Zumdahl’s Chapter 20 Transition Metals

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 Configurations  d – block transition metals  ns 2 (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  ns 2 (n–1)d 0,1 (n–2)f X where n = 6,7  Lanthanides & Actinides are even more similar than members of d – block.

Oxidation States  Often lose e – to Rare Gas configuration.  But beyond Mn, transition metal ions do not achieve that high.  Because the 8 th IP is prohibitively expensive! ScTiVCrMnFeCoNiCuZn 3 2,3 4 1,2,3 4,5 1,2, 3, 4, 5, 6 1, 2, 3,4, 5,6, 7 2,3, 4,5,6 1, 2,3,4 1, 2 3,4 1, 22

Coordination Compounds  Often 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 Number  The 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 :  C 12 H 24 N 8 Co 2+  Or the one we used in lab, MgEDTA 2–  C 10 H 12 O 8 N 2 Mg 2–

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 – < H 2 O < NH 3 < en < NO 2 – < CN – halides ethylene diamine

Naming Anionic Names  Anions 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: SpeciesCl – NO 2 – CN – As ion:chloridenitritecyanide As ligand:chloronitrocyano

Naming Neutral Names  But ligands needn’t be anions; many neutral molecules are Lewis bases.  And they too get new names appearing as ligands in coordination complexes: SpeciesH2OH2ONH 3 CO Normal:waterammoniacarbon monoxide As ligand:aquaamminecarbonyl

Name That Complex, Oedipus  [ Cr Br 2 (en) 2 ] Br  Anion, bromide, is named last (no surprise)  chromium(III) is named next-to-last  Ligands named 1 st in alphabetical order:  Number of a ligands is shown as Greek prefix: dibromo …  Unless it already uses “di” then use “bis” Dibromo bis(ethylenediammine) …  Dibromobis(ethylenediammine)chromium(III) bromide

Charge Overrun  Since ligands are often anions, their charge may swamp the transition metal, leaving the complex ion negative!  Na 2 [ PbI 4 ] (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 [ AgCl 2 ], lithium dichloroargentate

Isomeric Complications  dichlorobis(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 Isomers  Unique to coordination complexes  [ Pb (en) 2 Cl 2 ] Br 2 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 Br 2 (en) 2 ] Cl 2 dibromobis(ethylenediammine)lead(IV) chloride

Optical Isomers  We 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 Complexes  Colors 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 Degeneracy  5 d orbitals in a tetrahedral charge field split as a doublet (E) and a triplet (T). TdTd E8 C 3 3 C 2 6 S 4 6  d h=24 A1A x 2 +y 2 +z 2 A2A2 111 –1–1 –1–1 E2 –1–1 200(2z 2 –x 2 –y 2, x 2 –y 2 ) T1T1 30 –1–1 1 –1–1 T2T2 30 –1–1 –1–1 1(xy, xz, yz)

Symmetry Tells Not All  While 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 for a visual argument why d xy,d xz,d yz are lower energy.

Other Ligand Symmetries  Octahedral, O h, (6-coordinate, Fig )  E g symmetic species for (2z 2 –x 2 –y 2, x 2 –y 2 )  T 2g symmetric species for (xy, xz, yz)  Square Planar, D 4h (Fig a)  A 1g symmetric species for z 2  B 1g symmetric species for x 2 –y 2  B 2g symmetric species for xy  E g symmetric species for (xz, yz)

Consequences  Degeneracies 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 – < H 2 O < NH 3 < en < NO 2 – < CN –