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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 p 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. F example: For l Co3+ Cl Co3+ Check out this website http://www.chem.purdue.edu/gchelp/cchem/struct2.html 1 3+ NH3 H3N NH3 Co H3 N NH3 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. NH3 Each N atom donates a pair of electrons to the Co3+ metal ion, i.e. each NH3 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. H3 N NH3 NH3 NH3 H3N H3N NH3 NH3 NH3 NH3 In reality the situation is a little more complex: NH3 Co or Co H3N NH3 NH3 NH3 H 3N or Co H 3N 3+ 3+ 3+ NH3 + 3+ 3+ NH3 + NH3 + H3 N NH3 Co3+ + + H3N +NH NH3 H 3N Co3NH3 H 3N NH3 NH3 3 (a) (b) Coordinate bonds are formed by lone pair donation from the ligands the Co3+ centre. It implies transfer of charge from ligand to metal and figure (a) shows the resulting charge distribution. This is unrealistic since the Co3+ 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 NH3 (b), 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 + 1/2 3+ NH3 1/2 + + 1/2 H3N NH3 Co0 1/2 + H3 N + 1/2 1/2 + NH 3 NH3 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 Co3+ metal centre should be close to zero . In order to satisfy this the Co3+ 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. Co3+ Cl Co3+ This representation shows that a bridging chloride ion donates two pairs of electrons to two Co3+ metal ions which are the Lewis acids, accepting the lone pairs. In reality, lit when h n thinking thinkin about b t th the b bonding, ndin th the formal f m l charge h on n the th chloride ion is not actually -1, which means also that the formal charge on the two Co3+ 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 (3+3-1 = 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 Cδ+–Clδ-. 3 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 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. [FeCl4]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: CH3OCH3 (dimethylether) and CH3CH2OH (ethanol). C4H8 cyclobutane l b t and d 11-butene b t H2 C CH2 H2 C CH2 CH3CH2CH=CH2 4 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 l in different d ff geometries (d (different ff b bond d angles l or different distances between nonbonded atoms, for example). Organic example: cis- and trans-2-butene, CH3CH=CHCH3 H3C H H3C H H H CH3 cis 2. CH3 trans 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 C F Cl H I C I Cl F mirror images non superimposable 5 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. g inside the coordination sphere p 1. Ionization isomers - Ligands 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 CrCl3.6H2O. 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(H2O)6]Cl3 (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(H2O)5Cl]Cl2.H2O (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(H2O)4Cl2]Cl.2H2O (deep green) Thus the three ionzation isomers are [Cr(H2O)6]Cl3 (violet), [Cr(H2O)5Cl]Cl2.H2O (grey-green), and [Cr(H2O)4Cl2]Cl.2H2O (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 2. Linkage isomers - Linkage isomers can exist when one or more ambidentate ligand is bonded to a metal ion. NO2- An 18 electron system O N O O N O an ambidentate ligand A compound with the formula CoCl2(NO2).5NH ) 5NH3 has two isomers, 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(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2. 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 NO2- Mn+ Nitro linkage Mn+ Nitrito linkage A resonance hybrid, showing the N-O bonds iin th the nitrite it it iion h have a bond b d order d of f about b t 1.5, leaving most of the single negative charge shared between the terminal oxygen atoms 7 3. Coordination isomers - involve ligand exchange between coordination spheres of two metal ions that are part of the same compound. [Pt(NH3)4]2+ [PtCl4]2- and [Pt(NH3)3Cl]+[Pt(NH3)Cl3]- Formula, both atoms contain Pt2Cl4(NH3)4 [Pt(NH3)2Cl2] 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 p for square q planar p 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 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 p on what you y are trying y g 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 1. Ionization 2. Linkage 3. Coordination Stereoisomers – same connectivities, different spatial arrangement of atoms Geometric cis/trans Optical (enantiomers) 9