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Formal Oxidation State versus Partial ChargeЧA Comment.

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Formal Oxidation State versus Partial Charge-A
Martin Kaupp* and Hans Georg von Schnering
In a communication in this journal."] Snyder recently claimed
that the oxidation state of the copper atom in the square-planar
anion [Cu(CF,)J is Cu', and not Cu"' as inferred by Naumann
et al.''' from synthesis and properties, and from structural arguments which up to now have been generally accepted. Snyder
based his conclusion on NBO population analysis from a b initio
pseudopotential calculations. These gave charges on copper of
only y = 1 .
Evidently, there must be a gross misunderstanding. Snyder
has confused two important but fundamentally different concepts: the formal oxidation state, and partial charges as obtained, for example, from population analyses. To derive a formal oxidation state. a completely ionic bonding description is
chosen deliberately; that is, the bonding electron pairs are assigned exclusively to the more electronegative bonding partner.
Thus, if one accepts that the CF, ligand is more electronegative
than the copper atom, that each CF, ligand is bound to the
central atom, that there are no additional bonds between the
ligands, and that all CF, ligands are equivalent, there is no way
around an assignment of an oxidation state of 111 to the copper
atom. (An alternative. completely homolytic distribution of the
bonding electrons would lead to the unrealistic oxidation state
- I.) The validity of none of the above assumptions has been
touched in any way by Snyder's calculations.
Snyder's population analysis yields a significant electron population in the d,, >,. atomic orbital of copper. This merely tells
us that significant covalent interactions contribute to the Cu-C
bonds. Therefore, the formal oxidation state III differs appreciably from the calculated partial charge in this case. However. this
holds generally true for significantly covalent bonds.
The square-planar structure also clearly supports the original
description['] as a Cu"' complex. A four-coordinate Cu' complex should be tetrahedral. The examples for square-planar Cu'
species erroneously quoted by Snyder turn out to be either obviously Cu"
or (slightly distorted) tetrahedrally coordinated system^.'^. 'I
German vcrsinn: A i i g r i i ~Chcin. 1995, 107. 1076
[ l ] J. P. Snyder. i I / i w i i ~
C/iivii. 1995, 107. 113: A i i g ( w . C%ciii.
Ed. E/i,c/. 1995. 34.
[ 2 ] D. Naumann. T. Roy. K.-F. Tebbc. W. Cruinp,
Aiigrii.. C/IOJI.
1993, 105. 1555:
A ~ ~ ~ I ' CI I' /,I.P I I ~b .i t . Ed. h'iix/. 1993, 32. 1482.
[i] B. C'hiui. 0. Piovesiin;~,T. Tarantelli. P. F. Zanarn. bioi:q. C/ir,irr.1984, 73, 2547.
[4] .I.C. Dyason. P. C. Healy, L. M. Engclhardt, C. Pakawatchai. W. A. Patrick.
A. H. White. J. Chcnr. Soc. Dnlron naris. 1985. 839.
IS] M . Hakansson. S. Jagner. M. Nilsson. J O r p m m i , / . C i i m 1987. 336. 279.
[*] Dr. M. Kaupp, Prof. Dr. H. G. von Schnering
Max-Planck-Institut fur Festkiirperthrschung
Heisenbergstrasse 1, D-70569 Stuttgart (Germany)
Telefax: Int. code (71 1)689-1562
Distinguishing Copper d8 and d10 Configurations in a Highly Ionic Complex;
A Nonformal Metal Oxidation State
James P. Snyder"
Since Pauling's introduction of the concept over 40 years
ago."] assignment of,/brnza/ oxidation numbers and oxidation
states has proved to be useful for teaching elementary chemical
concepts and for cataloguing trends in the periodic table."]
Nonetheless, apart from isolated monoatomic ions, designation
of a formal oxidation state is entirely arbitrary and often accom[*] P r d Dr. J. P. Snyder
Emerson Center for Scientific Computation
Department of Chemistry. Emory University
1515 Pierce Drive, Atlanta. G A 30322 (USA)
Telefax: I n t . code + (404)727-6586
e-mail: snyderrrr
panied by assumptions as underscored by Kaupp and von
S ~ h n e r i n g . Assignment
is accomplished by employing one of
the many predefined but not entirely consistent electron-counting schemes.[41Unfortunately. uncritical application of the resulting oxidation values can lead to confusing and physically
unrealistic descriptions for organic, organometallic. and inorganic species alike."]
The report by Naumann et aLC6'classified the metal in 1 as
Cu'" and designated it as d8, suggesting an intimate link between
[Cu(CF:,),]- 1
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forma, oxidation, state, versus, partial, chargeчa, comments
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