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Photochemical and Magnetic Properties of Complex and Nuclear Chemistry

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Photochemical and Magnetic Properties of Complex and Nuclear Chemistry • Programme: MSc in Chemistry • Course Code: MSCHE2001C04 • Course Title: Photochemical and magnetic properties of complex and nuclear chemistry • Unit: Electronic spectra of coordination compounds • Course Coordinator: Dr. Jagannath Roy, Associate Professor, Department of Chemistry, Central University of South Bihar Note: These materials are only for classroom teaching purpose at Central University of South Bihar. All the data/figures/materials are taken from text books, e-books, research articles, wikipedia and other online resources. 1 Course Objectives: 1. To make students understand structure and propeties of inorganic compounds 2. To accuaint the students with the electronic spectroscopy of coordination compounds 3. To introduce the concepts of magnetochemistry among the students for analysing the properties of complexes. 4. To equip the students with necessary skills in photochemical reaction transition metal complexes 5. To develop knowledge among the students on nuclear and radio chemistry Learning Outcomes: After completion of the course, learners will be able to: 1. Analyze the optical/electronic spectra of coordination compounds 2. Make use of the photochemical behaviour of complexes in designing solar cells and other applications. 3. Design and perform photochemical reactions of transition metal complexes 4. Analyze the magnetic properties of complexes 5. Explain the various phenomena taking place in the nucleus 6. Understand the working of nuclear reactors 2 Lecture-1 Lecture Topic- Introduction to the Electronic Spectra of Transition Metal complexes 3 Crystal field theory (CFT) is ideal for d1 (d9) systems but tends to fail for the more common multi- electron systems. This is because of electron-electron repulsions in addition to the crystal field effects on the repulsion of the metal electrons by the ligand electrons. To deal with this we introduce the concept of an electronic state. The Electronic state refers to energy levels available to a group of electrons, just as electronic configurations refer to the way in which the electrons occupy the d orbitals. 1 When describing electronic configurations, lower case letters are used, eg t2g etc. For electronic states, upper case (CAPITAL) letters are used and by analogy, a T state is triply degenerate and an E state is doubly degenerate. Quantum Numbers: n = Principal quantum numbers l = Azimuthal quantum numbers ml = Magnetic quantum numbers ms = Spin quantum numbers L 0 1 2 3 Shape of orbital s p d f No of ml 1 3 5 7 ml is a subset of l, where ml take values from l…. 0….-l. There are thus (2l +1) values of ml for each l value, i.e. for the 1 s orbital (l = 0), 3 p orbitals (l = 1), 5 d orbitals (l = 2), etc. 4 Russell-Saunders (R-S) coupling The ways in which the angular momenta associated with the orbital and spin motions in many- electron-atoms can be combined together are many and varied. In spite of this seeming complexity, the results are frequently readily determined for simple atom systems and are used to characterise the electronic states of atoms. The interactions that can occur are of three types: • spin-spin coupling (S) • orbit-orbit coupling (L) • spin-orbit coupling (J) There are two principal coupling schemes used: • Russell-Saunders (or L - S) coupling • j - j coupling. In the Russell Saunders scheme it is assumed that: • spin-spin coupling > orbit-orbit coupling > spin-orbit coupling. • This is found to give a good approximation for first row transition series where J coupling can generally be ignored, however for elements with atomic number greater than thirty, spin-orbit coupling becomes more significant and the j-j coupling scheme is used. Spin-Spin coupling S - the resultant spin quantum number for a system of electrons. The overall spin S arises from adding the individual ms together and is as a result of coupling of spin quantum numbers for the separate electrons. The total spin is normally reported as the value of 2S + 1, which is called the multiplicity of the term: S = 0 ½ 1 3/2 2 5/2 2S + 1 1 2 3 4 5 6 singlet doublet triplet quartet quintet sextet 5 Orbit-Orbit coupling L - the total orbital angular momentum quantum number defines the energy state for a system of electrons. These states or term letters are represented as follows: By analogy with the notation s, p, d, … for orbitals with l = 0, 1, 2, …, the total orbital angular momentum