Transition Metal Chemistry and Coordination Compounds

Alfred Werner FRENCH-BORN SWISS CHEMIST 1866 19191919 Winner of the 1913 Nobel Prize in chemistry, "in recognition of his work on the linkage of atoms in molecules by which he has thrown new light on earlier investigations and opened up new fields of research especially in inorganic chemistry." Transition Metal Chemistry and Coordination Compounds Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. BRIEF CHEM 1B REVIEW Transition Metals 3 1

Properties of the Transition Metals All transition metals are metals, whereas main-group elements in each period change from metal to nonmetal. Many transition metal compounds are colored and paramagnetic, whereas most main-group ionic compounds are colorless and diamagnetic. The properties of transition metal compounds are related to the electron configuration of the metal ion. 4 The Transition Metals 5 6 2

7 8 Writing Electron Configurations of Transition Metal Atoms and Ions PROBLEM: Write condensed electron configurations for the following: (a) Zr; (b) V 3+ ; (c) Mo 3+. (Assume that elements in higher periods behave like those in Period 4.) PLAN: We locate the element in the periodic table and count its position in the respective transition series. These elements are in Periods 4 and 5, so the general electron configuration is [noble gas]ns 2 (n 1)d x. For the ions, we call that ns electrons are lost first. SOLUTION: (a) Zr is the second element in the 4d series: [Kr]5s 2 4d 2 9 3

(b) V is the third element in the 3d series, so its configuration is [Ar]4s 2 3d 3. When it forms V 3+, it loses the two 4s e - first, then one of the 3d e - : [Ar]3d 2 (c) Mo lies below Cr in group 6B(6), so we expect the same exception as for Cr. The configuration for Mo is therefore [Kr]5s 1 4d 5. Formation of the Mo 3+ ion occurs by loss of the single 5s electron followed by two 4d electrons: [Kr]4d 3 10 11 12 4

Aqueous oxoanions of transition elements. +2 +6 +7 Mn 2+ MnO 2 4 MnO 4 The highest oxidation state for Mn equals its group number. +5 +6 +7 VO 3 4 Cr 2 O 2 7 MnO 4 Transition metal ions are often highly colored. 13 Oxidation States of the 1 st Row Transition Metals (most stable oxidation numbers are shown in red) 14 Ionization Energies for the 1 st Row Transition Metals 15 5

Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper 16 Colors of representative compounds of the Period 4 transition metals. titanium(iv) oxide sodium chromate potassium nickel(ii) nitrate zinc sulfate ferricyanide hexahydrate heptahydrate scandium oxide vanadyl sulfate dihydrate manganese(ii) chloride tetrahydrate cobalt(ii) chloride hexahydrate copper(ii) sulfate pentahydrate 17 Coordination Compounds A coordination compound typically consists of a complex ion and a counter ion. A complex ion contains a central metal cation bonded covalently to one or more molecules or ions. The molecules or ions that surround the metal in a complex ion are called ligands. A ligand has at least one unshared pair of valence electrons H O H H N H H Cl - C O 18 6

Coordination Compounds The atom in a ligand that is bound directly to the metal atom is the donor atom. O N H H H H H The number of donor atoms surrounding the central metal atom in a complex ion is the coordination number. Ligands with: one donor atom monodentate H 2 O, NH 3, Cl - two donor atoms bidentate ethylenediamine three or more donor atoms polydentate EDTA 19 Some Common Ligands in Coordination Compounds 20 Coordination Compounds bidentate ligand H 2 N CH 2 CH 2 NH 2 [Co(en) 3 ] 2+ or 21 7

Chelates Bidentate and polydentate ligands give rise to rings in the complex ion. A complex ion containing this type of structure is called a chelate because the ligand seems to grab the metal ion like claws. EDTA has six donor atoms and forms very stable complexes with metal ions. 22 polydentate ligand (EDTA) [PbEDTA] 2- Bidentate and polydentate ligands are called chelating agents 23 Some Common Ligands in Coordination Compounds 24 8

Chelates 25 Chelates Fe++ 26 Chelates Metals and Chelates in Living Systems 27 9

28 29 Chelates Metals and Chelates in Living Systems 30 10

Oxidation Numbers What are the oxidation numbers of the metals in K[Au(OH) 4 ] and [Cr(NH 3 ) 6 ](NO 3 ) 3? OH - has charge of -1 NO 3- has charge of -1 K + has charge of +1 NH 3 has no charge? Au + 1 + 4x(-1) = 0 Au = +3? Cr + 6x(0) + 3x(-1) = 0 Cr = +3 31 Oxidation Numbers Determine the oxidation numbers of the metals in K 3 [Fe(CN) 6 ] Fe = +3 [Ru(NH 3 ) 5 (H 2 O]Cl 2 Ru = +2 [Fe(CO) 5 ] Fe = 0 32 Naming Coordination Compounds Six Rules The cation is named before the anion. As in ionic compounds + or - Charge on complex ion doesn t affect this rule Ni(CO) 4 = nickel tetracarbonyl K 3 [Fe(CN) 6 ] = potassium hexacyanoferrate 33 11

