Chng 3. LIN KT TRONG PHN T. CU TO V TNH CHT 3.1. Mt s khi nim 3.1.1. Khi nim v phn t

Phn t l phn t nh nht ca mt cht c kh nng tn ti c lp m vn gi nguyn tnh cht ca cht .

Theo quan im hin i th: phn t bao gm mt s gii hn ca ht nhn v cc electron tng tc vi nhau v c phn b mt cch xc nh trong khng gian to thnh mt cu trc khng gian bn vng.

Phn t khng ch l cc phn t trung ho nh H2, Cl2, CO2 m cn bao gm cc ion phn t nh 2 , 3H NO+ ,

3.1.2. m in

m in () ca mt nguyn t l i lng c trng cho kh nng ca nguyn t nguyn t trong phn t ht cp electron lin kt v pha mnh.

Nh vy cng ln th nguyn t d thu electron. * Mt s quy lut: - m in ca nguyn t cng ln th kh nng ht cp electron cng mnh, tnh phi kim ca nguyn t cng mnh.

V d: Trong phn t HCl, gia hydro v clo c mt cp electron dng chung, chng b lch v pha nguyn t clo v clo c m in ln hn hydro nn trong phn t HCl th hydro mang in dng v clo mang in tch m.

H Cl + - Trong mt chu k, i t tri sang phi: m in ca cc nguyn t tng dn. - Trong mt nhm, i t trn xung: m in ca cc nguyn t gim dn.

Bng 1. m in ca mt s nguyn t theo Pauling

IA IIA IIIA IVA VA VIA VIIA

H 2,1

Li 1,0

Be 1,6

B 2,0

C 2,6

N 3,0

O 3,4

F 4,0

Na 0,93

Mg 1,3

Al 1,6

Si 2,0

P 2,2

S 2,6

Cl 3,0

K Ca Ga Ge As Se Br

31

0,82 0,95 1,8 2,0 2,4 2,1 2,7

Rb 0,82

Sr 0,95

In 1,8

Sn 2,0

Sb 2,4

Te 2,1

I 2,7

Cs 0,79

Ba 0,89

Tl 2,0

Pb 2,3

Bi 2,0

Po 2,0

At 2,2

3.1.3. Mt s c trng ca lin kt a. Nng lng lin kt

Nng lng ca mt lin kt ho hc l nng lng cn thit ph v lin kt v thnh nguyn t th kh.

V d: Nng lng ca lin kt (H-H) trong phn t H2 chnh l hiu ng nhit ca qu trnh.

H2 (k) H (k) + H (k) = 104,2 kcal/mol Nng lng lin kt c trng cho bn ca lin kt, nng lng lin kt cng

ln th lin kt cng bn. b. Gc lin kt

Gc lin kt l gc to bi 2 na ng thng xut pht t ht nhn ca mt nguyn t v i qua ht nhn ca 2 nguyn t khc lin kt trc tip vi nguyn t trn.

V d: Gc lin kt trong cc phn t sau: H2O, CO2, C2H4

c. di lin kt

di lin kt c xc nh bi khong cch gia hai nhn ca hai nguyn t khi hnh thnh lin kt.

Nu cc nguyn t tham gia lin kt nh nhau, th lin kt cng bn khi di lin kt cng b.

Bng 2. di lin kt v nng lng lin kt ca mt s lin kt

Lin kt Phn t r0(A0) E (Kcal/mol)

C H CH4 1,09 98,7

C Cl CHCl3 1,77 75,8

C F CH3F 1,38 116,3

C C C2H6 1,54 79,3

32

C = C C2H4 1,34 140,5

H H H2 0,74 104,0

O = O O2 1,21 118,2

O H H2O 0,96 109,4

d. Mmen lng cc ca phn t Trong nguyn t, cc electron bao quanh ht nhn c phn b ton ton i

xng khp mi pha nn trng tm ca cc in tch trng vi tm ca ht nhn (trng tm ca in tch dng).

phn cc ca lin kt c nh gi qua mmen lng cc (muy), c n v o l Debye (D). Trong phn t, trng tm ca cc in tch m v dng c th trng nhau hoc khng trng nhau.

- Nu trng tm in tch dng v trng tm in tch m trong phn t trng nhau ta c phn t khng phn cc.

