ws 8 ak

WS 8:The Valance Bond iiodel and Hybridizotion
The Lewis theory of the chemicol bond ond its extension VSEPR, ossume thot eoch bonding electron
poir is locoted between the two bonded otoms. From chopter 6,we know thot electrons ore not
locoted at precise positions but ore spraod over o region defined by the atomic orbitals they
occuPy. Our tosk is to see how to combine the Lewis opprooch with the description of o molecule in
terms of orbitols. We will use volence bond theory (VBT) for this. Volonce-bond theory is o
description of bond formotion in terms of merging of otomic orbitsls in the volence shells of
neighboring atoms ond of poiring of the spins of the electrons lhot occupy these orbitqls.
Types
of
bonds in VBT:
There are two types of bonds thot we consider in volence bond theory: the sigmo bond (o) ond the
pi bond (n). Lets consider overlap between pure otomic orbitols, for example, ls orbitols ond 2p
orbitals. The sigma bond (o), or sigma type overlap occurs when the orbitols lie along the internuclear axis, the line thot joins two atoms. The following overlops form s o bond: s-p, s-s, p-p (end
to end). The sigmo bond is usually a single bond or o skeletol bond. I
A multiple bond consists of o o bond ond one or two n bonds. Tha side-to-side overlop of
two p orbitols forms o n bond. The two lobes of o n bond lie an either side of the axis. To achieve
side-to-side overlop, the p orbitols must be oligned. This meons thot the molecule is not free to
rotote obout o multiple bond oxis os it is about a single bond axis. A a bond is weoker thot o o bond
becouse its overlop is not os efficient. A single bond consists of one o bond between two atoms; a
double bond consists of a o bond ond one n bond between two otoms; o triple bond consists of a o
bond and two n bond between two otoms.
Toble 1; DrowirE the bond:
Drow a Zpx orbitol
oo
Drow o 2pz orbitol
Drow o
ls orbital.
B
o@
Draw o Zpx orbitol with 1
electron overlapping o ls
Drow two Zpx orbitols with
1
electron each showing o bond
Drow two ?pz orbitols with one
electron eoch showing
r bond
orbital with 1 electron forming
a o bond
Notice that the two 2px orbitals overlop on the inter-nucleor oxis, while fhe two Zpz orbitals form
of electron density obove ond below the inter-nucleor oxis.
o plane
1
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Port 2: The fundomental bosis of volence Bond Theoqy;
Volonce bond theory states
thot o bond forms when two otomic orbitols come close enough
together thot they overlop. Actuolly orbitals inter-penetrote, but the term overlop is
commonly
used' Overlop implies that the atoms shore electrons. Eoch bond contoins of most fwo
electrons
with their spins poired (opposed). A high degree of overlap means that o strong bond is
formed.
Atomic orbitols with directionol chorqcter (those thot point in q certain direction)
overlap better
thon those with no directionol chorocter. We soy they overlop more efficiently
or there is s
greater degree of overlop.
An ottempt to explain the bonding in the CHa(methane) molecule provides
on exomple of the
need f or hybridization. The promoted, or excited, conf igurotio n of C permits
the formation of four
eguolC-H bonds.
C: lHelzs??p2
tu
I
?s
C[promotedl;
zsrzp3
t-rl
2s
t
?p
t
t
2p
Tf we examine the un-hybridized orbitols of carbon, we seea problem. A bond that would
result from the overlap of o holf-f illed 2p orbitol on corbon with o half-f illed orbitol on
the
hydrogen (1s) would hove o bond cngle of 9O". The filled 2s orbitol on cqrbon would
not form o bond
ot oll. These bonds would disogree in two woys crrith the experimentol resutts
for the bonding on
methone. In methone, the four bonds arethesome; the bonds we hove just
formed differ from
each other becouse they use diff erent orbitol overlop to form the bonds. One-bond
forms vio Zs-1s
overlap while three other bonds form vio 2p-1s overlop. Also, the just predicted
bond ongles ore
9o", compored to the bond angles of 109.5". However, we can .rmbin. the four pure otomic
orbitols
to produce four eguivolent hybrid orbitols. Notice that hybridization is used after we know
the
bond angles, which VSEPR theory con often predict.