Naming Coordination Compounds Within a complex ion, the ligands are named first in alphabetical order and the metal atom is named last. K 3 3[ [Fe(CN) 6 ] = potassium hexacyanoferrate [Cr(NH 3 ) 3 (H 2 O)]Cl 3 = triamminetriaquo chromium (III) chloride 34 Naming Coordination Compounds The names of anionic ligands end with the letter o. Neutral ligands are usually called by the name of the molecule. The exceptions are H 2 O (aqua), CO (carbonyl), and NH 3 (ammine). From Chang, Table 22.4 35 Naming Coordination Compounds Some examples K 3 [Fe(CN) 6 ] = potassium hexacyanoferrate Na 2 [NiCl 4 ] = sodium tetrachloronickelate (II) Na[PtCl 3 (NH 3 )] = sodium amminetrichloroplatinate (II) Note that the Greek prefixes mono-, di,- tri-, etc. do 36 not affect alphabetization of the ligands 12

Naming Coordination Compounds When several ligands of a particular kind are present, the Greek prefixes di-, tri-, tetra-, penta-, and hexa- are used to indicate the number. If the ligand already contains a Greek prefix, use the prefixes bis, tris, and tetrakis to indicate the number. 2 bis 6 hexakis 3ti tris 7h heptakis 4 tetrakis 8 octakis 5 pentakis 9 ennea [Co(en) 3 ]SO 4 = tris(ethylenediamine)cobalt(ii) sulfate is the same as tris(h 2 NCH 2 CH 2 NH 2 )cobalt(ii) sulfate and as tris(en)cobalt(ii) sulfate 37 Examples of Some Ligands Containing Prefixes 38 Naming Coordination Compounds The oxidation number of the metal is written in Roman numerals following the name of the metal. What is the systematic name of [Cr(H 2 O) 4 Cl 2 ]Cl? tetraaquadichlorochromium(iii) chloride 39 13

Naming Coordination Compounds If the complex is an anion, its name ends in ate. e.g., [Zn(OH) 4 ] 2- tetrahydroxozincate(ii) 40 41 M. Escher 42 14

43 44 Icosahedral Symmetry in Viruses Icosahedral symmetry involves 6 five-fold rotation axes, 10 three-fold, and 15 two-fold. Robijn Bruinsma, PhD, Professor Physics and Astronomy, Virus Research Group at UCLA http://virus.chem.ucla.e du/node/8 45 15

Yum! 20,000 : 1 Humans (globally) 89 : 11 46 47 48 16

Gian Lorenzo Bernini s piazza in front of St. Peter's in Rome 49 50 51 17

Stereochemistry refers to the three-dimensional structure of a molecule. 52 53 ISOMERS 54 18

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Although everything has a mirror image, mirror images may or may not be superimposable. Some molecules are like hands. Left and right hands are mirror images, but they are not identical, or superimposable. They are chiral 58 59 We can now consider several molecules to determine whether or not they are chiral. 60 20

A and B are stereoisomers specifically, they are enantiomers. A carbon atom with four different groups is a tetrahedral chiral center. 61 The two 2-bromobutanes are not identical (superimposable): They are image and mirror image. The two isomers are called enantiomers. Molecules that lack reflection symmetry are chiral. *Greek: cheir,, hand handedness 50:50 Mix of enantiomers : Racemic mix or racemate 62 In general, a molecule with no stereogenic centers will not be chiral. With one stereogenic center, a molecule will always be chiral. 63 21

Summary of the Basic Principles of Chirality: Every object has a reflection, therefore, a mirror image. Are the molecule and its mirror image superimposable? YES, then achiral NO, then chiral In general, a chiral molecule must have one or more chiral (stereogenic) centers. The presence of a plane of symmetry makes a molecule achiral. 64 Different conformations 65 Structure of Coordination Compounds Coordination number 2 4 6 Structure Linear Tetrahedral or Square planar Octahedral 66 22

