Page 1 - Interaction of Multiple Bonded and Unsaturated Heavier Main Group Compounds with Hydrogen, Ammonia, Olefins, and Related Molecules

Interaction of Multiple Bonded and Unsaturated
Heavier Main Group Compounds with

Hydrogen, Ammonia, Olefins, and Related
Molecules

PHILIP P. POWER*
Department of Chemistry, University of California, One Shields Avenue, Davis,
California 95616, United States
RECEIVED ON MARCH 18, 2011

CONSPECTUS

e showed in 2005 that a digermyne, a main group compound with a digermanium core and aromatic substituents, reacted
W directly with hydrogen at 25 °C and 1 atm to give well-defined hydrogen addition products. This was the first report of a
reaction of main group molecules with hydrogen under ambient conditions. Our group and a number of others have since shown
that several classes of main group molecules, either alone or in combination, react directly (in some cases reversibly) with hydrogen
under mild conditions. Moreover, this reactivity was not limited to hydrogen but also included direct reactions with other important
small molecules, including ammonia, boranes, and unactivated olefins such as ethylene. These reactions were largely unanticipated
because main group species were generally considered to be too unreactive to effect such transformations.
In this Account, we summarize recent developments in the reactions of the multiple bonded and other open shell derivatives of
the heavier main group elements with hydrogen, ammonia, olefins, or related molecules. We focus on results generated primarily
in our laboratory, which are placed in the context of parallel findings by other researchers. The close relationship between
HOMOLUMO separations, symmetry considerations, and reactivity of the open shell in main group compounds is emphasized, as
is their similarity in reactivity to transition metal organometallic compounds.
The unexpectedly potent reactivity of the heavier main group species arises from the large differences in bonding between the
light and heavy elements. Specifically, the energy levels within the heavier element molecules are separated by much smaller gaps
as a result of generally lower bond strengths. In addition, the ordering and symmetries of the energy levels are generally different
for their light counterparts. Such differences lie at the heart of the new reactions. Moreover, the reactivity of the molecules can often
be interpreted qualitatively in terms of simple molecular orbital considerations. More quantitative explanations are accessible
from increasingly sophisticated density functional theory (DFT) calculations.
We open with a short description of the background developments that led to this work. These advances involved the synthesis
and characterization of numerous new main group molecules involving multiple bonds or unsaturated configurations; they were
pursued over the latter part of the last century and the beginning of the new one. The results firmly established that the structures
and bonding in the new compounds differed markedly from those of their lighter element congeners. The knowledge gained from
this fundamental work provided the framework for an understanding of their structures and bonding, and hence an understanding
of the reactivity of the compounds discussed here.

1. Introduction: Heavier Group 14 Element and stabilization of heavier group 14 element alkyne
Alkyne Analogues, Related Group 13 Dime- analogues REER (E = SiPb, R = large organic or silyl
tallenes, and Other Low Valent Group 13 and substituent) were a significant part of these advances.
2
14 Element Species Beginning with the diplumbyne Ar*PbPbAr* (Ar* = C 6 H 3 -
i
The new millennium has seen major developments in 2,6(C 6 H 2 -2,4,6-Pr 3 ) 2 ) in 2000, 2d stable tin, 2b germanium, 2c
1
multiple bonded heavier main group chemistry. The synthesis and silicon 2a,e analogues had been prepared by 2004. This
Vol. 44, No. 8 ’ 2011 ’ 627–637 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 627
Published on the Web 06/10/2011 www.pubs.acs.org/accounts
10.1021/ar2000875 & 2011 American Chemical Society