In the Process of hybridization, pure atomic orbitals ore combined to produce hybrid
orbitols thqt hove the correct geometry for the resulting molecule. The hybridizotion of o
species
con be predicted by starting from the Lewis structure. Ther e o?e as rnony hybrid
orbitols os there
ore totol electron poirs on the centrol otom. This implies thot there is conservotion of orbitals.
Whot goes in must come out. Recoll thot the total numbe r of electron poirs is found by odding
the
number of lone poirs to number of bonding poirs. The pure otomic orbitols ore used in order of
increosing energY- Thus, if the totol number of electronpoirs is two, ond one s ond one p
orbitol are
combined or hybridized to form two^sp hybrid orbitols. If on s ond two p orbitols
hybridize (three
otomic orbitols), they form three sp2-hybridized orbitols . When hybrid orbitals are ormed,
f
those
thot ore to hold lone poirs are tull, while those that ore to form bonds hove one electron. The
exception is the coordinate covolent bond. Whenever we consider the origin of bonding, we must
ossess whether the overoll outcome of aseries of changes is o net lowering of energy. fn corbon,
not only is the energY of promotion or formation of hybridized orbitols very smoll ond for bonds ore
tormed in ploce of the two bonds of the un-promoted electrons in the pure orbitols, but the sigmo
bonds ore stronger thon in the obsence of hybridization becouse the hybrid orbitots
ore strongly
directional ond hove a large overlap urith the orbitols of the neighboring otoms.
The properties of hybrid orbitols depend on the properties of the pure atomic orbitals
thot
were combined to form them. Consider the direction of the orbitol. A p orbitol points
olong o line
on an axis (xyz). Thus, if on s orbitol is combined nrith o p orbitol the
tnro sp rryurids formed must
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olso point olong o line.
In similor foshion, sp2 orbitols are confined to o plane; they are made from ?
orbitals interocting with on s orbitol oll in the some plone; they must be flot. When three p
orbitals qnd qn s orbital interoct, the result is o three-dimensional structure becouse eoch of the p
orbitols lies on a diff erent oxis orthogonol to each other. Therefore, the four sp3 orbitols are
three-dimensional. When the number of bonds pairs fotals five or six, d orbitals must be included
in the hybridizotion scheme, since the mixing orbitols are s, p, ond d otomic orbitals. This mokes
the hybrid orbitals sp3d ond sp3d2 respectively.
p
Selecting a hybridization scheme:
1.
2.
3.
4.
5.
6.
7-
8.
9.
Drow the most plousible Lewis structure for the species.
For each centrol otom, count the totol number of bonds ond lone poirs. Thesearetheregions
of electron density or electron domoins.
The total number of electron domains determines the electron domoin geometry bosed on
VSEPR ond the number of hybrid orbitols needed for thot geometry. Use the following
informotion to determine the hybridizotion of the centrol atom:
Write the ground stote orbital diogram of the centrol otom (subtrocting or adding electrons to
occount for the chorge, if ony, of the species.
Each electron domoin represents one hybrid orbital needed. For eoch hybrid orbifol need,one
atomic orbifol is used. 2 electron domoins = 2 hybrid orbitols 2 atomic orbitals.
=
Consider the n-level for the centrol otom. All n=2 atoms hove only s ond p orbitols to hybridize,
while n=3 ond obove hove s, p, and d orbitols to use.
Form the hybrid orbitols needed- Fill with the valence electrons, one electron per orbitol. Keep
in mind that if the otom has lone pairs, those lone pair electrons must go in hybrid orbitols.
Overlap two half-filled orbitols , onz froma ierminol (out side) otom ond one from the centrol
otom for eoch bond.
overlop one holf-filled p orbitol on a bonding otom with one half filled p orbitol on o centrol
m bond. a bonds ore mode from un-hybridrzedorbitols.
otom to form eoch
Toble
2:
Electeon-Poir
Eln.
