Transition Metal Chemistry and Coordination Compounds

Transition Metal Chemistry and Coordination Compounds Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. The Transition Metals Notice that Zn, Cd and Ag Are not included 3 1

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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 (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 9 3

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. Oxidation States of the 1 st Row Transition Metals (most stable oxidation numbers are shown in red) 12 4

Ionization Energies for the 1 st Row Transition Metals 13 Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper 14 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 5

Coordination Compounds A coordination compound typically consists of a complex ion and a counter ion. A complex ion contains a central cation bonded to one or more molecules or ions. The molecules or ions that surround the metal in a complex ion are called has at least one unshared pair of valence electrons H O H H N H H Cl - C O 16 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 H 2 O, NH 3, Cl - two donor atoms ethylenediamine three or more donor atoms EDTA 17 Some Common Ligands in Coordination Compounds 6

Coordination Compounds bidentate ligand H 2 N CH 2 CH 2 NH 2 [Co(en) 3 ] 2+ or 19 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. polydentate ligand (EDTA) [PbEDTA] 2- Bidentate and polydentate ligands are called agents 21 7

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 28 Oxidation Numbers Determine the oxidation numbers of the metals in K 3 [Fe(CN) 6 ] [Ru(NH 3 ) 5 (H 2 O]Cl 2 [Fe(CO) 5 ] Fe = Ru = Fe = 29 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 30 10

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 ] = [Cr(NH 3 ) 3 (H 2 O)]Cl 3 = 31 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 32 Naming Coordination Compounds Some examples K 3 [Fe(CN) 6 ] = potassium hexacyano Na 2 [NiCl 4 ] = sodium tetrachloro Na[PtCl 3 (NH 3 )] = sodium amminetrichloroplatinate (II) Note that the Greek prefixes mono-, di,- tri-, etc. do not affect alphabetization of the ligands 33 11

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 34 Examples of Some Ligands Containing Prefixes 35 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? 36 12

Naming Coordination Compounds If the complex is an anion, its name ends in ate. e.g., [Zn(OH) 4 ] 2- tetrahydroxozincate(ii) 37 Structure of Coordination Compounds Coordination number 2 4 6 Structure or 38 39 13

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. 40 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. 41 Structure of Coordination Compounds isomers 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. 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 ] 42 14

Structure of Coordination Compounds trans cis -[Co(NH 3 ) 4 Cl 2 ] -[Co(NH 3 ) 4 Cl 2 ] Are these additional geometric isomers of [Co(NH 3 ) 4 Cl 2 ]? 43 Isomerism Stereoisomerism Copyright 1999, PRENTICE HALL 44 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 45 15

Structure of Coordination Compounds Chiral molecules are optically active. Rotate the plane of polarized light. Polarimeter 46 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. 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). 16

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 ] +. 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. 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. 17

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. 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 Bonding in Coordination Compounds Valence Bond Theory bonding takes place when the filled atomic orbital on the overlaps an empty atomic orbital on the ion explain geometries well, but doesn t explain properties 54 18

55 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 56 Splitting of d Orbital Energies due to Ligands in a Octahedral Complex 57 19

Bonding in Coordination Compounds d-orbital orientation relative to x, y, and z axes All equal in energy in the absence of ligands! 58 Strong and Weak Field Splitting 59 Bonding in Coordination Compounds Isolated transition metal atom Bonded transition metal atom Crystal field splitting ( ) is the energy difference between two sets of d orbitals in a metal atom when ligands are present 60 20

Bonding in Coordination Compounds E = h 62 Complex Ion Color the observed color is the complimentary color of the one that is absorbed 63 21

Complex Ion Color and Crystal Field Strength the colors of complex ions are due to electronic transitions between the split d sublevel orbitals the wavelength of maximum absorbance can be used to determine the size of the energy gap between the split d sublevel orbitals E photon = h = hc/ = 64 The absorption maximum for the complex ion [Co(NH 3 ) 6 ] 3+ occurs at 470 nm. What is the color of the complex and what is the crystal field splitting in kj/mol? Absorbs blue, will appear orange. = h = hc (6.63 x 10-34 J s) x (3.00 x 10 8 m s -1 ) = = 4.23 x 10-19 J 470 x 10-9 m (kj/mol) = 4.23 x 10-19 J/atom x 6.022 x 10 23 atoms/mol = 255 kj/mol 65 Ligand and Crystal Field Strength the strength of the crystal field depends in large part on the ligands strong field ligands include: CN > NO 2 > en > NH 3 weak field ligands include H 2 O > OH > F > Cl > Br > I crystal field strength increases as the charge on the metal cation increases 66 22

The Spectrochemical Series The complexes of cobalt (III) show the shift in color due to the ligand. (a) CN, (b) NO 2, (c) phen, (d) en, (e) NH 3, (f) gly, (g) H 2 O, (h) ox 2, (i) CO 3 2. Bonding in Octahedral Coordination Compounds Spectrochemical Series I - < Br - < Cl - < OH - < F - < H 2 O < NH 3 < en < CN - < CO Weak field ligands Small Strong field ligands Large 68 Paramagnetic and Diamagnetic Properties weak ligand field strong ligand field Obeys Hund s rule 69 23

Bonding in Octahedral Coordination Compounds maximum stability, higher E cost weak ligand field high spin less stability, lower E cost strong ligand field low spin 70 Nature makes compromises Orbital Diagrams for High Spin and Low Spin Octahedral Complexes 71 24

Crystal Field splitting in a Square Planar Complex 73 Crystal Field splitting in a Tetrahedral Complex 75 25

Chemistry In Action: Coordination Compounds in Living Systems 76 26