Zumdahl’s Chapter 20 Transition Metals Chapter Contents e – configuration Oxidation #s & IP ...
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Transcript of Zumdahl’s Chapter 20 Transition Metals Chapter Contents e – configuration Oxidation #s & IP ...
Chapter Contents e– configuration Oxidation #s & IP Coordination
Compounds Coordination # Ligands Nomenclature
Isomerism Structural Isomerism Stereoisomerism
Bonding in Complex Ions Crystal Field Theory
Octahedral Tetrahedral
Electronic Configurationsd – block transition metals
ns2 (n–1)d X where n = 4,5,6,7 Potential for high spin (Hund’s Rule) Ions lose s electrons first.
f – block transition elements ns2 (n–1)d0,1 (n–2)f X where n = 6,7 Lanthanides & Actinides are even more
similar than members of d – block.
Oxidation StatesOften lose e– to Rare Gas configuration.
But beyond Mn, transition metal ions do not achieve that high.
Because the 8th IP is prohibitively expensive!
Sc Ti V Cr Mn Fe Co Ni Cu Zn
3 2,3
41,2,3
4,51,2,
3,4,
5,6
1,2,3,4,5,
6,7
2,3,4,5,6
1,2,3,4
1,23,4
1,2 2
Coordination CompoundsOften complex ions (both cat– and an–)
But neutrals possible if ligands exactly balance metal ion’s charge.
Often highly colored Since MO energy separations match visible
light photon energies, absorb visible light.
Often paramagnetic Duhh! These are transition metals, no?
Dative bonded by e– donating ligands.
Coordination NumberThe number of ligand bonds
Usually 6 (octahedral) but as few as 2 (linear) and as many as 8 (prismatic or antiprismatic cube).
Here’s Gd bonding
to a ligand called
DOTA 6 ways …
But to only one
of many solvent
water molecules.
For a bizarre
7 coordination.
Sane Coordination Numbers 6-coordinated
metals like cobalt sepulchrate : C12H24N8Co2+
Or the one we used in lab, MgEDTA2– C10H12O8N2Mg2–
Ligands From Latin ligare, “to bind”
Must be a Lewis base (e– donor)Could, as does EDTA, have several
Lewis base functionalities: polydentate! If monodentate, should be small enough
to permit others to bind.Relative bonding strengths:
X– < OH– < H2O < NH3 < en < NO2– < CN–
halides ethylene diamine
Naming Anionic NamesAnions that electrically balance cationic
coordination complexes can also be present as ligands in that complex! So they need different names that identify
when they’re being used as ligands:
Species Cl– NO2– CN–
As ion: chloride nitrite cyanide
As ligand: chloro nitro cyano
Naming Neutral NamesBut ligands needn’t be anions; many
neutral molecules are Lewis bases. And they too get new names appearing as
ligands in coordination complexes:
Species H2O NH3 CO
Normal: water ammonia carbon monoxide
As ligand: aqua ammine carbonyl
Name That Complex, Oedipus
[ Cr Br2 (en)2 ] Br Anion, bromide, is named last (no surprise) chromium(III) is named next-to-last Ligands named 1st in alphabetical order:
Number of a ligands is shown as Greek prefix: dibromo …
Unless it already uses “di” then use “bis” Dibromobis(ethylenediammine) …
Dibromobis(ethylenediammine)chromium(III) bromide
Charge OverrunSince ligands are often anions, their
charge may swamp the transition metal, leaving the complex ion negative!
Na2 [ PbI4 ] (from Harris p. 123)
Sodium tetraiodoplumbate(II) While lead(II) is the source, the Latin root is
used for the complex with “ate” denoting anion.
Li [ AgCl2 ], lithium dichloroargentate
Isomeric Complicationsdichlorobis(diethylsulfide)platinate(II)
would appear to be the name of the square planar species above, but The square planar configuration can have
another isomer where the Cl ligands are on opposite sides of the platinum, so it’s really
cis-dichlorobis(diethylsulfide)platinate(II) and this is not the only way isomers arise!
Complex Isomerization Simplified
Stereoisomers preserve bonds Geometric (cis-trans) isomers Optical (non-superimposable mirrors)
Structural isomers preserve only atoms Coordination isomers swap ligands for
anions to the complex. Linkage isomers swap lone pairs on the
ligand as the bonding site.
Coordination IsomersUnique to coordination complexes [ Pb (en)2 Cl2 ] Br2
bis(ethylenediammine)dichlorolead(IV) bromide
Only 1 of 3 possible coordination isomers The other 2 are
[ Pb Br (en)2 Cl ] Br Cl bromobis(ethylenediammine)chlorolead(IV)
bromide chloride
[ Pb Br2 (en)2 ] Cl2 dibromobis(ethylenediammine)lead(IV) chloride
Optical IsomersWe need to compare the mirror image
of a sample complex to see if it can be superimposed on the original.
These views of cobalt sepulchrate and its
Mirror image demonstrate non-superimposition.
They are optical isomers.
Colorful ComplexesColors we see everywhere are due, for
the most part, to electronic transitions. Most electronic transitions, however, occur
at energies well in excess of visible h. d-electrons transitions ought not to be
visible at all, since they are degenerate. But, in a complex, that degeneracy is
broken! Transition energies aren’t then 0.
Breaking Degeneracy5 d orbitals in a tetrahedral charge field
split as a doublet (E) and a triplet (T).
Td E 8 C3 3 C2 6 S4 6 d h=24
A1 1 1 1 1 1 x2+y2+z2
A2 1 1 1 –1 –1
E 2 –1 2 0 0 (2z2–x2–y2, x2–y2)
T1 3 0 –1 1 –1
T2 3 0 –1 –1 1 (xy, xz, yz)
Symmetry Tells Not AllWhile the symmetry tables assure us
that there are now 2 energy levels for d orbitals instead of 1, we don’t know the energies themselves. That depends upon the field established by
the ligands and the proximity of the d s. See Zumdahl’s Fig. 20.26 for a visual
argument why dxy,dxz,dyz are lower energy.
Other Ligand SymmetriesOctahedral, Oh, (6-coordinate, Fig. 20.20)
Eg symmetic species for (2z2–x2–y2, x2–y2)
T2g symmetric species for (xy, xz, yz)
Square Planar, D4h (Fig. 20.27a)
A1g symmetric species for z2
B1g symmetric species for x2–y2
B2g symmetric species for xy
Eg symmetric species for (xz, yz)
ConsequencesDegeneracies work in Hund’s favor to
separate e– pairs and maximize spin.With high enough energy separations,
, Aufbau (lowest level) wins instead. High field case, large, e– pairs in lower
energy states. Low field case, small, e– unpaired as
much as feasible.
Symmetry and tetrahedral = (4/9) octahedral (same ligands)
As a consequence of symmetry. If some ligand was 9/4 as strong as the
weakest to give octahedral strong field, then strong field (low-spin) tetrahedral might exist. But none does.
Field strengths of ligands vary as: X– < OH– < H2O < NH3 < en < NO2
– < CN–