Crystal Field Theory Focus: energies of the d orbitals Assumptions 1. Ligands: negative point charges 2. Metal-ligand bonding: entirely ionic strong-field (low-spin): large splitting of d orbitals weak-field (high-spin): small splitting of d orbitals

_ _ _ _ _ _ _ _ _ _ Octahedral crystal field d orbital energy levels  metal ion in octahedral complex Octahedral crystal field d orbital energy levels dz2 dx2- y2 _ _  _ _ _ E dxy dxz dyz isolated metal ion _ _ _ _ _ Metal ion and the nature of the ligand determines  d-orbitals

_ _ _ _ _ _ _ _ _ _ d-orbital energy level diagram for tetrahedral  dyz dxz dxy _ _ _  _ _ E dz2 dx2- y2 _ _ _ _ _ isolated metal ion d-orbitals only high spin

_ _ _ _ _ d-orbital energy level diagram square planar dx2- y2 dxy dz2 __ dx2- y2 __ dxy __ dz2 E __ __ _ _ _ _ _ isolated metal ion d-orbitals dxz dyz only low spin

square planar Examples: Pd2+, Pt2+, Ir+, and Au3+. Crystal-Field Theory square planar Examples: Pd2+, Pt2+, Ir+, and Au3+.

Tetrahedral Complexes

High spin Low spin

Spectrochemical Series: An order of ligand field strength based on experiment: Weak Field I-  Br- S2- SCN- Cl- NO3- F-  C2O42- H2O NCS- CH3CN NH3 en  bipy phen NO2- PPh3 CN- CO Strong Field

Colors of Transition Metal Complexes Compounds/complexes that have color: absorb specific wavelengths of visible light (400 –700 nm) wavelengths not absorbed are transmitted and appear as color

Color and Magnetism Color Color of a complex depends on; (i) the metal, (ii) its oxidation state & (iii) ligands (i.e., everything) For example, pale blue [Cu(H2O)6]2+ versus dark blue [Cu(NH3)6]2+. Partially filled d orbitals usually give rise to colored complexes because they can absorb light from the visible region of the spectrum.

Color and Magnetism Color

Visible Spectrum wavelength, nm 400 nm 700 nm higher energy (Each wavelength corresponds to a different color) 400 nm 700 nm higher energy lower energy White = all the colors (wavelengths)

Complexes and Color The larger the gap, the shorter the wavelength of light absorbed by electrons jumping from a lower-energy orbital to a higher one.

Absorbs in green yellow. Looks purple. [Ti(H2O)6]3+ Absorbs in green yellow. Looks purple. Figure: 24-26 Title: The color of [Ti(H2O)6]3+. Caption: (a) A solution containing the [Ti(H2O)6]3+ ion. (b) The visible absorption spectrum of the [Ti(H2O)6]3+ ion.

the spectrum for [Ti(H2O)6]3+ has a maximum absorption at 510 nm Absorbs green & yellow, transmits all other wavelengths, the complex is purple.

Crystal-Field Theory [Ti(H2O)6]3+

Electronic Configurations of Transition Metal Complexes d orbital occupancy depends on  and pairing energy, P e-’s assume the electron configuration with the lowest possible energy cost If  > P ( large; strong field ligand) e-’s pair up in lower energy d subshell first If  < P ( small; weak field ligand) e-’s spread out among all d orbitals before any pair up

d-orbital energy level diagrams octahedral complex

d-orbital energy level diagrams octahedral complex

d-orbital energy level diagrams octahedral complex

d-orbital energy level diagrams octahedral complex low spin  > P high spin  < P

d-orbital energy level diagrams octahedral complex low spin  > P high spin  < P

d-orbital energy level diagrams octahedral complex low spin  > P high spin  < P

d-orbital energy level diagrams octahedral complex low spin  > P high spin  < P

d-orbital energy level diagrams octahedral complex

d-orbital energy level diagrams octahedral complex

d-orbital energy level diagrams octahedral complex

Coordination complexes: isomers Isomers: same atomic composition, different structures Different composition! We’ll check out the following types of isomers: Hydrate Linkage Cis-trans Optical (Enantiomers)

Hydrate isomers: Water in outer sphere (water that is part of solvent) Water in the inner sphere water (water is a ligand in the coordination sphere of the metal)

Structural Isomerism 1 Coordination isomerism: Composition of the complex ion varies. [Cr(NH3)5SO4]Br and [Cr(NH3)5Br]SO4

Coordination-Sphere Isomers Example [Co(NH3)5Cl]Br vs. [Co(NH3)5Br]Cl Consider ionization in water [Co(NH3)5Cl]Br  [Co(NH3)5Cl]+ + Br- [Co(NH3)5Br]Cl  [Co(NH3)5Br]+ + Cl-

Structural Isomerism 2 Ligand isomerism: Same complex ion structure but point of attachment of at least one of the ligands differs. [Co(NH3)4(NO2)Cl]Cl and [Co(NH3)4(ONO)Cl]Cl

Linkage Isomers

Linkage isomers Example: Bonding to metal may occur at the S or the N atom Bonding occurs from N atom to metal Bonding occurs from S atom to metal

Linkage Isomers [Co(NH3)5(NO2)]Cl2 [Co(NH3)5(ONO)]Cl2 Pentaamminenitrocobalt(III) chloride [Co(NH3)5(ONO)]Cl2 Pentaamminenitritocobalt(III) chloride

Stereoisomers Stereoisomers Geometric isomers Isomers that have the same bonds, but different spatial arrangements Geometric isomers Differ in the spatial arrangements of the ligands

Stereoisomerism 1 Geometric isomerism (cis-trans): Atoms or groups arranged differently spatially relative to metal ion Pt(NH3)2Cl2

Geometric Isomers cis isomer trans isomer Pt(NH3)2Cl2

Geometric Isomers cis isomer trans isomer [Co(H2O)4Cl2]+

Stereoisomers: geometric isomers (cis and trans) Cl- Cl-

Stereoisomers Optical isomers isomers that are nonsuperimposable mirror images said to be “chiral” (handed) referred to as enantiomers A substance is “chiral” if it does not have a “plane of symmetry”

Stereoisomerism 2 Optical isomerism: Have opposite effects on plane-polarized light (no superimposable mirror images)

Two coordination complexes which are enantiomers

Chirality: the absence of a plane of symmetry Enantiomers are possible A molecule possessing a plane of symmetry is achiral and a superimposible on its mirror image Enantiomers are NOT possible Are the following chiral or achiral structures? No plane of symmetry Chiral (two enantiomer) Plane of symmetry Achiral (one structure)

Enantiomers: non superimposable mirror images A structure is termed chiral if it is not superimposable on its mirror image Mirror image Of structure Structure Two chiral structures: non superimposable mirror images: Enantiomers!

Which are enantiomers (non-superimposable mirror images) and which are identical (superimposable mirror images)?

Mirror images [Co(en)3] 5 1 5 3 2 3 4 1 6 4 2 6

Enantiomers: non superimposable mirror images A structure is termed chiral if it is not superimposable on its mirror image Mirror image Of structure Structure Two chiral structures: non superimposable mirror images: Enantiomers!

Enantiomers A molecule or ion that exists as a pair of enantiomers is said to be chiral.

Figure: 24-20-01UNEx6 Title: Sample Exercise 24.6 Caption: trans-[Co(en)2Cl2]+.

Figure: 24-20-02UNEx6 Title: Sample Exercise 24.6 Caption: cis-[Co(en)2Cl2]+.