) also forms a complex with the phenanthroline ligand which forms protonated complexes which do not carry charges.

C) Instrumental deviation - polychromaticity and unabsorbed stray light errors

Chemical causes of deviation - every photon must have equal chances of aborption

every photon must pass through equal wavelength