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Bonding in Coordination Compounds Crystal Field Theory Assume that metal ion and ligands interact electrostatically -- metal ion exists in a field created by ligands electrons. This field affects the energy of the metal's d orbitals. In absence of ligands (applying crystal field), d orbitals are degenerate (have the same energy) ___ ___ ___ ___ ___ dxy dxz dyz dz2 dx2-y2 In the presence of the ligands, orbitals interact with ligands differently, depending on the geometry of the orbital and the ligands. Interaction depends on the geometrical arrangement of ligands about the central metal ion d-Orbital Splitting in Octahedral Complexes Draw shapes of d orbitals How will these orbitals interact with electrons from the ligands at an octahedral geometry? Draw splitting of d orbitals in octahedral field. Splitting in Octahedral Complexes: Size of depends on ________________________________________________. Ligands, which interact strongly with the metal: cyanide, ammonia, _________ Ligands, which interact weakly with the metal: water, chlorine,______________________________ In order to determine properties of transition metal complexes, we must place electrons in the orbitals Electron counting for transition metal ions: 1. Determine the oxidation state of the metal. 2. Determine the number of d electrons by subtracting the oxidation state from the number of s and d electrons in the neutral metal. Fe(O) has 2 4sand 6 3d electrons. If ion is +3, Fe(III) has 5 3d electrons Placing electrons into orbitals. Remember, orbitals are split by the ligand field. For Fe(III), there are two possible arrangements of electrons: ___ ___eg ___ ___eg ___ ___ ___t2g ___ ___ ___ t2g Arrangement depends on the magnitude of If is large, electrons will pair up in t2g orbitals, complex is low spin. (6 3d electrons for half-filled stability) If is small, electrons will occupy eg orbitals instead of pairing up, complex is high spin. (5 3d electrons are needed to achieve half-filled stability) Magnetism Note differences in magnetic properties (magnetism) for the two complexes above. Low spin complexes have the minimum number of unpaired electrons. High spin complexes have the maximum number of unpaired electrons. How can we predict if a complex will be high spin or low spin? Based on the value for . Crude Estimation of Can approximate value for using the color of a complex. Colors of Transition Metal Complexes Visible light: 400-700 nm Color of substance is color of light transmitted. Light absorbed is complement of color of light transmitted. Draw color wheel here. How does the color of the compound related to the color of light transmitted? Visible light is absorbed by transition metal complexes. This is why they appear colored. Light absorbed by octahedral complexes causes electrons in the t2g orbitals to be promoted to the eg orbitals. Color of absorbed light indicates spacing of orbitals (magnitude of ). What happens if is large? What happens if is small? Complexes with high values of tend to appear _____________________________, complexes with low values of tend to appear _______________________________. You have 2 complexes using the same metal, but one has six NH3 ligands and the other has six H2O ligands. The ammine complex is orange and the aqua complex is yellow. Which complex has the highest value? Explain. The orange complex absorbs higher energy blue light while the yellow complex absorbs the lower energy indigo light. Thus, the crystal field splitting of the ammine complex is larger. NH3 is a stronger ligand than water. Remember, reflects how strongly the ligand interacts with the metal's d orbitals. Can determine the ranking of ligands from average values for a number of complexes. This ranking is called the spectrochemical series Spectrochemical Series 2- X- < C2O4 < H2O < NH3 =en < phen < CN- small large weak field strong field Ionization energies generally increase from left to right across a transition series, with some exceptions. This trend correlates with an increase in effective nuclear charge and a decrease in atomic radius. The transition elements differ from most main-group metals in their larger number of oxidation states. Note the following important trends: Ions that have a transition metal in a high oxidation state tend to be good oxidizing agents. Early transition metal ions with the metal in a low oxidation state are good reducing agents. Divalent ions of later transition metals (on the right side of the periodic table) are poor reducing agent because of larger Zeff. Coordination Chemistry A coordination compound is a compound in which a central metal atom or ion is attached to a group of surrounding molecules or ions by coordinate covalent bonds. The molecules or ions that surround the central metal ion are called ligands, and the atoms that are attached directly to the metal are called donor atoms. The term metal complex, or simply complex, refers to neutral molecules such as Pt(NH3)2Cl2, and to complex anions such as [Fe(CN)6]3–, and complex cations such as [Ni(NH3)6]2+. The number of donor atoms that surround a metal ion in a complex is called the coordination compound of the metal. The most common coordination numbers are 4 and 6. The coordination number of a metal in a particular complex depends on the metal ion's size, the ion's charge and electron configuration, and on the size and shape of the ligands. The geometry of a metal complex depends on the coordination number. The charge on a complex is equal to the charge on the metal ion plus the sum of the charges on the ligands. All ligands are Lewis bases and have at least one lone pair of electrons available to form a coordinate covalent bond. Ligands can be classified by the number of donor atoms that bond to the metal. Monodentate ligands have a single donor atom, H2O, NH3, Cl–, CN–, CO, SCN–, OH–. Bidentate ligands have two donor atoms, C2O42-, NH2CH2CH2NH2 (ethylenediamine, en). Polydentate ligands are also known as chelating agents. The attachment of ligands to the metal ion resembles a claw grasping an object. Because of their complexity, a set of rules is used to name coordination compounds. 