Pairs
No.
Lineor
L
o-L
b9
LE
+o
uc)ctr
q)
o-.4
3
4
5
6
Trigonol
plonar
Tetrohedrql,
tetrohedron
Trigonal
bipyromidol
Octahedron
o
?
I
ca\
lrt
Bond ongles
of electron
190"
109.5"
L?O"
t20"
90"
eguotorial
90o qxiol
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Toble 3:Possible Shopes of AX6EI
x----+-X
AXz
xx
-'\'/^
AIA
XO
X- -X
\./
AX:
AXzE
x
-A
V
l
x,l\,
{l\*
Ax+
AXsE
x
AXzEz
x
X
*'yic
*,,,,i
x/l"-n
X
x.
D4x
X
l
X
Axa
X
D+-"
I
Ax+E
AXsEz
AXzEz
*,,"f,,o -r0\
*,,,,],,,,o ,,,.9,,n,.,1-*
*-&*
AXo
AX5E
AXaE2
Tqble 4
2
Concepts Eguivolent To Bond Order
Lewis theory
Single bond
Double bond
3
Triple bond
Bond order
1
Valencebond theory
One o bond
One o bond ond one zr bond
One o bond ond two n bonds
Table 5
Number
of
lone poirs
ond bonds
ieometry of hybrid orbitols
Geometry of hybrid
Hybridizction
2
sp
3
sP2
4
sps
5
sP'd
linear
triongulor
tetrohedrol
trigonal bipyromidal
sp'd'
octahedrol
Bond ongels
1900
L20"
109.5"
L20"
90"
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fn the toble below, fill in the types of atomic
ond hybrid orbitals used
for electrondomoin
geometry.
Formuta Exampre
ffi*r
Electrondomoin
geometry
AX,
BeCl..
lineor
Central otom s n ond types of orbitols
Use RED for regions of electron density
Use AO for otomic orbitals
Use HO for hybrid orbitols
a w=2,
a Two R.ED = 2 Ao wsed, to n*ake 2 t+o
a rhe *o n*ad,e are two esp orbLtaLs
o I *se the cs awd owe of the three z1
orbltaLs , &vo tto orbltal* weeded-to
w"a4e twa 2s? orbltaLs; &vo r,twused z7
orbltaLs rey,*alw for n overLap (doubLe or
trr,TLe bawds, or wot l+sed at aLL.
AX.
BF,
3
trigonol
plonar
lrr=L
J tr,FD
. t IO r*u-As
SO,
3
trigonol
plonar
3rE.f
t\ =7
3leeD
Do w{/L\}fic2
AXo
CFo
0
4
tetrohedrol
4 <fhe^M<otts
4??M
h=Z
4 onD
25?'
t+O "v!4Lo 4'
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Formuta Exomple
t*ffit
ErectrondomOin
geometry
AX3E
NF. 1 3
4
tetrahedrot
+
AxzE?
oF, ? z
4
centrarotom,s n ond types of orbitors
Use RED for regions of elec,tron density
Use AO for otomic orbitals
Use HO for hvbrid orbitols
4<6D= 4,Aro = 4rl.
L zs + sz?-s
eSt= ot\,; k&
terrohedrot
4RfD = +A'Sa*+O
4?-s t-3 2#
4 2'?3
AXu
PCts
O 5
5
!o'*A{
6r+0
trigonol SREID =Erro=
bipyromidol
{s, + 3 3qi + 4 3"1
ffi 4- =*1*P 5 rsfl)*" Frb:[4,[J *-'l.