of an atomic term is denoted by the equivalent upper case letter: Total Orbital Momentum: L 0 1 2 3 4 S P D F G Spin-Orbit coupling Coupling occurs between the resultant spin and orbital momenta of an electron which gives rise to J the total angular momentum quantum number. Multiplicity occurs when several levels are close together and is given by the formula (2S+1). The multiplicity is written as a left superscript representing the value of L and the entire label of a term is called a term symbol. The Russell Saunders term symbol is then written as: (2S+1)L Examples: For a d1 configuration: S= +½, hence (2S+1) = 2 ; L=2 and the ground term is written as 2D (doublet dee) For an s1p1 configuration: S= both +½, hence (2S+1) = 3; or paired hence (2S+1) = 1 ; L=0+1=1 and the terms arising from this are 3P (triplet pee) or 1P (singlet pee) 6 The Ground Terms are deduced by using Hund's Rules 1. The Ground Term will have the maximum multiplicity. 2. If there is more than 1 Term with maximum multipicity, then the Ground Term will have the largest value of L. Thus for a d2 the rules predict 3F < 3P < 1G < 1D < 1S Note the idea of the "hole" approach: Hole formalism in quantum chemistry states that, one may consider systems with e or (n-e), the number of unoccupied sites or “holes”, to be equivalent. A d1 configuration is treated as similar to a d9 configuration. d1 has 1 electron more than d0 and d9 has 1electron less than d10 i.e. a hole. Calculating d-ion Ground Terms dn 2 1 0 -1 -2 L S Ground Term d1 ↑ 2 1/2 2D d2 ↑ ↑ 3 1 3F d3 ↑ ↑ ↑ 3 3/2 4F d4 ↑ ↑ ↑ ↑ 2 2 5D d5 ↑ ↑ ↑ ↑ ↑ 0 5/2 6S d6 ↑↓ ↑ ↑ ↑ ↑ 2 2 5D d7 ↑↓ ↑↓ ↑ ↑ ↑ 3 3/2 4F d8 ↑↓ ↑↓ ↑↓ ↑ ↑ 3 1 3F d9 ↑↓ ↑↓ ↑↓ ↑↓ ↑ 2 1/2 2D The overall result shown in the Table is that: 4 configurations (d1, d4, d6, d9) give rise to D ground terms, 4 configurations (d2, d3, d7, d8) give rise to F ground terms d5 configuration gives an S ground term. 7 Terms for dn free ion configurations The Russell Saunders term symbols for all the free ion configurations are given in the Table below. 8 The ligand field splitting of Transition Metal complex ground terms So far our discussion refer only to free atoms, we will now take it a step further to the domain of transition metal complex ions. The treatment will be restricted to complex ions in an Oh crystal field. The ground term energies for transition metal complexes are affected by the influence of a ligand field. The effect of a ligand field is that: d orbitals split into t2g and eg subsets. By analogy the D ground term states split into T2g and Eg. f orbitals are split to give subsets known as t1g, t2g and a2g. By analogy, the F ground term when split by a crystal field will give states known as T1g, T2g, and A2g. S orbitals are never split, hence S ground terms are also not split. Note that whenever the ground state is an F term there will be a P term found at higher energy with the same spin multiplicity. 9 Lecture-2 n Lecture Topic- Spectroscopic ground states and Splitting of d terms 10 Splitting of dn terms in an octahedral field is represented in the follwing table: Term Components in an octahedral field S A1g P T1g D Eg+T2g F A2g+T1g+T2g G A1g+Eg+T1g+T2g H Eg+T1g+T1g+T2g I A1g+A2g+Eg+T1g+T2g+T2g 11 d1 and d9 case Octahedral Crystal Field The ground state free ion term for this configuration can be calculated as follows: Total orbital angular momentum value = L = Σml = 2 Total spin angular momentum value = S = Σms = 1/2 Spin multiplicity = (2S+1) = (2(1/2)+1) = 2 Thus the free ion ground state term for d1 and d9 configuration comes out to be 2D. Therefore, splitting of d9 case is just the inverse of d1 case in octahedral crystal field 12 d2 and d8 case Octahedral Crystal Field The ground state free ion term for this configuration can be calculated as follows: Total orbital angular momentum value = L = Σml = 2+1 =3 Total spin angular momentum value = S = Σms =2 x ½= 1 Spin multiplicity = (2S+1) = (2 x 1+1) = 3 Thus the free ion ground state term for d2 and d8 configuration comes out to be 3F. Therefore, splitting of d8 case is just the inverse of d2 case in octahedral crystal field. 13 d3 and d7 case Octahedral Crystal Field The ground state free ion term for this configuration can be calculated as follows: Total orbital angular momentum value = L = Σml = 2+1+0=3 Total spin angular momentum value = S = Σms =3 x ½= 3/2 Spin multiplicity = (2S+1) = (2 x 3/2 +1) = 4 Thus the free ion ground state term for d3 and d7 configuration comes out to be 4F. Therefore, splitting of d7 case is just the inverse of d3 case in octahedral crystal field.
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