V d: phn t H2, N2, F2 - Nu trng tm in tch m v dng khng trng nhau (xut hin 2 nguyn

t c s chnh lch v m in) th ta c phn t phn cc. Lc ny phn t xut hin mt mmen lng cc.

V d: phn t H-Cl

3.2. Lin kt ion theo Kossel 3.2.1. Khi qut

Lin kt ion l lin kt c hnh thnh do lc ht tnh in gia cc ion mang in tch tri du.

Lin kt ion l lin kt c hnh thnh t hai nguyn t ca hai nguyn t c m in rt khc nhau, mt bn l kim loi in hnh c m in rt b, mt bn l phi kim in hnh c m in rt ln, nh trng hp gia cc kim loi kim hoc kim loi kim th v halogen hoc oxi. Khi xy ra s chuyn dch electron t mt nguyn t c tnh dng in mnh sang mt nguyn t c tnh m in mnh to thnh cation v anion, cc ion ngc du ht nhau bng lc ht tnh in. Vy bn cht ca lin kt ion l lc ht tnh in gia cc ion tri du.

V d: phn t NaCl, electron c chuyn t Na sang Cl to thnh Na+ v Cl-. Na + Cl Na+ + Cl- NaCl 2s22p63s1 3s23p5 2s22p6 3s23p6

Khi cc ion Na+ v Cl- c hnh thnh bi s sau: Na - e Na+ Cl + e Cl-

33

Nh vy, s hnh thnh trong lin kt ion, l do s phn b li cc ion mt cch u n trn cc nt mng li tinh th v gia chng tn ti lc tng tc tnh in v mi pha. 3.2.2. iu kin to thnh lin kt ion

m in ca 2 nguyn t tham gia lin kt phi khc nhau nhiu (hiu s m in 2). 3.2.3. c im ca lin kt ion - Mi ion to ra in trng xung quanh n, nn lin kt ion xy ra theo mi hng hay thng ni lin kt ion l lin kt khng c hng. - Khng bo ho, ngha l mi ion c th lin kt c nhiu ion xung quanh n. - Lin kt rt bn.

Do hai tnh cht ny m cc phn t hp cht ion c khuynh hng t kt hp li mnh m, cc phn t ion ring l ch tn ti nhit cao. Cn nhit thng mi hp cht ion u tn ti trng thi rn, c cu trc tinh th v ton b tinh th c xem nh mt phn t khng l. 3.2.4. Ho tr ca nguyn t trong hp cht ion

Ho tr ca mt nguyn t trong hp cht ion c gi l in ho tr hay ho tr ion ca nguyn t .

V d 1: Na+, Cl- th Na v Cl u c ho tr 1 i vi ion a nguyn t: ta khng ni n ho tr ca tng nguyn t m ni

n ho tr ca c ion.

V d 2: c ho tr 1 ,4+ ,3S 4Cl

c ho tr 2 ,24S ,24

3.3. Lin kt cng ho tr Trong 2 kiu lin kt chnh ca phn t, lin kt ion c gii thch bng thuyt

tnh in ca Kossel cn lin kt cng ho tr l i tng nghin cu ca nhiu nh khoa hc sut t th k 19 n nay. Trong s cc thuyt ny c th k n 2 quan im ch yu l quan im ca Lewis v quan im ca c hc lng t.

3.3.1. Thuyt Lewis v lin kt cng ho tr a. Khi qut

L loi lin kt bng cp electron chung hnh thnh gia cc nguyn t ging nhau hay khng khc nhau nhiu v m in. Chng s dng cc electron lm thnh cp electron dng chung cho c 2 nguyn t, khi chng cng c c cu hnh bn vng ca kh him, lin kt ny gi l lin kt cng ho tr - mi cp electron dng chung to thnh mt lin kt. Vy lin kt cng ho tr l lin kt bng cp electron chung.

