Five Hydrogen Probe Experiments
main group elemental hydrides as a set are subjected to a
series of experiments with nature's simplest chemical probes:
H+, H & H•. Patterns
of structure and reactivity behaviour are found.
The Probe Experiments
The first 36 main group elemental
hydrides, H2 to BaH2, are
probed in five experiments involving hydrogen species. These five experiments
Add a proton or
Remove a proton, H+
Add a hydride ion or two, H
Remove a hydride ion or two, H
Remove a hydrogen radical or two,
To and/or from the first
36 main group elemental hydrides:
The main group elemental hydrides
and the products of the five hydrogen probe experiments can be grouped
into periodic "arrays", or as we shall call them "congeneric arrays". There are 23 such arrays:
are 1x1 dots
4 arrays are 1x4 series
6 arrays are 1x5 series
(Group 1 and 2 cation series are counted only once)
5 arrays are 4x4 planars
Arrays always consist
of sets of isoelectronic species:
"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." IUPAC
It follows that, isoelectronic
species have the same Lewis structure and/or frontier molecular orbital
(FMO) topology, as discussed on this page of the Chemogenesis webbook.
This is important because species
can only be expected to have similar, mutually predictable, reaction chemistry
if they are isoelectronic. And isoelectronic arrays usually, but do not always, exhibit
linear reactivity traits, something we shall explore in detail over the
next few web pages.
Arrays which are linear
in both structure and reactivity are defined as being:
(or "of the same family" or here)
The 23 arrays associated with
the main group elemental hydrides and generated by the five hydrogen probe
experiments exhibit a few, general types of reactivity behaviour:
9 arrays are Lewis acids
8 arrays are
2 arrays are Lewis
2 arrays are
2 arrays are metallic
These are colour coded:
Why Are The
Main Group Elemental Hydrides and The Five Hydrogen Probe Experiments So
the species generated by the five hydrogen probe experiments are common
with well established structure and reaction chemistry.
- The species
generated by the five hydrogen probe experiments make excellent surrogates
for other cations, anions and radicals. For example, the [BH4]
ion has many similarities with the [BF4]
H+, hydride ions, H, and hydrogen radicals,
H, make excellent structural and reactivity probes because they
are electronically simple moieties. Hydrogen interactions minimise the
often convoluted orbital/orbital interactions observed with heavier
cations, anions and radicals.
species have such a low mass that they are able to partially quantum
tunnel between species (states). Transfer reactions involving H+,
H and H are kinetically fast and the thermodynamic
product product is formed. Complicating activation energy effects are
acid reaction, proton or H+, transfer, is a thermodynamically
equivalent nucleophilic substitution, R+ transfer, is kinetically
may require heating in a polar solvent.
Side reactions are likely.
The kinetics may be first order, SN1,
or second order, SN2.
- The supply
and removal of H+ to/from a species equates with Brønsted
acidity and the pKa of a Brønsted
acid gives information about both the Brønsted acid and about the conjugate [Brønsted AND Lewis] base. The
fact that methane has a pKa 46 tells us
about the proton donating ability of CH4 AND about
the proton abstracting ability of the conjugate base, H3C. There is a
- Weak Brønsted
acids have strong conjugate Brønsted bases.
- Strong Brønsted acids have weak conjugate Brønsted
is such a weak protonating [Brønsted] acid that many would
not consider it to be acidic at all. But the conjugate base the methyl
anion, H3C, in the form of
methyl lithium, LiCH3, is amongst the strongest
proton abstracting [Brønsted] bases known. It is a "super-base".
- The element
to hydrogen bond length, X-H, is a subtle and versatile proxy for a
kind of reactivity.
because the parameter often tells us something about the intrinsic
hardness of an atomic centre X, a term which will be strictly
defined later, here.
because the X-H bond can be disconnected , as indicated
by the disconnection arrow =>, in three ways to give information
about conjugate anions, X, cations, X+,
and radicals, X.
- The XH molecular
distance (bond-length) parameter is difficult to obtain by physical
experiment because hydrogen atoms cannot be seen in x-ray crystal
analysis, the most common direct method of determining the three dimensional
structure of molecules. To observe hydrogen atoms, and hence XH
distances, it is necessary to perform neutron diffraction experiments.
- However, XH
distances can be readily and accurately calculated using ab initio
molecular orbital (MO) software such as Spartan by Wavefunction.
- The five
hydrogen probe experiments are a subset of the 10 simplest probe experiments,
Group Elements & Hydrides
Dots, Series & Planars
© Mark R. Leach 1999-
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