Transcript Zumd20
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 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 States Often lose e– to Rare Gas configuration. Sc Ti V Cr Mn 3 2,3 1,2,3 4 4,5 3,4, 3,4, 5,6 5,6,7 1,2, 1,2, Fe Co Ni Cu Zn 2,3, 1,2,3 ,4 1,2 1,2 2 4,5,6 3,4 But beyond Mn, transition metal ions do not achieve that high. Because the 8th IP is prohibitively expensive! Coordination Compounds Often 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 complex ions (both cat– and an–) paramagnetic Duhh! These are transition metals, no? Dative bonded by e– donating ligands. Coordination Number The But to only one of many solvent water molecules. 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 … 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” 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: Must X– < OH– < H2O < NH3 < en < NO2– < 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: Species Cl– As ion: chloride As ligand: chloro NO2– CN– nitrite cyanide nitro cyano 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: Species H2O NH3 CO Normal: water ammonia As ligand: aqua ammine carbon monoxide 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 Overrun Since ligands are often anions, their charge may swamp the transition metal, leaving the complex ion negative! Na2 [ PbI4 ] (from Harris p. 123) Sodium Li tetraiodoplumbate(II) While lead(II) is the source, the Latin root is used for the complex with “ate” denoting anion. [ AgCl2 ], 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 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 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). 8 C3 3 C2 6 S4 6 d h=24 Td E A1 1 1 1 1 1 A2 1 1 1 –1 –1 E 2 –1 2 0 0 T1 3 0 –1 1 –1 T2 3 0 –1 –1 1 x2+y2+z2 (2z2–x2–y2, x2–y2) (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. 20.26 for a visual argument why dxy,dxz,dyz are lower energy. Other Ligand Symmetries Octahedral, Oh, (6-coordinate, Fig. 20.20) symmetic species for (2z2–x2–y2, x2–y2) T2g symmetric species for (xy, xz, yz) Eg Square Planar, D4h (Fig. 20.27a) symmetric species for z2 B1g symmetric species for x2–y2 B2g symmetric species for xy Eg symmetric species for (xz, yz) A1g Consequences Degeneracies work in Hund’s favor to separate e– pairs and maximize spin. With high enough energy separations, , Aufbau (lowest level) wins instead. field case, large, e– pairs in lower energy states. Low field case, small, e– unpaired as much as feasible. High 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–