Download NAME: Chem 1b, 2005, 3rd

NAME:__SOLUTIONS_____ Chem 1b, 2005, 3rd. midterm examination
A periodic table and useful data are provided on the insert. You may use the insert for
scratch work but enter all work to be graded in the space with each question.
1) (58 points) For each of the following species, give its IUPAC chemical name and the
hybridization of the transition-metal atom. Draw the structure of the complex portion of
each. If geometrical and optical isomers are possible, draw the structure of all isomers.
a) [Cr(H2NCH2CH2NH2)3](NO3)3
name: tris(ethylenediamine)chromium(III) nitrate. Chromium(III) is d3 and none of the
ligands are halide ions so expect octahedral coordination. The ligand is bidentate. (3
ligands)(2 metal-ligand bond per ligand) = 6. hybridization: d2sp3. The complex lacks a
center and planes of symmetry hence there is a pair of optical isomers.
NN
N Cr N
N N
b) [Pd(NH3)2(CN)2]H2O (Pd, Z = 46)
name: diamminedicyanopalladium(II) monohydrate. Palladium(II) is d8 so expect
square-planar coordination. There are 4 monodentate ligands. hybridization: sp3
The complex has cis (Z) and trans (E) isomers.
O
H3N Pd NH3
O
H3N Pd
O
NH3
O
c) [FeS4-5] (Hint: The ligand S-2, sulfido, acts as a pseudohalide.)
name: tetrasulfidoferrate(III)
Iron(III) is d5 and the ligands are pseudohalides so expect tetrahedral symmetry.
hybridization: sp3
S
S
Fe S
S
2) (9 points) A top view of a single crystal of the mineral tourmaline is given below.
Which elements of symmetry are revealed by this view? To which crystal system does
tourmaline belong?
The crystal has a three-fold axis of symmetry and three planes of
symmetry hence it belongs to the hexagonal/trigonal crystal system.
3) (43 points) Tungsten (W,atomic number 74) forms a 8-coordinate complex, W(CN)8-4.
a) Suggest a reason for the expanded coordination number.
tungsten (wolfram) is found in the third row of transition metals. The resulting
larger size of the W(IV) ion will accommodate more ligands. The linear cyanide
ligand is not bulky and therefore fits well.
b) Give the number of bonding MO's in the complex: ____________
8. Cyanide is monodentate. In our simple model, each monodentate ligand has a
lone pair of electrons which it shares with the metal. 8 ligands morph into 8
bonding MO's upon formation of the complex. These 8 MO's hold 16 electrons.
c) Does the most stable bonding MO have any nodes? Briefly explain.
The most stable bonding MO will exhibit complete constructive interference
between the metal and ligand orbitals and therefore will have no nodes between
the ligands and the W.
d) Give the electronic configuration of W in the complex: __________________
W in the complex has an oxidation number of IV. The electron configuration of
elemental W is [Xe](4f)14(6s)2(5d)4 and W(IV) is [Xe](4f)14(5d)2
e) Determine the number of unpaired electrons in the complex. The MO's derived
from the metal orbitals are given below. Suggest a color for the complex.
______ ______ E3*
______ ______ E1*
_______ A1*
The electron configuration is d2. Cyanide is a very strong-field ligand so
the gaps between the orbitals will be large. The two electrons will be
placed into the A1* MO and will be paired. The complex is diamagnetic.
The complex will absorb at the blend end of the spectrum and the
transmitted light will be at the red end. The complex has a center of
symmetry and a small extinction coefficient is expected.
4) (20 points) The mineral spinel, MgAl2O4, forms lovely octahedral crystals.
a) What is the arrangement of the anion anions in a crystal structure of spinel?
Cloest packing of oxide anions is expected. Since the crystal has octahedral
(cubic) symmetry, expect cubic closest packing (ccp).
b) The magnesium cations are found in tetrahedral holes. What fraction of the
tetrahedral holes is filled?
Spinel has 1/4 magnesium per oxide but there are two tetrahedral holes per oxide
anion in the ccp lattice so 1/8th of the tetrahedral holes are filled.
c) The aluminum cations are found in octahedral holes. What fraction of the
octahedral holes is filled?
Spinel has 1/2 aluminum per oxide and there is one octahedral hole per oxide
anion so half of the octahedral holes are filled.
c) Is the experimentally observed filling of holes by the two cations the expected
outcome? Briefly discuss. The ionic radii of the Mg+2, Al+3, and O-2 ions are 0.78
Å, 0.45 Å, and 1.32 Å, respectively.
Consider the ratio of ionic radii. R(Mg) = 0.78/1.32 - 0.59. this lies between 0.73
(the cubic cutoff) and 0.41 (the octahedral cutoff) so packing of the magnesium
cations in octahedral holes is expected. Similarly, R(Al) = 0.45/1.32 = 0.34. This
is less than the octahedral cutoff but greater than the trigonal cutoff (0.23) so
packing in tetrahedral holes is expected. There are sufficient numbers of holes to
accommodate this packing. Yet the reverse to one's surprise is observed.
5) (20 points) RNA is an important biopolymer that plays coding, catalytic, and structural
roles. Structurally, the backbone of the molecule consists a chain or strand with ribose
sugar molecules covalently linked by phosphate groups. One of four bases-A, U, G, and
C-is bound to each sugar molecule.
a) RNA frequently is found in a form in which two strands are linked and form a
helical staircase. Suggest an experimental method that would demonstrate the
helical character of double-stranded RNA.
X-ray diffraction is the method of choice. A single crystal is required. Other
diffraction methods, e.g. neutron and electron diffraction, could be employed.
With a great deal of sweat, NMR spectroscopy might reveal helicity.
b) Various chemical reactions are employed to synthesize polymers. What
general class of reaction is employed by living organisms to synthesize the
backbone of RNA? A sugar molecule is an alcohol and has multiple -OH groups.
Synthetically, RNA can be viewed as the product of substituted ribose with
phosphoric acid. This is a condensation reaction.
c) The structure of a section of double-stranded RNA showing the base G
(guanine) on one strand in close proximity to the base C (cytosine) on the other
strand is given below. Suggest the basis for the close proximity and pairing of G
and C.
The two bases interact via three hydrogen bonds. The hydrogen bonds are close
to ideal, i.e. 180 degree bond angles, and will be strong as hydrogen bonds go.
The observation that hydrogen bonds between DNA bases were possible played a
major role in the discovery of the double helix by Watson and Crick.