Ax4esFoL45''il'fl'l"|5{?f-D*5*o-FBtdD
4 as r B SPts +
+ 3d-arb,(*IJ +
uu\
Ax3E" crF3-
d
s--l
5
s. rsf d
*..
ti
k-0s
1 3'l
J
2 3 5
o,lffili", Sersu = fro = BL+o
3s+3*Rb +,{ ts/
t
4 3r\o.\r) \r^b
)C
U
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,tc\s<y'
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i
Sp
*n
Li+rr,L
Alscher
t^rL
No. of reg)ons
Formulo Example lone bonding totql Electron-
Central otom's n ond 'types of orbitols
Use RED for regions of electron density
Use AA for otomic orbitols
Use HO for
domoin
geometry
XeF..
AXzE3
trigonol
bipyramidol
3
6
SP,Fb
sFu
AX5E
octohedro
sf d
I 6Ri€lO a
SLfD
r
srLuL'Qs -"'-&'
(srtO *- 6 [i'O
6fl.f0: G*a 3 UW
4s + s 4Fb +
6 * sf d,' ,r"r,*U
XeFo
AX4Ez
octohedrol GEtrJ
?
6r+
G
How
4,c4
Ls .t- 3)e'' + 2 3dJ
G 3sfd'- t'""&
octohedral
BrFu
=.
6's f 3 6ft
G
AX,
=$
2
?- 6Jo =- 61fr
+ zcas
3
cf'
6 sgsd'*4
to do o hybridizotion scheme for o molecule.
Consider the nitrite ion NQ
Following the steps listed obove,
I o-,t<- I
ws
I
nitrite.
resonanceforma
I o:+rr--O:
I
resonance form b
1.
Drow the best lewis structure
2.
determine the regions of electron density oround the otoms of interest For example,
nitrogen hos three regions of electron density: 2 bonding regions ond I lone poir.
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3.
4.
5.
Determine the electron geometry: Triangulor planor for the oxygen and nitrogen, one
oxygen could be considered sp3'
Now we consider the electron conf igurction and how electrons ore distributed in
hybridizotion.
spz hybr;dizqtion on N
6round state N: [He] 2s
sp2 hybridized N [Hel: spz
zp
ro tr E E
oround stsre o: t{el2s N zp ro tr n
o [He]: spz ru |T| Fl zp ffl
o: LHells E-+-EEn sp2hybridizedo [He]:sp3ro M E E
spz hybridized
Ground state
o Bond Descriptions
structure a= N (spz)-o (2p) N (sp2)-o qsp21
structure A= N (spz)- o (sp,) N (sp,)-o (ep1
fn Structure A,the r bond forms between N ond O#2-Tn Structure B, the n bond forms between
N and O #t. Volence bond theory, strictly opplied, still does not exploin resononce structures. The
problem is tronsfered from lines between otoms to pictures of p orbitols. Notice also thot the
number of boxes (or lines) rep?esents the numbers of orbitols and the grouping of the boxes
represents their relotive energies. fn the un-hybridized otom, the boxes form distinct groups for
the s, p (ond d-but not for these otoms) orbitols. After hybridizotion, the boxes form two groups
consisting of hybridized orbitols at an energy that is broadly the average of the contributing
orbitols, ond the unused P- (or d) orbitals. These orbitols cqn hove electrons, or con be un occupied.
Structure A
Structure
oFt
6/c *J*'l*iWl
/
'1w
,y,
''r-q,
B
, S?' '
',1,',4
4'
+EED
s?*r
lO-N:O
a-
e
Qgestions
1.
0:N-0,
e
,,i,-fus
to help you think.
For the following molecules (OFz,XeFz, BrF3):
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o) Drow the Lewis structure.
b) Determine the electron domains obout the centrol otom.
c) What is the e-lcpected hybridizotion of eoch molecule.
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d)
e)
Whqt is the moleculor geomelry?
Predict the bond ongle(s).
