Transition Metals utilize their valence d-orbitals to form coordination complexes, which have characteristics important to industry, technology, and medicine. Coordination complexes exist in every color of the rainbow and can be found in jewelry, steel, paints, anticancer drugs, and photographic films. Most catalysts contain transition metal complexes and they are commonly used in the pharmaceutical industry. There two areas are vitally important to research chemists and are rapidly growing. A better understanding of the fundamentals of coordination complexes will help you understand current materials and will help chemists improve how these materials function.
In the early days, chemists were fascinated by transition metal complexes due to their color. The color of these complexes depends heavily on the d electron count of the transition metal, and the energy levels in the complex ions.
23.1 Transition Metals and Coordination Complexes
Being able to properly interpret the properties of the electromagnetic spectrum will allow us to interpret how our eyes detect color.
23.1 The Electromagnetic Spectrum and Color
The discreet nature of the energy levels in molecules allows us to determine the energy it would take to excite an electron from an energy level of lower energy to one of higher energy. If this excitation falls in the visible range of the electromagnetic spectrum, our eye will detect color.
23.1 Orbital Energies
Chemists are able to measure the wavelength of light a material absorbs using a UV-Vis Spectrometer, and by measuring which wavelengths of radiation are absorbed and which are not, we can gain valuable information on a molecules electronic excitations, or electronic structure.
23.1 UV-Vis Spectroscopy
23.1 UV-Vis Spectroscopy (cont.)
Keep in mind that a transition metal atom has very different properties than when it is the central atom in a coordination complex. This is due to the difference in how the electrons fill the orbitals to achieve the lowest energy configuration possible.
23.1 Electron Configurations of Transition Metal Complexes
Coordination Compounds consist of a transition metal complex ion (transition metal center with ligands attached) and counter ions. The relative energies of the d orbitals in these transition metal complexes lead to some unique physical properties, including color and magnetism.
23.2 Chemistry of Coordination Complexes
In 1893 Alfred Werner proposed a theory that successfully explaining the difference in color for various transition metal complexes. This theory is still used today, but before we go into the modern day theory, lets take a look into how Werner developed his theory.
23.2 Coordination Complexes Before 1893
23.2 Modern Day Formulas for Transition Metal Complexes
Once the modern day formulas were introduced, chemists began to investigate how ligands surrounded the transition metal center. The term isomer is given to complexes that have the same overall composition, but different structures. This prompted chemists to introduce two new terms: coordination sphere and coordination number.
23.4 Arranging Ligands Around the Transition Metal Center: Introducing Isomers
Once Werner showed coordination complexes could exist as isomers, chemists began to investigate various isomers and in this class we will cover two types of structural isomers (linkage and coordination sphere) and two types of stereo isomers (geometric and optical).
23.4 Isomer Overview
23.4 Linkage Isomers
23.4 Coordination Sphere Isomers
23.4 Geometric Isomers
23.4 Optical Isomers
At this point we really havn’t scratched the surface of why a transition metal wants to form a bond with a ligand.
23.2 Complex Formation: The Metal-Ligand Bond
When you are given a transition metal complex you will need to know how to determine its oxidation state, its coordination number, and its geometry.
23.2 Transition Metal Complexes: Oxidation States, Coordination Number, and Geometry
When a transition metal complex has a coordination number of 4 it can exist as either a tetrahedral or square planar molecule. There are a set of empirical observations that can allow you to predict whether a complex with a coordination number of 4 will exist as a tetrahedral molecule or a square planar molecule.
23.2 Determining if an ML4 complex is Td or Square Planar.
23.2 Example Problem: Determining if an ML4 complex is Td or Square Planar.
Transition metal complexes bond to a variety of surrounding atoms/molecules which we refer to as ligands. The nature of the ligand that binds to the transition metal center has a large influence on the properties of the transition metal complex.
We have discussed that transition metal complexes exhibit a wide variety of colors and have varying magnetic properties. In order to explain the similarities and differences in these properties we need to look into the bonding theories of transition metal complexes.
23.6 Bonding Theories of Transition Metal Complexes
23.6 Crystal Field Theory
In order to understand the bonding theories of transition metal complexes you need to visualize how the ligands interact with the 5 different d orbitals. You cannot visualize how the d orbitals interact with the ligands if you do not know the shapes of the d orbitals like the back of your hand.
23.6 Shapes of d Orbitals
By observing the colors of various transition metal complexes, we can start to see how the nature of the ligand bound to the transition metal center plays a big role in determining the resulting color of a transition metal complex.
23.6 Orbital Overlap and Orbital Energies in Crystal Field Theory
23.6 Crystal Field Theory
23.6 The Spectrochemical Series
23.6 High Spin/Low Spin Co Complexes
23.6 Octahedral Field Splitting vs. Tetrahedral Field Splitting
23.6 Octahedral Field Splitting vs. Square Planar Field Splitting
23.3 The Chelate Effect