Page 58 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

A COLLECTION OF BASIC CONCEPTS
38
1.22 ARROW PUSHING: ORGANIC PARADIGMS

Organic chemists have codified a number of rules and guidelines as aids to arrow pushing.
We’ll summarize a few of them here as a starting point for exploring the more diverse world
of inorganic mechanisms.

1. Stable molecules, the final products of reactions, typically have noble gas configura-
tions on all atoms, that is, octets on all atoms B through F.
2. Nucleophilic attack on a center with an inert gas configuration (an octet) must lead to
bond breakage, that is, departure of a leaving group.
3. Stable organic molecules strive toward charge neutrality for all atoms, although
charged species such as carbocations may arise as intermediates. Like charges on
two adjacent atoms are a taboo in organic chemistry!
4. Good leaving groups are relatively nonnucleophilic and nonbasic. Thus, N is a superb
2
leaving group, and water (H O) and alcohols (ROH) are very good leaving groups as
2
−
−
−
well. By contrast, OH and RO are lousy leaving groups and H (hydride) and R −
(alkyl anions) are far worse.
For us, the key question is: how well do these guidelines carry over to inorganic chem-
istry? Not too well, it turns out, as we discuss below!

1.23 INORGANIC ARROW PUSHING: THINKING LIKE A LONE PAIR

Generally speaking, the mechanisms of p-block element reactions are not particularly con-
sistent with the rules outlined above. The reason for this boils down to the so-called first-row
anomaly, where both first- and second-period elements (H–Ne) are all somewhat unrea-
sonably lumped together as first row. The expression means that the chemical properties
of first-row elements are anomalous relative to those of their heavier congeners. Let us go
through the above four rules one by one and see how well they hold up in a main-group
inorganic context.

1. The octet rule breaks down routinely as soon as one goes down to period 3.
Main-group centers with more than eight electrons in their valence shells abound
for period 3 and below. Molecules containing such centers are called hypervalent.
−
Well-known examples include SiF 6 2− ,PF ,PF ,SF ,SF ,BrF ,IF ,IF ,XeF ,
4
5
6
7
2
5
6
3
XeF ,XeF , and XeF 2− . Even this short list of paradigmatic examples should show
4 6 8
with complete clarity that hypervalent molecules are anything but unusual; they are
ubiquitous for p-block elements in period 3 and below. Not surprisingly, therefore,
the octet rule is essentially irrelevant for these elements. (Note: The bonding in the
hypervalent molecules listed above might seem puzzling at first sight, but we will
address that issue over the next few sections.)
2. As mentioned in Section 1.9, nucleophilic attack on a heavier main-group center
with a noble gas configuration does not necessarily lead to immediate bond break-
age. Instead, the first step may be an A reaction leading to a hypervalent intermediate
with an expanded octet. The leaving group would them depart in a subsequent D pro-
cess. As mentioned, this two-step process is known as S 2-Si. In this book, we have
N