Re: CCL:isolobal vs. isoelectronic

[Guosheng Wu <gwuxi (- at -) chem.nwu.edu>]

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