Reaction of FeCl2 with phenanthroline (phen) and two equivalents of K[NCS] yields the octahedral iro — Analytical Chemistry Chemistry Question
Magnetic Complexes
Reaction of FeCl2 with phenanthroline (phen) and two equivalents of K[NCS] yields the octahedral iron (II) complex Fe(phen)2(NCS)2 (A). At liquid nitrogen temperature A has a magnetic moment of 0.0 B.M. but a magnetic moment near 4.9 B.M. at room temperature. [The effective magnetic moment, µeff, for a complex containing n unpaired electrons is given by: µeff = \sqrt{n(n+2)} Bohr magnetons, [B.M.] (mangled in the source text as µeff = ( * + *2)n n*)]
[VISUAL]
The ligand Hacac (B, C5H8O2) is shown below. Treatment with NH3 yields the anion acac– (C) whose C–O bond lengths are longer than those in B and whose 1H NMR exhibits just two peaks. Addition of three equivalents of acac– to an aqueous solution of FeCl3 yields a bright red octahedral complex (D) of composition C15H21O6Fe with an effective magnetic moment of 5.9 B.M.
[VISUAL]
Draw structures of the possible isomers of A
Model Answer
The NCS ligand could bond either through the sulfur, or through the nitrogen atom. Representing the bidentate phenanthroline ligand as a line between two adjacent sites on the octahedral complex gives the following isomers: [VISUAL]
Determine the number of valence electrons which occupy the d-orbitals of A
Model Answer
Since the iron is in the +2 state, the number of d electrons the Fe contributes is 6.
Draw electronic configurations for the d-orbital occupancy consistent with the high temperature and low temperature magnetic behaviour of A [You should determine the expected effective magnetic moment in each case]
Model Answer
µeff = 4.9 B.M. = \sqrt{n(n+2)}. Solving gives n = 4.
[VISUAL]
Which of the following statements is/are consistent with the low temperature magnetic data:
Hund’s Rules are obeyed [YES | NO | INSUFFICIENT DATA]
The Pauli Exclusion Principle is obeyed [YES | NO | INSUFFICIENT DATA]
Model Answer
Unfortunately, this question is rather vague. Hund's Rules of maximum multiplicity apply only to degenerate orbitals; the Pauli exclusion principle is always obeyed.
Which of the following statements is/are consistent with the low temperature magnetic data:
Hund’s Rules are obeyed [YES | NO | INSUFFICIENT DATA]
The Pauli Exclusion Principle is obeyed [YES | NO | INSUFFICIENT DATA]
Model Answer
Unfortunately, this question is rather vague. Hund's Rules of maximum multiplicity apply only to degenerate orbitals; the Pauli exclusion principle is always obeyed.
Draw the anion acac– (C) and determine a resonance structure to explain the difference in C–O bond lengths between B and C.
Model Answer
Resonance structures explaining the difference in C-O bond lengths: [VISUAL]
Draw the structures of B and C and clearly label the hybridisation state at each carbon in each case.
Model Answer
Structures of B and C with hybridisation states labeled: [VISUAL]
Draw possible isomers of D and predict the d-orbital occupancy in light of the observed magnetic data.
Model Answer
µeff = 5.9 B.M. = \sqrt{n(n+2)}. Solving gives n = 5.
[VISUAL]