Transition Metal Complexes

Transcription

1 2P32 Principles of Inorganic Chemistry Dr. M. Pilkington Lecture 4 - Transition Metal Complexes Transition Metal Complexes: Definitions and Terminology. Isomerism in Transition Metal Complexes: Structural Isomers and Stereoisomers. 1. Transition Metal Complexes: Definitions and Terminology. General Convention The word ligand is derived from the Latin verb ligare meaning to bind. In a complex we have a Lewis Acid Base interaction: An arrow is used to show the donation of an electron pair from a neutral ligand to an acceptor. A line is used to denote the interaction between an anionic ligand and the acceptor. Often however, this convention is ignored and a line to denote both types of interaction is used. For example: Co 3 Cl Co 3 Check out this website 1

2 Co 3 Review your Acid/Base Interactions When a Lewis base donates a pair of electrons to a Lewis acid, a coordinate bond is formed and the resulting species is an adduct or coordination complex. Each N atom donates a pair of electrons to the Co 3 metal ion, i.e. each molecule is a Lewis base while the metal ion is the Lewis acid. We think of the metal to ligand interaction as being essentially covalent, but in reality this is not entirely true as the character of metal-ligand interactions varies with the nature of the metal ion and the ligand. Co or Co or Co In reality the situation is a little more complex: H 3N Co 3-3 H 3N Co 3 3 NH3 H 3N (a) (b) Coordinate bonds are formed by lone pair donation from the ligands the Co 3 centre. It implies transfer of charge from ligand to metal and figure (a) shows the resulting charge distribution. This is unrealistic since the Co 3 centre becomes more negatively charged than would be unfavorable given its electropositive nature. At the other extreme, consider bonding in terms of an ionic model (b), the 3 charge remains localized on the cobalt and the six ligands remain neutral. However this model also does not agree with experimental studies on this complex. So this model is flawed. Neither model is appropriate. 2

3 1/2 1/2 1/2 Co 0 1/2 1/2 1/2 3 We have to apply Pauling s Electroneutrality Principle which states that the distribution of charge on a molecule or ion is such that the charge on a single atom is within the range 1 to -1 (ideally close to zero). In this case the net charge on the Co 3 metal centre should be close to zero. In order to satisfy this the Co 3 ion can accept a total of only 3 electrons from the six ligands, thus giving the charge distribution above. This model is actually 50% ionic and 50% covalent. Co 3 Cl Co 3 This representation shows that a bridging chloride ion donates two pairs of electrons to two Co 3 metal ions which are the Lewis acids, accepting the lone pairs. In reality, when thinking about the bonding, the formal charge on the chloride ion is not actually -1, which means also that the formal charge on the two Co 3 metal ions are not strictly 3. either. What is important is that we have a complex above which has an overall charge of 5. In order to easily determine the overall charge of the complex, the above representation is easy to use (33-1 = 5). With respect to thinking about the coordinate bond it does not however accurately represent the formal charges on the metal ions and the ligands. This is analogous to a C-Cl bond in organic chemistry, we write C-Cl but in reality this does not accurately describe the bonding interaction since the electrons are not evenly shared and the truth is C δ Cl δ-. 3

4 Determination of Formal Oxidation States of Metals in Coordination Complexes To figure out the oxidation state or oxidation number of the central metal atom in a complex is very important. Proceed as follows: identify the charges on the ligands look at the total charge on the molecule charge of ligands formal oxidation state = total charge e.g. [FeCl 4 ] 2- has 4 Cl - ligands and overall 2 - charge, so it must contain Fe 2 or Fe(II). Review of Isomerism - Structural Isomers and Stereoisomers. Isomers Compounds with the same formula but different properties that result from different structures. There are two broad classes of isomers: structural isomers and stereoisomers. 1. Structural isomers have the same molecular formula but different molecular structures (different connectivities or different numbers and kinds of chemical bonds. Organic examples of structural isomers: CH 3 OCH 3 (dimethylether) and CH 3 CH 2 OH (ethanol). C 4 H 8 cyclobutane and 1-butene H 2 C CH 2 CH 3 CH 2 CH=CH 2 H 2 C CH 2 4

