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Appendix 7 Tanabe-Sugano Diagrams and Some Illustrative Spectra

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Appendix 7 Tanabe-Sugano Diagrams and Some Illustrative Spectra Appendix Consider the ligand ethylenediamine, en, which is typical of many. When it chelates to a metal atom a five-membered ring is formed, as shown in Fig. A 1.1. Is this ring planar? The answer is no, it is puckered. It is easy to see why this should be so. In a regular pentagon each internal angle is 108°, not far from the regular tetrahedron value of 109.SO. However, if the chelate ring is part of an octahedral complex then the N- M- N angle will be close to 900. This will open up the bond angles at the four atoms of the en ligand in the ring to an average of 112.5°; add to this the fact that the M- N bonds will be longer than the C--C and C- N and it is clear that some steric strain is likely to exist. This strain can be relieved at little or no cost if the ring puckers. Having recognized the possibility of puckering, the next thing is to recognize that two different puckered forms exist; much of this appendix is concerned with them. Any three points in space define a plane, unless the three points lie in a straight line. Consider the plane defined by the metal atom and the two nitrogen atoms of an en ligand in a complex. Our concern is with the two carbon atoms in the ligand. Relative to the plane, these can be arranged in either of the two ways shown in the top part of Figure A1.2. The two configurations are denoted i, and c5, as indicated. An easy way of remembering which configuration is which is to view the chelate ring with the metal atom closest to the observer, as shown in Figure A 1.2. From this viewpoint, in the c5 configuration the bonds involving the two carbon atoms roughly trace out a c5 and for the i. configuration they trace out a i .. This is shown at the bottom of Figure A 1.2. This is not the end of the conformational possibilities that have to be considered. In an octahedral complex there are two different ways of arranging three chelate rings, denoted by the upper case letters A and tl.. They are shown in the upper part of Figure A 1.3. The simplest way of remembering the difference between A and tJ. is to put arrows on the tJ. in the natural, clockwise sense, as is done at the bottom of Figure A 1.3. These arrows serve as a reminder that the tJ. configuration resembles a normal, right-handed screw (the A configuration resembles a left-hand screw). Finally, the possible configurations of the three individual ligands (i,, c5) Fig. Al.l Typical puckered five-membered ring are brought together with the two possible arrangements of a set of fonmed by chelated en ligand. three of them in an octahedral complex (A, tl.). A set of eight possible Appendix 1 1 433 c c Q:,b \I M r1;.0 c \ cI FIJI. AL2 The 2 and b ring configurations. =P- __\' __ L-CJ---~---~' L--~-~--~---~\ A Fig. AL3 The A and 6 configurations of three chelate rings in an octahedral complex. ( J configurations is obtained, all of which must be assumed to be present in a solution of a complex such as [Co(enh]3+: A(J.,V,) A(Mb) A(J.J.b) t.(bbJ.) A(lM) A( b).).) A(bM) t.().I.J.) The first member of each pair (those on the same line) is the mirror image of the second (they are enantiomorphic pairs). Generally speaking, members of families of such compounds rapidly interconvert. However, they have been extensively studied, usually by NMR (often with somewhat more exotic ligands than en) because they provide an opportunity of investigating the 434 I Appendix 1 effect of steric and other influences on the relative stabilities of octahedral complexes. In crystals of salts of the [Co(enh]J+ cation, only one of the eight forms listed above will usually be present. But which, and why? These questions do not seem to have general answers, but they are fascinating nonetheless. For simplicity, this appendix has considered only five-membered rings. Six-membered chelate rings are also quite common and follow a similar pattern; Figure Al.2 would be adapted to a six-membered ring by inclusion of an additional atom in the plane shown in each of the diagrams in this figure. The subsequent discussion would have been essentially unchanged. Finally, the ligand that has been considered is a rather symmetrical one, en. A related but less symmetrical ligand has one C-H bond of en replaced by a C-CH3. This ligand is 1,2-diaminopropane (usually denoted by pn); it contains an asymmetric carbon atom (that bonded to the methyl group). In principle, therefore, it is capable of separation into optically active isomers and our discussion could be elaborated to include this. This is not some academic exercise, a study of an