1. If the compound is a salt, name the cation first and then the anion. K2[Fe(CN)6] is potassium hexacyanoferrate (III). 2. Name the ligands before the metal. Names of anionic ligands end in -o. (Note the changes in anion endings from -ide to -o and from -ate to -ato) Notice that the names of the ligands differ slightly from their chemical names; water as a ligand is called "aquo," and so on. The name of a complex is one word, with no space between the ligand names and no space between the names of the last ligand and the metal. 3. If the complex contains more than one ligand of a particular type, use Greek prefixes (di-, tri-, tetra-, etc.) to indicate how many of each ligand are present. The ligands are listed in alphabetical order, and the prefixes are ignored in determining the order. For example, the name for [Cr(H2O)4Cl2]Cl is tetraaquadichlorochromium (III) chloride. 4. If the name of a ligand contains a Greek prefix (such as ethylenediamine), put the ligand name in parentheses and use an alternative prefix to specify the number of ligands, such as bis- (2), tris-(3), tetrakis-(4), and so forth. e.g.[Co(en)3]Cl3 is named tris(ethylenediamine)cobalt(III) chloride. 5. A Roman numeral in parentheses follows the name of the metal to indicate the metal's oxidation state. 6. For an anionic complex, name the metal using the ending -ate. Isomers Compounds with the same formula but a different arrangement of atoms are called isomers. A. Structural isomers have different connections among their constituent atoms. (1) Linkage isomers occur when a ligand can bond to a metal through either of two donor atoms. (2) Ionization isomers differ with respect to the anion that is bonded to the metal ion. [Cr(H2O)3Cl3].H2O and [Cr(H2O)4Cl2]Cl. Each isomer yields different ions in solution. B. Stereoisomers have the same connections among atoms, but a different arrangement of atoms in space. In coordination chemistry there are 2 kinds of stereoisomers: enantiomers and diastereoisomers. Diastereoisomers (also called geometric isomers) have different spatial arrangements in the metal–ligand bonds. In the cis isomer, identical ligands occupy adjacent corners of the square, whereas in the trans isomer the identical ligands are across from each other. In general, cis-trans isomers can exist for: Square planar complexes of the type MA2B2 and MA2CB (where M is a metal ion with ligands A, B, and C) Octahedral complexes MA4B2 Enantiomers are molecules or ions that are non-identical mirror images. Objects having a handedness are said to be chiral. An object that has a symmetry plane with identical mirror image halves is achiral. The physical properties of enantiomers are identical except for their reactions with other chiral substances and their effect on plane-polarized light. Enantiomers are sometimes called optical isomers because of their ability to rotate plane-polarized light. The labels (+) and (–) are used to indicate the direction of rotation. A 50:50 mixture of both isomers is called a racemic mixture, and produces no net optical rotation. Colors Most transition metal complexes exhibit beautiful colors that depend on the identity of both the ligands and the metal. When white light strikes a colored substance, some wavelengths are transmitted while others are absorbed. A metal complex absorbs light by undergoing an electronic transition from its ground (lowest) energy state to an excited (higher) energy state. The wavelength of light absorbed depends on the energy separation (ΔE) between the two states. An absorbance spectrum plots the absorbance (amount of light absorbed by a substance) as a function of wavelength. According to valence bond theory coordinate covalent bonds arise in metal complexes when a filled ligand orbital containing a pair of electrons overlaps a vacant hybrid orbital on a metal. The hybrid orbitals that a metal ion uses to interact with ligands are those that point in the direction of the ligands (for maximum overlap). Crystal field theory is a model of the bonding in complexes. Ion–dipole attractions are responsible for the stability of complexes formed with neutral dipolar ligands. The crystal field theory enables us to explore the origin of color in complexes. Incoming ligands interact with the metal ion's d orbitals. The d orbitals are repelled by the negatively charged ligands to different degrees. The dz2 and dx2-y2 orbitals, which interact most directly with the incoming ligands, are raised to a higher energy level than the dxy, dxz , and dyz orbitals that point between the ligands. The energy splitting between the two sets of d orbitals is called crystal field splitting, abbreviated Δ. The energy gap between the three lower-energy d orbitals and the two higher-energy orbitals corresponds to wavelengths of visible light. The colors of complexes can be attributed to transitions between the lower- and higher-energy sets of d orbitals. The wavelength of light absorbed depends on the magnitude of the crystal field splitting, which in turn depends on the nature of the ligands. A spectrochemical series lists ligands in order of increasing crystal field splitting. Ligands that produce a relatively small value of Δ are called weak-field ligands, while ligands that produce a relatively large value of Δ are known as strong-field ligands.
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