Subject: RE: isolobal vs. isoelectronic

From: "Shobe, Dave" <[email-masked]>
Date: Thu, 18 Oct 2001 09:47:59 -0400

From: Krzysztof Radacki [mailto:[email-masked]]
> So he had argued that BH_3 has only one free orbital (not involved in
> B-H bond) and CH_2 two of them. So BH_3 is isolobal to CH_3(+) but only
> isoelectronic to CH_2.

I would have said that all three (BH3, CH3+, and CH2) were isoelectronic
since this is just a matter of counting electrons (8 e- and 6 valence e- in
each case).  The # valence electrons has to be counted as well as the total,
otherwise you reach conclusions such as Zn being "isoelectronic" to Ne3 and
C5.  

However, CH2 would not be "isolobal" to BH3 or CH3+ because CH2 has a lone
pair as well as an empty orbital.  

I forget the name for the relationship among for example {HCN, HSiP, HCAs,
...} where atoms are replaced by others in the same column of the periodic
table.

--David Shobe

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From: Roy Jensen <[email-masked]>
Date: Thu, 18 Oct 2001 08:56:54 -0700 (PDT)

> >...} where atoms are replaced by others in the same column of the periodic

isovalent, I believe.

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From: "Kieran F Lim (Lim Pak Kwan)" <[email-masked]>
Date: Fri, 19 Oct 2001 10:32:21 +1000

>pair as well as an empty orbital.

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.

by extension, "isolobal" would mean same number/arrangement of lobes
(ie near-same symmetry). CH4, NH3 (with lone pair) and H2O (with 2 lone pairs)
are both isoelectronic and isolobal.

note that this does not mean same symmetry since these molecules
are Td, C3v and C2v.

CH2 is a special case because it has two close electronic states
singlet (lone pair and empty orbital) and doublet (diradical)

BH3, CH3+ and singlet CH2 are both isoelectronic and isolobal.

doublet CH2 has a very different electronic configuration.

BH3, CH3+ and singlet CH2 are isoelectronic but not isolobal.

>isovalent, I believe.

Does "isovalent" refer to "valency"? Are NaCl and AgCl isovalent?
I am not sure.

I had heard (cannot remember where) and use "valence isoelectronic"
but am very happy to be corrected if wrong.

Kieran

------------------------------------------------------------
  Dr Kieran F Lim             Biol. and Chemical Sciences
  (Lim Pak Kwan)              Deakin University
  ph:  + [61] (3) 5227-2146   Geelong          VIC   3217
  fax: + [61] (3) 5227-1040   AUSTRALIA
  mailto:[email-masked]    http://www.deakin.edu.au/~lim
------------------------------------------------------------

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

> 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