67 CUBIC CLOSEST-PACKED (CCP) - Way in which atoms (considered as hard spheres) pack together to fill space. In cubic closest-packing, there are three alternating hexagonal layers, a, b, and c, offset from one another so that the spheres in one layer sit in the small triangular depressions of neighboring layers. Each sphere is touched by 12 neighbors, 6 in the same layer, 3 in the layer above, and 3 in the layer below. 68 Notice that in NaCl, each Cl ion is also surrounded by 6 Na ions in octahedral coordination. So, again, the 1/6 of a positive charge from each Na reaches the Cl ion and thus the Cl ion sees 6*1/6 = 1 positive charge, which exactly balances the -1 charge on the Cl. 69 23

Structure of Coordination Compounds Stereoisomers are compounds that are made up of the same types and numbers of atoms bonded together in the same sequence but with different spatial arrangements. Geometric isomers are stereoisomers that cannot be interconverted without breaking a chemical bond. cis-[pt(nh 3 ) 2 Cl 2 ] trans-[pt(nh 3 ) 2 Cl 2 ] 70 Structure of Coordination Compounds trans cis cis-[co(nh 3 ) 4 Cl 2 ] trans-[co(nh 3 ) 4 Cl 2 ] Are these additional geometric isomers of [Co(NH 3 ) 4 Cl 2 ]? 71 Isomerism Stereoisomerism 72 24

Structure of Coordination Compounds Optical isomers are nonsuperimposable mirror images. cis-[co(en) 2 Cl 2 ] trans-[co(en) 2 Cl 2 ] optical isomers chiral not optical isomers achiral 73 Structure of Coordination Compounds Chiral molecules are optically active. Rotate the plane of polarized light. Polarimeter 74 75 25

Geometric (cis-trans) isomerism. The cis and trans isomers of [Pt(NH 3 ) 2 Cl 2 ]. In the cis isomer, identical ligands are adjacent to each other, while in the trans isomer they are across from each other. The cis isomer (cisplatin) is an antitumor agent while the trans isomer has no antitumor effect. 76 Geometric (cis-trans) isomerism. The cis and trans isomers of [Co(NH 3 ) 4 Cl 2 ] +. Note the placement of the Cl - ligands (green spheres). 77 Optical isomerism in an octahedral complex ion. Structure I and its mirror image, structure II, are optical isomers of cis-[co(en) 2 Cl 2 ] +. 78 26

Optical isomerism in an octahedral complex ion. The trans isomer of [Co(en) 2 Cl 2 ] + does not have optical isomers. Structure I can be superimposed on its mirror image, structure II. 79 Sample Problem Determining the Type of Stereoisomerism PROBLEM: Draw stereoisomers for each of the following and state the type of isomerism: (a) [Pt(NH 3 ) 2 Br 2 ] (square planar) (b) [Cr(en) 3 ] 3+ (en = H 2 NCH 2 CH 2 NH 2 ) PLAN: We determine the geometry around each metal ion and the nature of the ligands. If there are different ligands that can be placed in different positions relative to each other, geometric (cis-trans) isomerism occurs. Then we see whether the mirror image of an isomer is superimposable on the original. If it is not, optical isomerism also occurs. 80 Sample Problem SOLUTION: (a) The square planar Pt(II) complex has two different types of monodentate ligands. Each pair of ligands can be next to each other or across from each other. Thus geometric isomerism occurs. These are geometric isomers; they do not have optical isomers since each compound is superimposable on its mirror image. 81 27

Sample Problem (b) Ethylenediamine (en) is a bidentate ligand. The Cr 3+ ion has a coordination number of 6 and an octahedral geometry, like Co 3+. The three bidentate ligands are identical, so there is no geometric isomerism. However, the complex ion has a nonsuperimposable mirror image. Thus optical isomerism occurs. mirror N N Cr N N N Cr N N N N N N N rotate not the same as N N Cr N N N N 82 Linkage or Ionization Isomers NO 2 - SO 3 2- SCN - NCS - 83 fac- isomer mer-- isomer 84 28

Coordination Isomerism [Co (NH 3 )][Cr (CN) 6 ] [Cr(NH 3 )][Cr(CN) 6 ] 85 Bonding in Coordination Compounds Valence Bond Theory bonding takes place when the filled atomic orbital on the ligand overlaps an empty atomic orbital on the metal ion explain geometries well, but doesn t explain color or magnetic properties 86 The Transition Metals 87 29

