Advanced Inorganic Chemistry ADVANCED INORGANIC CHEMISTRY UV visible infrared 3 3 A2g → T2g 3 1 A2g → Eg 2+ [Ni(NH3)6] ? υ, cm-1 A MOLECULAR ABSORPTION PROCESSES DVANCED -18 ~10 J I NORGANIC • Electronic transitions • UV and visible wavelengths C • Molecular vibrations HEMISTRY Increasing energy • Thermal infrared wavelengths • Molecular rotations • Microwave and far-IR wavelengths -23 ~10 J • Each of these processes is quantized • Translational kinetic energy of molecules is unquantized A طیفی ترم طیفی DVANCED ELECTRONIC (UV-VISIBLE) SPECTROSCOPY I NORGANIC C HEMISTRY Electronic XPS UPS UV-visible Ferdowsi University of Mashhad 5 A طیفی ترم طیفی DVANCED ELECTRONIC (UV-VISIBLE) SPECTROSCOPY I NORGANIC C HEMISTRY c = n . l With energy of photons E = h . n Ferdowsi University of Mashhad 6 A طیفی ترم طیفی DVANCED UV-visible spectroscopy I ligand p* NORGANIC (1) metal-metal (d-d) transition metal d s* metal-ligand (2) charge transfer (MLCT) C ligand-metal HEMISTRY metal d (LMCT) n ligand p n (3) ligand-centered transition s s s*, n s*, n p*, and p p* Ferdowsi University of Mashhad 7 A طیفی ترم طیفی DVANCED UV-visible spectroscopy I NORGANIC (1) metal-metal (d-d) transition metal-ligand (2) charge transfer (MLCT) C HEMISTRY ligand-metal (LMCT) (3) ligand-centered transition s s*, n s*, n p*, and p p* Ferdowsi University of Mashhad 8 ADVANCED INORGANIC CHEMISTRY 9 Ferdowsi University of Mashhad of University Ferdowsi A DVANCED There are three types of electronic transitions: - p, s, and n electrons - d and f electrons I NORGANIC - charge transfer electrons C single bonds → sigma (s) orbitals → s electrons HEMISTRY double bond → a sigma (s) orbital and a pi (p) molecular orbital Pi orbitals are formed by the parallel overlap of atomic p orbitals Ferdowsi University of Mashhad 10 Selection Rules 1. Spin selection rule: DS = 0 only one electron is involved in any transition allowed transitions: singlet singlet or triplet triplet forbidden transitions: singlet triplet or triplet singlet Changes in spin multiplicity are forbidden • Spin-forbidden transitions – Transitions involving a change in the spin state of the molecule are forbidden – Strongly obeyed – Relaxed by effects that make spin a poor quantum number (heavy atoms) Selection rules 2. Laporte selection rule (or parity rule): there must be a change in the parity (symmetry) of the complex DL = ±1 Electric dipole transition can occur only between states of opposite parity. Laporte-allowed transitions: g u or u g Laporte-forbidden transitions: g g or u u g stands for gerade – compound with a center of symmetry u stands for ungerade – compound without a center of symmetry Selection rules can be relaxed due to: vibronic coupling (interaction between electron and vibrational modes) spin-orbit coupling geometry relaxation during transition • Symmetry-forbidden transitions – Transitions between states of the same parity are forbidden – Particularly important for centro-symmetric molecules (ethene) – Relaxed by coupling of electronic transitions to vibrational transitions (vibronic coupling) A طیفی ترم selection rules DVANCED e electronic transition Laporte allowed (charge transfer) 10000 I NORGANIC (1000—50000) Laporte forbidden (d-d transition) spin allowed; noncentrosymmetiric 100—200 C (200—250) HEMISTRY spin allowed; centrosymmetric 5—100 (20—100) spin forbidden 0.01—1 (< 1) Ferdowsi University of Mashhad 15 The Selection rules for electronic transitions Charge-transfer band – Laporte and spin allowed – very intense 3 1 A2g → Eg Laporte and spin forbidden – very weak a, b, and c, Laporte 2+ a [Ni(H2O)6] forbidden, spin allowed, inter- mediate intensity 3 3 A2g → T2g b c ADVANCED INORGANIC CHEMISTRY A طیفی ترم طیفی DVANCED I NORGANIC C HEMISTRY 2- [CoCl4] 2+ [Co(H2O)6] 2+ [Mn(H2O)6] Ferdowsi University of Mashhad 18 d-d transition crystal field splitting A DVANCED I NORGANIC C HEMISTRY Do size and charge of the metal ion and ligands 4d metal ~50% larger than 3d metal 5d metal ~25% larger than 4d metal 5d > 4d > 3d 19 d-d transition crystal field splitting A DVANCED I NORGANIC C HEMISTRY crystal field stabilization energy (CFSE) spin-pairing energy 4 7 high-spin/low spin configuration d ~ d 4 d 20 A طیفی ترم طیفی Tetrahedral DVANCED I NORGANIC Dt = 4/9 Do tetrahedron octahedron elongated square C octahedron planar HEMISTRY Ferdowsi University of Mashhad 21 ADVANCED INORGANIC CHEMISTRY 22 ترم طیفی Ferdowsi University of Mashhad of University Ferdowsi ADVANCED INORGANIC CHEMISTRY 23 ترم طیفی Ferdowsi University of Mashhad of University Ferdowsi A DVANCED Crystal Field Theory An energy diagram of the orbitals shows all five d orbitals I are higher in energy in the forming complex than in the free NORGANIC metal ion, because of the repulsions from the approaching ligands Crystal Field Splitting Energy C HEMISTRY Forming Complex Ligand field theory combines an electrostatic model of metal-ligand interactions (crystal field theory) and a covalent model (molecular orbital theory). 