Question

Provide a reasonable explanation for the following experimental observations.

a) The wavelength of the absorption maximum in the UV/vis spectra of Cr(III) complexes decreases in the order [CrCl6]3-, [Cr(NH3)6]3+, [Cr(CN)6]3-.

b) Low spin Fe(III) complexes have stronger d-d transitions (are more intensely colored) than the corresponding high spin complexes.

c) The initial rate of Cl- substitution is 108 times faster for [Fe(H2O)6]2+ (μeff = 5.9 μB) than for [Ru(H2O)6]2+ (μeff = 0 μB) and is independent of the concentration of Cl- in the solution.

d) The reaction of [Pt(CO)Cl3]1- with ammonia yields trans-[Pt(NH3)(CO)Cl2] while the same reaction with [PtCl3(py)]1- yields cis-[Pt(NH3)Cl2(py)] (py = pyridine).

e) The complex [Ru(bpy)3]2+ (bpy = 2,2’-bipyridine) is more stable than [Ru(py)6]2+ (py = pyridine). f) The carbon-oxygen bonds in [V(CO)6] are shorter than the ones in [V(CO)6]1-.

g) The addition of PEtPh2 (Et = ethyl, Ph = phenyl) to NiBr2 in solution at low temperature gives a red complex with the formula [Ni(PEtPh2)2Br2], which is converted to a green isomer upon warming to room temperature. The red complex is diamagnetic, but the green complex has a magnetic moment of 3.2 B.

Answer 1

a) the UV-vis absorption by any complex is due to d-d transition. so any effect which alters the d-d transition will alter the absorption of wavelength in Uv-visible region.

The splitting of d-orbitals depends upon the ligan present

stronger the ligand more the splitting hence higher the absorption of light and lower the wavelenth absorbed.

the order of strength of ligand is

Halides< NH3 < CN-

so the order of absroption of light wavelenght of the given complex is

[CrCl6]3- > [Cr(NH3)6]³⁺ > [Cr(CN)6]3-.

b) Low spin Fe(III) complex will absorb higher wavelength as the splitting will be less as compared to high spin complexes

c) The complexes are classified as labile and inert

In labile complexes the ligands are loosely attached and can be removed easily while in case of inert complex they are strongly bonded.

so Fe forms a labile complex with water as compared to Ru

d) We know that in case of square planar geometry complexes the cis effect is smaller than the trans effect.

e) Due to stronger chelate effect of bipyridine as compared to pyridine as ligand

f) The charge on the two complexes are different hence the charge on metal Vanadium is +1 in case of [V(CO)6]1-