increasingly remote problem. The naturally occurring ligands of bioinorganic chemistry are invariably optically active and an understanding of the properties of their complexes would not be possible if the possibilities for as ligand such as pn were not well understood. The subject matter of this appendix is comprehensively treated by C. J. Hawkins in Absolute Configuration of Metal Complexes, Wiley, New York, 1971. Appendix2 Valence shell electron pair repulsion (VSEPR) model This appendix contains an outline of the most successful of the simple approaches to molecular structure, one that is described in most current introductory and inorganic texts. In contrast to the treatment given in such texts, the present is more concerned with an analysis of it, in order to assess whether it correctly highlights the key factors involved in determining molecular geometry. Whether, for main group complexes at least, it may be said to provide an explanation (as opposed to a prediction) of their geometries. The model is based on the postulate that repulsion between valence electron pairs surrounding an atom lead to the observed atomic arrangement around this atom, an arrangement in which these electron pairs are as far apart as possible. That is, it is postulated that bonding forces are not the vital factor in determining molecular geometry. This view finds support in the observation that relatively little energy is required to excite those vibrations which correspond to bond angle changes. These, of course, change the geometry around the central atom. The actual energy is about one half of that required to excite the corresponding bond stretching vibrations. The simplest way of looking at the VSEPR approach is to regard each valence electron pair as being represented by a point, the points being constrained to move over the surface of a sphere drawn around the central atom. As the points (i.e.valence shell electron pairs) repel each other, the most stable molecular arrangement will be that in which the points are as far apart as possible. If each valence shell electron pair is called P, then the following geometries of the Ps around the central atom M are at once predicted: MP2 linear MP3 equilateral triangular MP4 tetrahedral MP6 octahedral Comparison with the geometries which are discussed in Chapter 3 will show that for main group elements these predictions are rather good. So, [BF3 · NMeJ, in which there are four valence electron pairs around the boron, is approximately tetrahedral, the distortion being consistent with the 436 1 Appendix 2 fact that one of the valence electron pairs (that involved in the B-N bond) would not be expected to have exactly the same spatial distribution as the other three, so that (B-N)--(B-F) bond repulsions will be slightly different from (B-F)--(B-F) repulsions. Similarly, extending the discussion to a compound which would not normally be regarded as a complex, the oxygen atom in water is surrounded by four electron pairs (two from the 0-H bonds and two from the lone pairs on the oxygen atom). Repulsion between these electron pairs leads to a tetrahedral distribution and, in accord with this, the H--()-H bond angle in water has roughly the tetrahedral value, the deviation from regularity being in accord with the suggestion that the centre of gravity of the electron density in the 0-H bonds is further away from the oxygen atom than is the case for the lone pair electrons. Such arguments lead to the prediction that the relative magnitude of the electron repulsion is in the order: lone pair-lone pair> lone pair-bonding pair> bonding pair-bonding pair an order which almost invariably explains distortions from symmetric structures. So far, coordination numbers five, seven, eight, nine and above have not been discussed. The case of nine-coordination is simple, for the predicted geometry is that discussed in Section 3.2.7, the three-face-centred trigonal pyramid, and is the one commonly observed. The others are not so clear-cut, because the valence electron pair arrangements which involve minimum electron repulsion are not intuitively obvious. This difficulty can be removed by assuming some form for the repulsive energy between the points constrained to move over the surface of a sphere (the points which represent the valence electron pairs). For instance, it could be assumed that the energy varies as 1/r", where r is the interpoint distance, and the arrangement of lowest energy thus determined. Unfortunately, the results depend to some extent on the value given ton, (a value of between 6 and 12 is usually chosen) and in any case it is not certain that the real energy varies as 1/r"-an exponential might be more appropriate, for instance.
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