24 OH TD Octahedral 3d Complexes Δo ≈ P (pairing energy) Both low-spin (Δo ≤ P) and high-spin (P ≥ Δo ) complexes are found Tetrahedral Complexes ΔTd = 4/9 Δo hence P >> ΔTd and tetrahedral complexes are always high spin ELECTRONIC STRUCTURE OF HIGH-SPIN AND LOW-SPIN OH COMPLEXES NOTE: SOME FACTORS INFLUENCING THE MAGNITUDE OF Δ-SPLITTING Oxidation State 3+ 2+ Δo (M ) > Δo(M ) e.g. Δo for Fe(III) > Fe(II). The higher oxidation state is likely to be low-spin 5d > 4d >3d e.g. Os(II) > Ru(II) > Fe(II) All 5d and 4d complexes are low-spin. A DVANCED Crystal Field Theory I *Crystal Field Splitting Energy - The d orbital energies are NORGANIC “split” with the two dx2-y2 and dz2 orbitals (eg orbital set) higher in energy than the dxy, dxz, and dyz orbitals (t2g orbital set) C HEMISTRY *The energies of the d orbitals in different environments determines the magnetic and electronic spectral properties of transition metal complexes. *Strong-field ligands, such as CN- lead to larger splitting energy *Weak-field ligands such as H2O lead to smaller splitting energy 30 A DVANCED Crystal Field Theory Explaining the Colors of Transition Metals I NORGANIC Diversity in colors is determined by the energy difference (D) between the t2g and eg orbital sets in complex ions C HEMISTRY When the ions absorbs light in the visible range, electrons move from the lower energy t2g level to the higher eg level, i.e., they are “excited” and jump to a higher energy level D E electron = Ephoton = hv = hc/l The substance has a “color” because only certain wavelengths of the incoming white light are absorbed 11/21/2012 31 A DVANCED Crystal Field Theory Example – Consider the [Ti(H O) ]3+ ion – Purple in 2 6 I aqueous solution NORGANIC Hydrated Ti3+ is a d1 ion, with the d electron in one of the three lower energy t2g orbitals The energy difference (DA) between the t2g and eg orbitals C corresponds to the energy of photons spanning the green HEMISTRY and yellow range These colors are absorbed and the electron jumps to one of the eg orbitals Red, blue, and violet light are transmitted as purple 32 A DVANCED Crystal Field Theory For a given “ligand”, the color depends on the oxidation I NORGANIC state of the metal ion – the number of “d” orbital electrons available 2+ A solution of [V(H O) ] ion is violet 2 6 C 3+ HEMISTRY A solution of [V(H2O)6] ion is yellow For a given “metal”, the color depends on the ligand 3+ [Cr(NH3)6] (yellow-orange) 2+ [Cr(NH3)5] (Purple) Even a single ligand is enough to change the color 33 A DVANCED Crystal Field Theory Spectrochemical Series I The Spectrochemical Series is a ranking of ligands with NORGANIC regard to their ability to split d-orbital energies For a given ligand, the color depends on the oxidation state of the metal ion C For a given metal ion, the color depends on the ligand HEMISTRY As the crystal field strength of the ligand increases, the splitting energy (D) increases (shorter wavelengths of light must be absorbed to excite the electrons 34 MAGNETIC PROPERTIES OF TRANSITION METAL COMPLEXES A DVANCED The splitting of energy levels influence magnetic properties I Affects the number of unpaired electrons in the metal ion NORGANIC “d” orbitals According to Hund’s rules, electrons occupy orbitals one at a time as long as orbitals of “equal energy” are available C HEMISTRY When “all” lower energy orbitals are “half-filled (all +½ spin state)”, the next electron can Enter a half-filled orbital and pair up (with a –½ spin state electron) by overcoming a repulsive pairing energy (Epairing) or Enter an empty, higher energy orbital by overcoming the crystal field splitting energy (D) The relative sizes of Epairing and (D) determine the occupancy of the d orbitals 36 MAGNETIC PROPERTIES OF TRANSITION METAL COMPLEXES A DVANCED The occupancy of “d” orbitals, in turn, determines the I number of unpaired electrons, thus, the paramagnetic NORGANIC behavior of the ion Ex. Mn2+ ion ([Ar] 3d5) has 5 unpaired electrons in 3d orbitals of equal energy C In an octahedral field of ligands, the orbital energies split HEMISTRY The orbital occupancy is affected in two ways: Weak-Field ligands (low D) and High-Spin complexes Strong-Field ligands (high D) and Low-Spin complexes (from spectrochemical series) 37 A DVANCED Crystal Field Theory Explanation of Magnetic Properties I Weak -Field ligands and High-Spin complexes NORGANIC 2+ 2+ 5 Ex. [Mn(H2O)6] Mn ([Ar] 3d ) A weak-field ligand, such as H2O, has a “small” crystal field splitting energy (D) C HEMISTRY It takes less energy for “d” electrons to move to the “eg” set (remaining unpaired) rather than pairing up in the “t2g” set with its higher repulsive pairing energy (Epairing) Thus, the number of unpaired electrons in a weak-field ligand complex is the same as in the free ion Weak-Field Ligands create high-spin complexes, those with a maximum of unpaired electrons Generally Paramagnetic 38 T C C E T OUNDS ON DINATI NTS ITION HEIR LEME RANS OMP OOR & A DVANCED Crystal Field Theory Explanation of Magnetic Properties I NORGANIC Strong -Field Ligands and Low-Spin Complexes 4- Ex.