88 89 90 30

Writing Electron Configurations of Transition Metal Atoms and Ions PROBLEM: Write condensed electron configurations for the following: (a) Zr; (b) V 3+ ; (c) Mo 3+. (Assume that elements in higher periods behave like those in Period 4.) PLAN: We locate the element in the periodic table and count its position in the respective transition series. These elements are in Periods 4 and 5, so the general electron configuration is [noble gas]ns 2 (n 1)d x. For the ions, we call that ns electrons are lost first. SOLUTION: (a) Zr is the second element in the 4d series: [Kr]5s 2 4d 2 91 (b) V is the third element in the 3d series, so its configuration is [Ar]4s 2 3d 3. When it forms V 3+, it loses the two 4s e - first, then one of the 3d e - : [Ar]3d 2 (c) Mo lies below Cr in group 6B(6), so we expect the same exception as for Cr. The configuration for Mo is therefore [Kr]5s 1 4d 5. Formation of the Mo 3+ ion occurs by loss of the single 5s electron followed by two 4d electrons: [Kr]4d 3 92 93 31

94 Aqueous oxoanions of transition elements. +2 +6 +7 Mn 2+ MnO 2 4 MnO 4 The highest oxidation state for Mn equals its group number. +5 +6 +7 VO 3 4 Cr 2 O 2 7 MnO 4 Transition metal ions are often highly colored. 95 Valence Bond Theory Linus Pauling and others applied the principles of quantum mechanics to molecules they reasoned that bonds between atoms would arise when the orbitals on those atoms interacted to make a bond the kind of interaction depends on whether the orbitals align along the axis between the nuclei, or outside the axis 96 32

Orbital Interaction as two atoms approached, the partially filled or empty valence atomic orbitals on the atoms would interact to form molecular orbitals the molecular orbitals would be more stable than the separate atomic orbitals because they would contain paired electrons shared by both atoms the interaction energy between atomic orbitals is negative when the interacting atomic orbitals contain a total of 2 electrons 97 Orbital Diagram for the Formation of H 2 S H H S bond 1s + S 1s 3s 3p H S bond H 98 Predicts Bond Angle = 90 Actual Bond Angle = 92 Valence Bond Theory - Hybridization one of the issues that arose was that the number of partially filled or empty atomic orbital did not predict the number of bonds or orientation of bonds C = 2s 2 2p x1 2p y1 2p z0 would predict 2 or 3 bonds that are 90 apart, rather than 4 bonds that are 109.5 apart to adjust for these inconsistencies, it was postulated that the valence atomic orbitals could hybridize before bonding took place one hybridization of C is to mix all the 2s and 2p orbitals to get 4 orbitals that point at the corners of a tetrahedron 99 33

Unhybridized C Orbitals Predict the Wrong Bonding & Geometry 100 Valence Bond Theory Main Concepts 1. the valence electrons in an atom reside in the quantum mechanical atomic orbitals or hybrid orbitals 2. a chemical bond results when these atomic orbitals overlap and there is a total of 2 electrons in the new molecular orbital a) the electrons must be spin paired 3. the shape of the molecule is determined by the geometry of the overlapping orbitals 101 Hybridization some atoms hybridize their orbitals to maximize bonding hybridizing is mixing different types of orbitals to make a new set of degenerate orbitals sp, sp 2, sp 3, sp 3 d,, sp 3 d 2 more bonds = more full orbitals = more stability better explain observed shapes of molecules same type of atom can have different hybridization depending on the compound C = sp, sp 2, sp 3 102 34

Hybrid Orbitals H cannot hybridize!! the number of standard atomic orbitals combined = the number of hybrid orbitals formed the number and type of standard atomic orbitals combined determines the shape of the hybrid orbitals the particular kind of hybridization that occurs is the one that yields the lowest overall energy for the molecule in other words, you have to know the structure of the molecule beforehand in order to predict the hybridization 103 Orbital Diagrams with Hybridization place electrons into hybrid and unhybridized valence orbitals as if all the orbitals have equal energy when bonding, bonds form between hybrid orbitals and bonds form between unhybridized orbitals that are parallel 104 Carbon Hybridizations Unhybridized 2s 2p sp hybridized 2sp 2p sp 2 hybridized 2sp 2 2p sp 3 hybridized 105 2sp 3 35

sp 3 Hybridization atom with 4 areas of electrons tetrahedral geometry 109.5 angles between hybrid orbitals atom uses hybrid orbitals for all bonds and lone pairs H sp 3 sp 3 H C N H s s H H 106 sp 3 Hybridization of C 107 108 36

Unhybridized atom sp 3 Hybridized Atoms Orbital Diagrams sp 3 hybridized atom 2s 2p 2sp 3 C 2s 2p N 2sp 3 109 110 Bonding in Coordination Compounds Crystal Field Theory bonds form due to the attraction of the electrons on the ligand for the charge on the metal cation electrons on the ligands repel electrons in the unhybridized d orbitals of the metal ion the result is the energies of orbitals the d sublevel are split the difference in energy depends the complex and kinds of ligands crystal field splitting energy strong field splitting and weak field splitting 111 37