V d: H. + .H = H : H hay H H * Phn loi lin kt:

34

- Nu hai nguyn t c m in nh nhau, cp electron lin kt s nm gia, ta c lin kt cng ho tr khng cc. V d Cl : Cl - Nu hai nguyn t c m in khc nhau, cp electron lin kt s nm lch v pha nguyn t c m in ln hn, ta c lin kt cng ho tr phn cc. V d H :Cl - Nu cp electron chung lin kt do mt trong hai nguyn t a ra cn nguyn t kia c dng chung: l hnh thnh lin kt phi tr. Mi cp electron dng chung c k hiu bng mt vch ngang gi l vch ho tr. b. iu kin to thnh lin kt

m in ca 2 nguyn t tham gia lin kt phi khc nhau nhiu (hiu s m in < 2). c. c im ca lin kt - Lin kt cng ha tr l lin kt c hng. - C tnh bo ho. - Lin kt km bn hn lin kt ion. d. Ho tr ca nguyn t trong hp cht cng ho tr

L s lin kt hnh thnh gia mt mt nguyn t ca nguyn t vi cc nguyn t khc trong phn t.

V d: Trong cc hp cht: CO2; NH3; HCl, th clo v hydro c ho tr 1, oxi c ho tr 2, nit c ho tr 3 v cacbon c ho tr 4.

Thuyt Lewis gii thch kh n gin, d hiu v s to thnh lin kt gia cc nguyn t trong phn t, gii thch c cc trng thi ho tr ca nguyn t trong cc hp cht. Tuy nhin thuyt ny cng gp mt s hn ch v khng gii thch c t tnh ca mt s cht.

3.3.2. Thuyt lin kt ho tr ca VB (Valence Bond).

a. Lin kt , Lin kt , lin kt . Thuyt VB dng s xen ph ca cc orbital nguyn t (AO) m t s to

thnh cc lin kt. Tuy theo tnh i xng ca vng xen ph gia cc AO tham gia lin kt i vi trc lin kt (trc vi tm 2 ht nhn), ngi ta phn bit lin kt xch ma (), lin kt (), v lin kt ().

* Lin kt c hnh thnh khi cc obitan nguyn t (AO ) tham gia lin kt xen ph dc

theo trc lin kt (theo trc ni gia hai ht nhn nguyn t).

Lin kt gia hai nguyn t

Lin kt c tnh i xng trc v kh bn.

35

http://vi.wikipedia.org/wiki/T%E1%BA%ADp_tin:Ligatio-covalens.jpg

H - H

F - F

H - F

C - H

C - C

Hnh 9. Lin kt

* Lin kt c hnh thnh khi c s xen ph cc AO ho tr v hai pha ca trc ni gia

hai ht nhn nguyn t tng tc.

p p ( p - d)

Hnh 10. Lin kt

Lin kt bn hn lin kt .

Lin kt lm thnh mt phng i xng vi trc lin kt. Ch : Gia 2 nguyn t lin kt vi nhau trong phn t bao gi cng ch tn ti mt lin kt , cn s lin kt c th bng 0, 1, 2.

Hnh 11. S xen ph xich ma v pi ca cc obitan nguyn t

Lin kt Lin kt ny t gp, l lin kt sut hin do s xen ph ca cc orbital d.

b. Lun im c bn ca thuyt VB

Bi ton phn t hydro. Khi p dng c hc lng t gii quyt vn bn cht ca lin kt ho hc,

nm 1927 hai nh bc hc W.Heiler v F. London gii bi ton tnh nng lng lin kt trong phn t hydr H2.

Kt qu vic gii bi ton ny cho thy:

36

Lin kt gia 2 nguyn t hydro c hnh thnh khi 2 electron ca 2 nguyn t hydro c spin ngc chiu nhau ghp i vi nhau.

Khi nng lng ca phn t hydro thp hn nng lng ca hai nguyn t hydro c lp v mc nng lng ca hai phn t thp nht khi khong cch gia 2 tm ca hai nguyn t hydro l 0,74A0.

Khi hnh thnh lin kt mt my electron khu vc khng gian gia hai ht nhn tng ln ging nh hai m my xen ph ln nhau, do mt in tch m ca khu vc tng ln, nn ht hai ht nhn li vi nhau v lin kt chng li tc l xut hin lin kt gia 2 nguyn t hydro to thnh phn t hydro (H2).

Lun im c bn ca thuyt VB: - Trong phn t cc electron vn chuyn ng trn cc AO. - Mi lin kt cng ho tr c to thnh do s ghp i 2 electron c thn c

spin tri du ca 2 nguyn t khc nhau tng tc vi nhau, cp electron ny c xem nh chung cho c 2 nguyn t.

