Subject: Re: CCL:isolobal vs. isoelectronic

From: Guosheng Wu <[email-masked]>
Date: Thu, 18 Oct 2001 22:21:50 -0500 (CDT)

On Fri, 19 Oct 2001, Kieran F Lim (Lim Pak Kwan) wrote:

> My text books state that isoelectronic means same number of electrons.
> USUALLY (not always) this implies same electronic configuration.
>
> by this definition, BH3, CH3+ and CH2 are isoelectronic.

If we check the textbooks for general chemistry with more sense of
science, for every 3-4 pages, we may find at least one statement needs to
be corrected. For "isoelectronic", it is just a very empirical word used
to help undergraduate understand chemistry. If we are really interested
in using it, maybe we need to be very careful to keep consistent with its
"original"(when it was first coined) meaning :-)

I spent a few minutes on the topic, and found much relevant info for the
defination of "isoelectronic". Here just pick three of them:

(1)Oxford online (2nd edition, using the NWU access)
   Isoelectronic: (Composed of atoms or molecules) having the same
   number of electrons.

< this is the same as in many textbooks, and it is obvious too simple to
be accurate.>

(2)Webster's New World (4th edition, 1999)
  -----supposed to "define the English Language for the 21st century"

   Isoelectronic: designating or of any of two or more atoms which have
   the same number of electrons around the nucleus and similar spectral
   and physical properties.

< By any means, this is much better, and it points out the "origination"
of this word (my guess): similar properties from same number of
electrons. Then CH4, BH3 and H2O can not be isoelectronics. Although they
have same electronic configurations, the number of atoms in these
molecules is not equal to each other, just like H2 and He should never be
called "isoelectronic" if this word has more meaning than just "same
number of electrons".

(2)IUPAC 1994

http://www.chem.qmw.ac.uk/iupac/gtpoc/I.html#34

GLOSSARY OF TERMS USED IN PHYSICAL ORGANIC CHEMISTRY
(IUPAC Recommendations 1994)

Isoelectronic: Two or more molecular entities are described as
isoelectronic if they have the same number of valence electrons
and the same structure, i.e. number and connectivity of atoms, but
differ in some of the elements involved.

<some examples are also given as following:>

(a) CO, N2 and NO+ are isoelectronic
(b) CH2=C=O and CH2=N=N are isoelectronic
(c) CH3COCH3 and CH3N=NCH3 have the same number of electrons,
but have different structures, hence they are not described as
isoelectronic.

<this is supposed to be the official defination. Here the "same structure"
is crucial, and "similar properties" from Webster defination is obviously
making-sense.>

Cheers.

Guosheng Wu

Department of Chemistry
Northwestern University