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exam question exchange Symmetry Questions in Some Organometallic Complexes
JOHN J. ALEXANDER University of Cincinnati CincinnakOH45Zl
containing the Mo atom and the phosphine ligands as the equatorial plane.
Aaustin Galindo ~e~artamento de Quimica lnorganica Universidad de Sevilla Aptdo 553 41071 Sevilla, Spain
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Classifvine ions and molecules into their ~ o i n.. ato u D. s is a common subject in a p a d u a t c i n o r g a n ~ cc h e r n ~ s t r y course The rorrect deduct~onbv the s t u d m t of the rnolrcular point group through the determination of symmetry elements is a n i m ~ o r t a n tto.~ i .cwhich . i s also used i n other advanced lcvels a s a usrful tool for several appl~cilt~ons ( 1 I. T h ~ oucstion s tests the student's abtlttv to detcrmme the molecuiar point group of some organomtkallic compounds and evaluates the accuracy of t h e student's knowledge of concepts such as dissymmetry, asymmetry, a n d optical activity, applied to some organometallic compounds (2-4). Question X-ray crystallographic studies carried out on complex trans-[M~(C~H~)~(PMe~)al 1showed t h e staggered-eclipsed geometry (the first term applies to t h e relative orientation of t h e olefin ligands while the second defmes t h e position of the axial olefin molecules relative to t h e trans-P-Mo-P vectors) for t h e ethylene ligands i n t h e solid state. Some chemical reactivity of the bis(ethylene1 derivative is displayed i n the following scheme.
1: There 1.; a n S, axis passing through the molybdenum atom,
2:
hiswring the C= C' bonds of the ethylrnr lignndr and perpendluulur to the rquntorinl plane. Howcwr, there arc also more symmetry elements so the point group S4 is ruled out. The higher-order proper axis is a Cz axis callinear with the S4axis. There are two more Cz' axes perpendicular to this one and biseding the P-Mo-P angles. There are no symmetry planes perpendicular ta the ~p axes, hut there are two vertical planes, which lie between the Cz' axes, perpendicular ta the equatorial plane and passing through the molybdenum and two phosphorous atoms. The group is thus D2+ Thereare no proper &improper axes. There is a symmetry plane perpendicular to the equatorial plane and intersectine it alone the P-Ma-CO vector. The molecule belones to the group Cs. Complex 3 does not possess an improper axis and the only proper axis is a Cz axis. Cz lies in the equatorial plane bisecting the ehelating phosphine organic chain. There are no more Cz axes and no symmetry planes are found, so the group is Cz. There is no improper axis. The highest-order axis is a Cz passing through the Mo atom, perpendicular ta the equatorial plane. There are no other Cz axes. Because it has two vertical planes (none perpendicular to Cz)containing the P-Mo-P and OC-Mo-CO vectors, respectively, 4 belongs ta the CZvgroup. No proper or improper rotation axes can be found. No symmetry planes exist. The group is C1.
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(1) Determine the molecular point group of all the complexes showed in the scheme. (Ignnre the alkyl groups of phosphine ligands). (2) Which of them are asymmetric? Are any of them dissymmetric? (3) Point out the optically active molecules.
Acceptable Solutions Question 1
To answer this point it i s necessary to follow t h e known flow chart for classifying molecular symmetry into point groups ( I , 5)for each complex. None of them belong to the special p u p s , and we will consider hereafter t h e plane
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Question 2
As pointed out by Hargittai (61, there still remains some confusion between t h e terms asvmmetw and dissvmmetry. Asymmetry means no-symmetry, so a n asymmetric molecule has no symmetry elements, t h a t is, it i s applicable only to a molecules belonging to point gmup CI. Com~ l e 5x is a n asvmmetric molecule. Dissvmmetw is t h e nonexistence of improper rotation a s a n element of symmetry i n a given group. A dissymmetric molecule has no improper rotation axis. All asymmetric molecules are dissymmetric, but t h e converse i s not true. Since improper rotation axes include the symmetry plane (S,) and the inversion center (Sz) only t h e complexes 3 and 5 are dissymmetric molecules. Volume 70
Number 4 A ~ r i l1993
325
Question 3
cally active, or enantiomers have the dissymmetry property Complexes 3 and 5 are optically active.
Optically active molecules are those that rotate the plane of polarized light, one to the left (laevo, 1or -) and the other to the right (dextro, d or +), optically adive molecules have the property of chirality, that is optical isomers (enantiomorphs or enantiomers) are pairs of molecules which are non-suoerirn~osablemirror images of each ~other. Any asymmetric molecule is chiral, but asymmetry for activity, the sense is not a necessary stated in question 2, all dissymmetric molecules are opti-
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326
Journal of Chemical Education
Literature Cited 1. cOn~~.F.A.Ckmiml4oplimtionsofOroupTk~om3rded.:W~eyIntersdence:Neu Ymk.1990. csmona.E.: ~ ~ r iJ nM.;, mveda.M. L.; ~ t - d . J. R O - ~ S , R D. J. AM. ckm. SO