- Khi xy ra s xen ph gia 2 m my electron lin kt, s xen ph cng mnh th lin kt cng bn.

V d: S hnh thnh lin kt trong phn t H2H: 1s1

H: 1s1H2

Nhng ng ln sng trn s biu th s to thnh lin kt cng ho tr. - Lin kt c phn b theo phng m ti s xen ph ln nhau gia cc

AO tham gia lin kt l ln nht, v nh vy s c nhng phng c u tin trong khng gian ph hp vi cu hnh khng gian ca phn t v vy lin kt cng ho tr c tnh nh hng.

- Lin kt cng ho tr c tnh bo ha, ngha l mi lin kt ch m bo bi 2 electron v mt nguyn t tham gia lin kt ch c mt s gii hn cc lin kt ho tr.

V d: N kt hp vi H to NH3, khng to ra cc phn t NH4, NH5 S lin kt vi H to H2S, khng to ra cc phn t H3S, H4S 3.3.3. Cc thuyt trong khun kh thuyt VB a. Ho tr ca cc nguyn t theo VB

Ho tr c th c ca mt nguyn t c tnh bng s electron c thn trong nguyn t ca nguyn t ( trng thi c bn hay kch thch).

V d: Xt cc nguyn t chu k 2:

Li (1) : 1s22s22p0

N (3) : 1s22s22p3

O (2) : 1s22s22p4

F (1) : 1s22s22p5

37

Be, B, C nhn nng lng, chuyn sang trng thi kch thch. Cp electron 2s s tch i v mt electron chuyn ln obitan 2p cn trng. S tiu tn nng lng ny c b li tha thi khi nguyn t to thnh cc lin kt ho hc.

Be : 1s22s22p0

Be* (2): 1s22s12p1E

B : 1s22s22p1

B* (3) : 1s22s12p2E

C : 1s22s22p2

C* (4) : 1s22s12p3E

V vy ho tr thng thng ca Be(2), B(3), C(4). Cn lu rng trng thi kch thch ca nguyn t to ra cc ho tr khc nhau ch xy ra trn cng mt lp. V d, trong trng hp nguyn t nit, lp electron ho tr ca n ch c hai phn lp 2s v 2p, nn nit khng th c trng thi ho tr nm, v s to ra nm electron c thn N phi chuyn mt electron ghp i 2s2 t lp trong ra lp ngoi i hi nng lng qu ln khng c b bng s to thnh lin kt. Phng php tm ho tr ca nguyn t nh trn ch ng vi cc nguyn t nhm A (tr kh him).

Nhn xt: Thuyt ho tr spin gip ta gii thch c ho tr ca nhiu nguyn t. Tuy vy n cng gp hn ch nhng khng gii thch c s hnh thnh lin kt trong cc ion phn t v t tnh ca mt s cht, khng gii thch c bn ca cc phn t, b. Thuyt lai ho orbitan v dng hnh hc ca phn t * Khi nim

Thuyt lai ho c Pauling a ra trong khun kh thuyt VB gii thch s hnh thnh lin kt trong phn t CH4, BeF2 v cc hp cht tng t.

Theo Pauling khi tham gia lin kt cc AO ho tr ca cc nguyn t khng tham gia ng thi bng cch trn ln vi nhau trc khi i vo lin kt. Hay ni cch khc l cc AO t hp vi nhau thnh cc t hp tt nht to lin kt bn nht. Cc t hp l cc hm sng tng ng nhau, c nng lng v hnh dng ging nhau v c nh hng r rt trong khng gian.

Vy S lai ho obitan l s t hp cc AO trong mt nguyn t to thnh cc AO lai ho ging nhau v nng lng, hnh dng v nh hng r rt trong khng gian.

V d: Xt s to lin kt trong phn t CH4. Nguyn t C trng thi ha tr 4 c 4e c thn: 1s2 2s2 . Bn e ny to thnh 4 lin kt C H, trong c 3 lin kt p- s, ngha l ba obitan 2p ca C xen ph vi 3 obitan 1s ca 3 nguyn t H,

1z

1y

1x p2p2p2

38

to thnh ba gc lin kt HCH = 900. Lin kt th 4: C H c to thnh do s xen ph gia obitan 2s ca C v 1s ca H khng c hng xc nh trong khng gian, v xen ph gia cc obitan s vi nhau l nh nhau theo mi hng.

