A transition metal complex containing diamagnetic ligands can be overall diamagnetic (all electrons — Organic Chemistry Chemistry Question
Magnetism of transition metal complexes
A transition metal complex containing diamagnetic ligands can be overall diamagnetic (all electrons are paired) or paramagnetic (having unpaired electron(s)) depending on the electronic configuration of the central metal ion, the nature of the ligand, and geometry of the ligand sphere. The magnitude of paramagnetism of a metal complex is commonly reported in terms of the effective magnetic moment (μ_eff) which can be obtained from the experimental measurement of molar magnetic susceptibility (χ_m) and is commonly expressed in Bohr magneton (BM).
Theoretically, the magnetic moment is contributed by two components, the spin angular momentum and the orbital angular momentum. For many complexes of first row d-block metal ions, however, the contribution of the second component can be ignored.
Thus, the so-called spin only magnetic moment can be determined by the number of unpaired electrons, n:
μ_(spin only) = √n(n + 2) (B.M.)
I. The observed effective magnetic moment of two octahedral complexes K4[Mn(CN)6] ⋅ 3 H2O and K4[Mn(SCN)6] are 2.18 B.M. and 6.06 B.M., respectively.
II. In practice, the experimentally observed μ_eff value of [Ni(H2O)6]Cl2 is 3.25 BM. This is not surprising due to the fact that magnetic moment of octahedral complexes of Ni2+ (d8) usually does not obey the spin only formula. In these cases, the contribution of orbital angular momentum should be taken into account. The simplification of spin-orbit coupling model can be applied to calculate their magnetic moment:
μ_eff = μ_(spin only) * (1 - 4λ / Δ_oct)
where λ is spin-orbit coupling constant of Ni2+ and has the value of -315 cm−1 and Δ_oct is the crystal-field splitting parameter.
III. Dibenzoylmethane (DBM) is a well known chelating κ-O,O-ligand which can form stable complexes with many transition metal ions.
DBM
[VISUAL]
Reaction of Ni(CH3COO)2 ⋅ 4 H2O with DBM in EtOH–H2O solution gives light green crystalline complex A which loses 6.8 % of mass on heating at 210 oC in the air to form green solid B. The substance B is quantitatively converted to brown prismatic crystals C by re-crystallization in dry toluene. B and C are two polymorphic forms and their inter-conversion is reversible. The X-ray single crystal structure of C shows a square planar geometry with the chemical composition of [Ni(DBM)2]. While B is paramagnetic with effective magnetic moment of 3.27 BM, the complex C is diamagnetic. When B and C are kept in the air, they slowly convert to A. This happens much faster in the presence of some organic solvents.
Calculate number of unpaired electrons in each complex. Which complex is low spin? Which complex is high spin?
Model Answer
Two compounds are octahedral complexes of Mn2+ (d5).
K4[Mn(CN)6] ⋅ 3 H2O is low spin, 1 unpaired electron.
K4[Mn(SCN)6] is high spin, 5 unpaired electrons.
Rationalize your answers by applying crystal field theory.
Model Answer
CN- is strong field ligand, electronic configuration is (t2g)5(eg)0.
SCN– is weak field ligand, electronic configuration is (t2g)3(eg)2.
Calculate the μ (spin only) of complex [Ni(H2O)6]Cl2.
Model Answer
Ni2+ (d8) in octahedral field has electronic configuration of (t2g)6(eg)2 with two unpaired electrons. The spin only μ_eff is 2.83 BM.
Calculate the effective magnetic moment of [Ni(H2O)6]Cl2 taking into account spin–orbit coupling. Δ_oct of [Ni(H2O)6]2+ is 8500 cm-1.
Model Answer
μ_eff = μ_(spin only) * (1 - 4λ / Δ_oct) = 2.83 * (1 - 4 * (-315) / 8500) = 3.25 BM
Draw the splitting diagram of the d orbitals of Ni2+ in C and confirm its diamagnetic property.
Model Answer
d8 in square planar field is diamagnetic.
What is the molecular formula of A? Assume that A is a mononuclear complex.
Model Answer
C is neutral, DBM is monoanionic form. Mc = 504 g mol–1. A should be hydrate form of C, MA = MB / 0.932 = 540.8 g mol–1, corresponding to two molecules of H2O per [Ni(DBM)2]. Thus, the formula is [Ni(DBM)2] ⋅ 2 H2O
The effective magnetic moment of A is 3.11 BM (Synth. React. Inorg. Met. Org. Chem., 2009, 39, 694-703). What is the most suitable molecular geometry of A? (Assuming that A is an octahedral complex, Δ_oct of A is similar to that of [Ni(H2O)6] 2+).
Model Answer
Water should coordinate to Ni center due to the change of color and magnetic property. μ_eff value of A is close to that of [Ni(H2O)6]Cl2. Thus, an octahedral complex is expected for A.
Draw all possible isomers of A.
Model Answer
There are three isomers, the trans isomer and two optical cis isomers.
What do you expect for the molecular geometry of B?
Model Answer
B should be an octahedral complex, due to the color and magnetic moment are similar to those of A. Octahedral geometry can be formed by oligomerization / polymerization of B on heating, the DBM may play as bridging ligand.