Orbitals and energitics
RDCH 702 Lecture 4: Orbitals and energeticsMolecular
symmetryBonding and structureMolecular orbital theoryCrystal field
theoryLigand field theory
Provide fundamental understanding of chemistry dictating
radionuclide complexes
Structure based on bondingCoordination important in defining
structureStructure related to spectroscopic behaviorElectron
configuration important in structured8 are square planard0 and d10
tetrahedral
4-#
Molecular symmetryEvaluation of point groupsDescription of
symmetry present in moleculesAxisPlanesInversionRotationUse with
group theory to determine spectroscopic properties
4-#
Method for determining molecular symmetry and point group
Use molecular geometry to follow chain
4-#
Non-linear, less than 2 unique C3 axis
4-#Identifying Point Group
Cs,C2,C3,D3,C2v,C3v,C3hC3D3h
4-#Point GroupsH2O point groupC2v symmetrySymmetry elementsE, C2
(180 rotation), 2 vertical mirror planes (sv)E, C2, sv, svNH3 point
groupC3v point groupElementsE, C3 (each N-H), three vertical mirror
plane through each N-H (3sv)E, C3, 3svApply to identification
tree
4-#Symmetry and spectroscopya1 vibration generates a changing
dipole moment in the z-directionb1 vibration generates a changing
dipole moment in the x-direction b2 vibration generates a changing
dipole moment in the y-directiona2 vibration does not generate a
changing dipole moment in any direction (no x, y or z in the a2
row). a1, b1 and b2 vibrations provide changes dipole moments and
are IR activea2 vibrations have no dipole moments IR
inactiveC2vEC2v (xz)v (yz) A11111zx2, y2, z2A211-1-1RzxyB11-11-1x,
RyxzB21-1-11y, Rxyz.
4-#Coordination numberGeometry strongly influence by
coordination numberCan assess information on potential structure
and geometry from coordination number (CN) CN=5Interconvertibility
between geometriesCompounds can vary between shapesTrigonal
bipyramid seems to be more commonCommon with metal pentachloride
species
PuOONHHHCNGeometries2Linear (Dh)Bent (C2v)3Planar (D3h)Pyramidal
(C3v)Some T-shaped forms (C2v)4Tetrahedral (Td)Square geometry
(C4h)One lone pair (C2v)5Trigonal bipyramid (D3h)Square pyramid
(C4v)BHHH CHHHHXeFFFF CuClClClClCl InClClClClCl
4-#Coordination NumberCoordination number 6Very common
coordination numberLigands at vertices of octahedron or distorted
octahedronOctahedron (Oh)Tetragonal octahedron (D4h)Elongated or
contracted long z axisRhombic (D2h)Changes along 2 axisTrigonal
distortion (D3d)
Oh ->D4hOh ->D2hOh ->D3dor
4-#Higher coordination: Relevant for actinides
7 coordinationPentagonal Bipyramidal, Capped Trigonal Prismatic
and Capped Octahedral.
8 coordination: Cubic structure, the Square Antiprism,
Dodecahedron
9 coordination: Tricapped trigonal
prismatichttp://www.d.umn.edu/~pkiprof/ChemWebV2/Coordination/CN8.html
4-#Hard and soft metals and ligandsBased on Lewis acid
definitionLigand acts as basedonates electron pair to metal ionHard
metal ion interact with hard basesHard ligands N, O, FSoft ligands
P, S, ClLigand hardness decreases down a groupHard metalsHigh
positive chargesSmall radiiClosed shells or half filled
configurationsSoft metalsLow positive chargesLarge ionic
radiusNon-closed shell configurationsTend to be on right side of
transition seriesLanthanides and actinides are hardActinides are
softer than lanthanidesLigands with soft groups can be used for
actinide/lanthanide separations
4-#
Hard
Intermediate
Soft
4-#Chelation and stabilityLigands with more than 1 complexing
functional groupCarbonate, ethylenediamineEnhanced stability
through chelation effectethylenediamine binding stronger than 2
ammonia groupsBidentateTridentateLigands can wrap around metal ion
forming stronger complex
4-#Effective atomic numberMetal bonding can be described with
effective atomic numberNumber of electrons surrounding metal is
effective atomic numberTransitions metal have 9 possible bonds5 d,
3p, 1 s18 electronsPossible to have effective atomic number
different than 18Few d electronsElectronegative ligands
4-#Effective atomic number16 electron Square planard8
configuration (Au, Pt)Greater than 18 electron8-10 d electrons
Expand metal-ligand interactions to exploit bonding and
geometryMolecular orbital theoryCrystal field theoryLigand field
theory
4-#Molecular orbital theoryMolecular orbitals are comprised from
the overlap of atomic orbitalsNumber of molecular orbitals equals
the number of combined atomic orbitalsDifferent type of molecular
orbitals bonding orbital (lower energy) Non-bonding (same energy as
atomic orbitals)Anti-bonding orbital (higher energy)Electrons enter
the lowest orbital available maximum number of electrons in an
orbital is 2 (Pauli Exclusion Principle) Electrons spread out
before pairing up (Hund's Rule)
4-#Molecular orbital
4-#Molecular orbitalsSigma, Pi, deltaGerade and ungeradeN
molecular orbitals from N atomic orbitalsN=8 in period 24 sigma, 4
PiPi degenerate bonding and antibonding
O and F
Li to N
4-#Molecular orbitalsMixture of different atomsSome bonding
characteristics dominateNonbonding orbitalsNo Pi from HHigh
occupied electron orbitalLowest unoccupied electron orbitalBond
orderOverall shared electronB=0.5(n-n*)
4-#Symmetry adapted orbitalsCombination of orbitals with
symmetry considerationsIf molecule has symmetry degenerate atomic
orbitals with similar atomic energy can be grouped in linear
combinationsgroups are known assymmetry-adapted linear
combinations
4-#20Crystal Field TheoryBehavior of electrons with
ligandschanges degenerate statesd and f electronsLone pair modeled
as pointRepels electrons in d or f orbitald orbitals have energy
differences due to pointResults in ligand field splittingAbout 10 %
of metal-ligand interactione and t orbitalsIgnores covalent
contributionEnergy difference is ligand field splitting parameter
(o)Can be determined from absorption spectrumeg t2g transition
4-#
Crystal Field TheoryTi(OH2)63+Absorbance at 500 nm, 20000
cm-11000 cm-1 = 11.96 kJ/molD0=239.2 kJ/mol D0 found to vary with
ligandFor metal ion increases with oxidation state and increases
down a groupI- < Br- < SCN- ~Cl- < F- < OH- ~ ONO-