Put your onswers in the chort on the below
XeFz
OFz
BrFs
Lewis
structure
tl
',
,a
?
sl
,'
tt
-o,' -.Y',
iF:
.t
1r
$,"t;
1,,
-^
(ijg.
ul -t:,:
, -*l
tl J
t,
#f
of
#of
RED
AO
l* of HO
Hybridization
of centrol
otom
5
4
5
+
a
r,
4
5
5
J10
s pt4
at
ry"4
Moleculor
geometry
tl*u*t'"rl
+ber&d
It'q/
brgY'ro'
L\'V'r
)/l't'
ndrl
TEr""4,1
Bond angles
oround central
otom
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OFz
Hybridizotion
&lVLL
for
scheme
XeFz
centrol otom
BrFs
lL 1,t/ fu
Sro
LL
s g'A
5
2.
s.+P
it-Sr -l{
.].
t
r
t+
P'4
Which type of otomic orbitsls of the centrsl atom mix to form hybrid orbitols in SiClH3
(which is central, 5i or ClZ) and ClzO (one Cl is c.entrol)
#
l,
I
H
I
i-!
1v L
lv
a-
Y(, r's b"A
3.
,i-frf
* +3 i+ ct is cewhrql
ur-vr
i, si
,t- -gr--H
t 3f
@l',*tJ**'(
Ll
gf3
For the molecule hydrogen sulfide:
o) Draw the Lewis structure.
b) Determine the electron domoins about the sulfur otom.
c) Predict the bond angles ZHSH.
d) Whot is the molecular geometry?
e) Whot is the expected hybridization of hydrogen sulfide.
t)
d
q) t;-fi
)
Drow o bonding description for hydrogen sulfide.
Show the hybridization scheme for the molecule.
\r) 1 €,D. c)t
1,0+^*o
lo1,
tr*
w'
o.
fu 1L
3,t'
75
4.
IL
I
UP
Use portiol orbitol diagroms to show how the otomic orbitsls
lF'
''lO
;r
=-)Ell
r%-
*
'l-,gFf,'
t-l
.t
6,tL€Q
_1!
t
l'
/
l*,P
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T
1v
tt
t
ro of
..
I
r',
\, rrM9
e uH-to$e*/
pase
'rQl
\ "
f,P3
lqika:
LLL L
sfl=d'
of the centrol otom leod to
eet,,
b^d {o
-4
(tr
tl/
l'o
,/
hybrid orbitols in IF+- and GeCl+.
rf
o=rr't-
!-a,t
-r*
t{
d) L^tu+ e) rP=
l'osskaan
to a
& +t
Cli
yl
€b,A
d
L LL!
Alscher
5.
Predict the shape, stote the hybridization of the centrol atom and givetheideol bond
ongle(s) ond ony acpected deviotion for: fFa*, BiF:z-, BrOs-, AsClq-.
E*
'sp'd
,'!.,
'rl ' lot [?n
,,
:!',h!'Y,: LossVfc
W-B?
\r
" \-
ntpu.I
u-rdA
lr.)
6.
,,1,
( n*
t
The compound 2,6'dimethylpyrozine gives chocolote its odor and is used in f lavorings. A
valid Lewis structure is shown below.
a) Which otomic orbitols mix to for the hybrids of N?
b) fn whot type of hybrid orbitol does the lone poirs of N reside? fs the hybridization
l -*
S 1:5
T'
c)
d)
d
H
;*T[';::i:;r1T:^*".,il:";#'
How many delocalized m bonds are present?
How many o bonds arepresenl?
c) +1M 1..J s
,', s p*
jT-E{
H
#rJ -c
of eoch c in the rins? Exproin?
4Ye
1F
L/,
t"F+ o$4
I
.-c
t) sf
\-t-,
nz
LP
."qtL(-
A.irt\L
3rt-r-o=3rro=3lYO z
gqtr
^4-t'o
4 nsn -toa
#,1r
s =6
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*d z+
h br.i{
\A/'$\^'t,t4
w
a.L J Q)s \-e,
P
u'lE\="9
, Ll\e C i r, -t*b i:
is
\,1-I,"', [,u- C N^ ru ]
C,r$
M
St3
Sp
5P'
z sP'
2v
3 ++ kl+.15 | 4
3re ct-b
c-c = lb d brdJ
)b
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c--t/ $