Coordination Chemistry II: Ligand Field Theory Continued

1 Coordination Chemistry II: Ligand Field Theory ContinuedChapter 10 Wednesday, November 25, 2015 Adding Metal Electronsegeg*a1ga1g*t1ut1u*t2gMetal ions typically have some valence electrons that can be accommodated in the metal dorbitals d0ions Ti4+, Zr4+, V5+, Ta5+, Cr6+, Mo6+, etc. d1ions Ti3+, V4+, Ta4+, Cr5+, Mo5+, etc. d2ions V3+, Ta3+, Cr4+, Mo4+, etc. d3ions V2+, Ta2+, Cr3+, Mo3+, Mn4+, etc. d4-d7 hold on d8ions Co1+, Ni2+, Cu3+, etc. d9ions Ni1+, Cu2+, etc. d10ions Cu1+, Zn2+, Spin and Low Spin ComplexesThe situation is a little more complicated for d4-d7metals: d4ions Cr2+, Mo2+, Mn3+, Fe4+, Ru4+, etc. d5ions Mn2+, Re2+, Fe3+, Ru3+, etc. d6ions Fe2+, Ru2+, Co3+, Rh3+, Pt4+, etc. d7ions Fe1+, Ru1+, Co2+, Rh2+, Ni3+, d4-d7electron counts: when o> total low spin when o< total high spinHIGH SPINLOW SPIN egeg*a1ga1g*t1ut1u*t2gegeg*a1ga1g*t1ut1u *t2gHigh Spin and Low SpinElectron configurations for octahedral complexes, [M(H2O)6]n+.

2 Only the d4through d7cases can be either high-spin or low spin. < > Weak- Field ligands:- Small , High spin complexesStrong- Field ligands:- Large , Low spin complexesElectron Pairing EnergyThe total electron pairing energy, total, has two components, cand e cis a destabilizing energy for the Coulombic repulsion associated with putting two electrons into the same orbital eis a stabilizing energy for electron exchange associated with two degenerate electrons having parallel spin total 3 e 0 ceg*t2gd4 HSeg*t2gd8eg*t2gd6LS total 7 e 3 c total 6 e 3 cLFSE 3 O O OLFSE 6 O O OLFSE 6 O O O eonly counts for electrons at the same energy!Using LFSE and Fe2+, d6Is the complex high spin or low spin?eg*t2gLow Spineg*t2gHigh SpinLFSE 6 O O 9350cm 1 22,440cm 1 LFSE 4 O O 9350cm 1 3740cm 1 19,600 2,000 9,350 3 6 319600 6 2000 22400 24,360 1 4 19600 4 2000 3740 7,860 < Aqua is a weak Field Ligand ; 3dhexaaquacomplexes almost always high spin MOs for Octahedral ComplexesThe reducible representation for the Ligand orbitals in Oh:E8C36C26C43C2 i6S48S63 h6 d 12000-400000T1g+ T2g+ T1u+ T2uirreducible representationsx and y axeson each ligandThe non-bonding t2gorbitals of an octahedral metal complex are oriented perfectly to form -bonds with ligands Donor vs Acceptor LigandsThe nature of the metal Ligand interaction is dependent on the type of Ligand .

3 -donor ligands are ligands with one or more lone pairs of electrons in porbitals on the donor atom that can donate to empty orbitals on the metal. preferred for metals with high oxidation states and low delectron count (d0-d3) -acceptor ligands ( -acidic ligands) are ligands with empty * orbitals on the donor atom that can accept electrons from the metal. preferred for metals with low oxidation states and high delectron count (d6or higher) donation of electron density from the metal to the Ligand * orbital results in weakening of the multiple Ligand bondExamples:Cl , Br , I , OR , SR , NR2 , O2 , NR2 , N3 Examples:CO, NO, CN-, pyridine back bonding -Effects in Octahedral Complexest2g o -only -donort2g -acceptort2g* oeg*t2gt2g*ega1ga1g*t1ut1u* ot2g*t2gM nbM L *M L M L M L *increasing Oadding delectrons populates the M L * orbitaladding delectrons populates the M L orbitaleg*eg* -Effects in Octahedral Complexesstrong Field ,low spinweak Field ,high spinSpectrochemical SeriesThe trend in Othat arises from -donor, -only, and -acceptor ligands is the basis for the Spectrochemical Series.

4 For [ML6]n+complexes:I < Br < Cl < OH < RCO2 < F < H2O < NCS < NH3< en < NO2 < phen < CN COI < Br < Cl < OH < RCO2 < F < H2O < NCS < NH3< en < NO2 < phen < CN CO donor ligands donor ligands only ligands only ligands acceptor ligands acceptor ligands weak- Field ligands high-spin complexes for 3dmetals* strong- Field ligands low-spin complexes for 3dmetals** Due to effect #2, octahedral 3dmetal complexes can be low spin or high spin, but 4dand 5dmetal complexes are always low OThe value of oalso depends systematically on the metal: 1. oincreases with increasing oxidation oincreases down a group. both trends are due to stronger metal- Ligand bonding. -MOs for Tetrahedral ComplexesFour-coordinate tetrahedral complexes are ubiquitous throughout the transition metals.

5 (n+1)s(n+1)pndz2x2 y2 ExyxzyzT2xyzT2sA1the irr. reps. of the metal valence orbitals are obtained directly from the character tableFor the Ligand orbitals we need to consider how the Lewis base pairs transform in the Tdpoint group. The result is: = A1+ T2A1T2Td -ML4 Tetrahedral MO Diagrame1a12a13t21t22t2A1T2(n+1)s(n+1)pn dz2x2 y2 ExyzT2sA1xyxzyzT2bondingM L very weakly M L *M non-bondingM dorbitals t t 49 o Metal dorbitals are split into a non-bonding Eset and a very weakly anti-bond T2set tetrahedral geometry can accommodate all delectron counts, from d0to d10 tis small compared to o: All tetrahedral complexes of the 3dtransition metals are HIGH SPIN! Tetrahedral complexes of the heavier transition metals are low Complexes ostrongly M L *M non-bondingt2geg*et2* tvery weakly M L *M non-bonding t 49 oTetrahedral Crystal Field Splittingbarycenter (spherical Field )t2orbitals point more directly at ligands and are point less directly at ligands and are splitting of octahedral fieldLLL t< obecause only 4 ligands and d orbitals point between ligands