5 2. Stereoisomers not only have the same formulas but also the same connectivities of their atoms. The spatial arrangements of the atoms are different. There are two examples: i. Geometric isomers have different spatial arrangement results in different geometries (different bond angles or different distances between nonbonded atoms, for example). Organic example: cis- and trans-2-butene, CH 3 CH=CHCH 3 H H H 3 C H H 3 C CH 3 H CH 3 cis trans 2. Optical isomers have the same geometrical parameters but are related as nonsuperimposable mirror images. (In other words, the molecule or ion is chiral.). Optical isomers get their names because they are able to rotate a planepolarized light beam to the left or to the right. Organic example: CHFClI. A carbon atom with four different groups attached to it has a nonsuperimposable mirror image. H H F C Cl I I Cl C F mirror images non superimposable 5

6 2. Isomerism in Transition Metal Complexes Structural Isomers There are many types of structural isomers in transition metal complexes. We will explore three of them. 1. Ionization isomers - Ligands inside the coordination sphere exchange places with ligands outside the coordination sphere. Ionization isomers are so-named because they give different ions when dissolved in water. Example: There are three compounds with the formula CrCl 3.6H 2 O. One is violet, one is grey-green, and the third is deep green. The violet isomer produces 3 moles of silver chloride upon reaction with silver nitrate, and does not lose water in a desiccator. [Cr(H 2 O) 6 ]Cl 3 (violet) The grey-green isomer gives 2 moles of silver chloride upon reaction with silver nitrate, and loses one mole of water when stored in a desiccator. [Cr(H 2 O) 5 Cl]Cl 2.H 2 O (grey-green) The deep green isomer gives 1 mole of silver chloride upon reaction with silver nitrate, and loses two moles of water when stored in a desiccator. [Cr(H 2 O) 4 Cl 2 ]Cl.2H 2 O (deep green) Thus the three ionzation isomers are [Cr(H 2 O) 6 ]Cl 3 (violet), [Cr(H 2 O) 5 Cl]Cl 2.H 2 O (grey-green), and [Cr(H 2 O) 4 Cl 2 ]Cl.2H 2 O (deep green). Note that the chloride ions that react with silver nitrate are the ones not bonded to the chromium(iii) ion, and the water molecules that are lost in a desiccator are the uncoordinated ones. 6

7 2. Linkage isomers - Linkage isomers can exist when one or more ambidentate ligand is bonded to a metal ion. NO 2 - An 18 electron system O N O O N O an ambidentate ligand A compound with the formula CoCl 2 (NO 2 )5NH ).5 has two isomers, one yellow and one red. Each precipitates two moles of silver chloride, therefore both chloride ions are outside the cobalt(iii) coordination sphere. Neither has an aqueous solution that is basic to ph paper, therefore all the ammonias are bonded to cobalt (III). The obvious possibility is that the ambidentate nitrite group is differently bonded in these two complexes: [Co( ) 5 NO 2 ]Cl 2 and [Co( ) 5 ONO]Cl 2. Today, we would assign the structures on the basis of infrared spectra: N- and O-bonded nitrite have different N-O stretching frequencies. The Ambidentate Nitrite ion NO 2 - M n Nitro linkage M n Nitrito linkage A resonance hybrid, showing the N-O bonds in the nitrite ion have a bond order of about 1.5, leaving most of the single negative charge shared between the terminal oxygen atoms 7

8 3. Coordination isomers - involve ligand exchange between coordination spheres of two metal ions that are part of the same compound. [Pt( ) 4 ] 2 [PtCl 4 ] 2- and [Pt( ) 3 Cl] [Pt( )Cl 3 ] - Formula, both atoms contain Pt 2 Cl 4 ( ) 4 [Pt( ) 2 Cl 2 ] has the same ratio of atoms, but does not have the same overall formula; hence it is not a coordination isomer of the above compounds. 2. Isomerism in Transition Metal Complexes Stereoisomers i. Geometric Isomers are found in square planar and octahedral complexes. Examples for square planar coordination are the cis- and trans-isomers of diamminedichloroplatinum(ii): Note the convention of drawing a square with the metal ion in the center and the ligands at the corners of the square. 8

9 An example of geometric isomers in octahedral complexes are the cis- and trans-isomers of the tetraamminedichlorocobalt(iii) ion: Note that there are several different ways to represent an octahedrally coordinated metal ion; which way you choose depends on what you are trying to show. Isomers compounds with same molecular formula but different properties Structural Isomers -Same molecular formula but different connectivities different numbers and kinds of chemical bonds Stereoisomers same connectivities, different spatial arrangement of atoms 1. Ionization 2. Linkage 3. Coordination Geometric cis/trans Optical (enantiomers) 9