318
Chapter 25 Some Additional Topics
2.
Structures for 3-Electron 3-Centre
Bonding Units
Although 3-electron 3-centre bonding units with three overlapping atomic orbitals
are not electron-rich, for them increased-valence structures of the general types
X R :Y and X : R Y (with atomic formal charges omitted) can be constructed.
With one atomic orbital (x, r and y) per atomic centre, their electrons occupy the
orbitals of Eqs. (2) and (3), respectively.
xr = x + lr, ry = r + k 1 y, yr = y + k 2 r
(2)
yr = y + kr, rx = r + l 1 x, xr = x + l 2 r
(3)
In Section 13-9, increased-valence structures are displayed for H 3 .
The mechanism for the radical transfer reaction X
+ R : Y X : R + Y
has
been formulated
6-8 (without atomic formal charges) as
X
R Y
[X R Y
+
X
R
Y]
X R + Y
Reactants
Reactant-like Complex
Product-like Complex
Products
With each pair of R:Y and X:R electrons singlet spin-paired, the reactant-like
and product-like wave-functions (RC) and (PC) wavefunctions are equivalent
to
6-8
(RC) = (Reactants) + l(Reactants*), i.e.
X
R
Y = X R Y
X
R
Y
(+)
(-)
and
(PC) = (Products) + k(Products*), i.e.
X
R
Y = X R Y
X
R
Y
(-)
(+)
As for S N 2 reactions (cf. Addendum, Chapter 21), without the formation of the
intermolecular one-electron bond that are present in the reactant-like and productlike complexes, the valence-bond mechanism for X
+ R : Y X : R + Y
involves
a 2-step mechanism
6c . The origin of electronic hypervalence for the reactant-like
and product-like complexes is associated
5,6c,9 with the contributions of the excited
state wavefunctions (Reactants*) and (Products*) to the (RC) and (PC),
respectively. The excited state wavefunctions are those for Pauling “3-electron
bonds”.
“Increased-Valence”
Chapter 25 Some Additional Topics
2.
Structures for 3-Electron 3-Centre
Bonding Units
Although 3-electron 3-centre bonding units with three overlapping atomic orbitals
are not electron-rich, for them increased-valence structures of the general types
X R :Y and X : R Y (with atomic formal charges omitted) can be constructed.
With one atomic orbital (x, r and y) per atomic centre, their electrons occupy the
orbitals of Eqs. (2) and (3), respectively.
xr = x + lr, ry = r + k 1 y, yr = y + k 2 r
(2)
yr = y + kr, rx = r + l 1 x, xr = x + l 2 r
(3)
In Section 13-9, increased-valence structures are displayed for H 3 .
The mechanism for the radical transfer reaction X
+ R : Y X : R + Y
has
been formulated
6-8 (without atomic formal charges) as
X
R Y
[X R Y
+
X
R
Y]
X R + Y
Reactants
Reactant-like Complex
Product-like Complex
Products
With each pair of R:Y and X:R electrons singlet spin-paired, the reactant-like
and product-like wave-functions (RC) and (PC) wavefunctions are equivalent
to
6-8
(RC) = (Reactants) + l(Reactants*), i.e.
X
R
Y = X R Y
X
R
Y
(+)
(-)
and
(PC) = (Products) + k(Products*), i.e.
X
R
Y = X R Y
X
R
Y
(-)
(+)
As for S N 2 reactions (cf. Addendum, Chapter 21), without the formation of the
intermolecular one-electron bond that are present in the reactant-like and productlike complexes, the valence-bond mechanism for X
+ R : Y X : R + Y
involves
a 2-step mechanism
6c . The origin of electronic hypervalence for the reactant-like
and product-like complexes is associated
5,6c,9 with the contributions of the excited
state wavefunctions (Reactants*) and (Products*) to the (RC) and (PC),
respectively. The excited state wavefunctions are those for Pauling “3-electron
bonds”.
“Increased-Valence”