Nu coi rng lin kt th 4 ny phi cch u 3 lin kt kia th gc lin kt

HCH th 4 phi bng 120014/. Kt qu ny dn n bn ca 1 lin kt C H (do s xen ph s s) khc vi bn ca 3 lin kt C H cn li (do s xen ph p s).

Tuy nhin thc nghim xc nhn rng bn gc lin kt HCH trong phn t CH4 u bng 109028/ (gc 4 mt u) v bn ca 4 lin kt C H u nh nhau.

gii quyt s khng ph hp gia suy lun vi thc nghim, Pauling v Slater ra thuyt lai ho. Theo , khi cc nguyn t tng tc vi nhau c th khng dng cc AO thun khit s, p, d m nhng AO ny c t hp thnh nhng obitan mi c nng lng, hnh dng, kch thc ging nhau, phn b i xng trong khng gian - gi l cc AO lai ha.

C bao nhiu AO tham gia lai ho th c by nhiu AO lai ho c hnh thnh v chng phn b i xng trong khng gian. Mun lai ho c vi nhau cc AO phi c mc nng lng xp x nhau. * iu kin cc AO lai ho bn l:

- Nng lng ca cc AO tham gia lai ho phi xp x nhau - Nng lng ca cc AO tham gia lai ho thp - xen ph cc AO lai ho vi cc AO ca cc nguyn t khc tham gia lin

kt phi ln. Nu khng cc iu kin trn th kh nng lai ho gim dn hoc khng c

s lai ho, kt qu dn n bn lin kt gim dn.

V d: T tri sang phi trong chu k 3, bn lin kt trong cc ion gim dn v cc obitan ho tr ca nguyn t trung tm u c lai ho

sp

4 34 4, ,SiO PO SO 2

4

3, nhng v t tri sang phi trong chu k hiu nng lng cc obitan 3p v 3s tng dn, dn n nng lng lin kt X O gim dn theo chiu trn. T trn xung trong nhm VA b lin kt v gc lin kt trong phn t NH3, PH3, AsH3, SbH3 gim dn v lai ho bn sp3 gim, do nng lng cc obitan ho tr tng dn t N n Sb trong nhm.

Phn t NH3 PH3 AsH3 SbH3

Nng lng lin kt kj. mol-1 380 323 281 256

Gc lin kt HXH 107,30 940 920 910

Tng t nh vy i vi cc phn t XH2 trong nhm VIA.

39

Phn t H2O H2S H2Se H2Te

Nng lng lin kt kj. mol-1 463 347 276 238

Gc lin kt HXH 104,50 920 910 900

Lu : Khi s nguyn t lin kt vi nguyn t trung tm A t hn s obitan lai ha ca A th gc lin kt trong phn t b thu hp li i cht so vi gc gia cc obitan lai ha, v cp e ha tr trn obitan lai ha cha tham gia lin kt c tc dng y mnh hn so vi cp e trn obitan lai ha tham gia lin kt. V d, gc (OSO) trong SO2 l 119,50 < 1200 gia cc lai ha sp2; Gc (HNH) trong NH3 l 107,30 nh hn 109028/ gia cc obitan lai ha sp3.

* Cc kiu lai ho gia cc AO ns v np - Lai ho sp

l s t hp gia mt AOs v mt AOp thuc cng mt lp nguyn t, to thnh 2 orbital lai ho c nng lng tng ng v hnh dng ging nhau, trc i xng ca 2 AO lai ho ny to vi nhau mt gc 1800.

s + p (sp)2 (sp)1 Hnh 12. Gin lai ha Sp Trng hp lai ho ny thng xy ra trong nguyn t khi to thnh cc hp

cht c dng ng thng nh BeF2, BeH2, BeCl2, CO2, C2H2, V d: Phn t BeCl2 c gc ha tr xc nh bng thc nghim Cl - Be - Cl l

1800. Trc khi to lin kt, hai obitan 2s v 2p ca Be lai ho vi nhau cho 2 obitan sp tng ng c trc nm trn cng mt ng thng v ngc chiu nhau. Hai electron ha tr ca beri c rp vo 2 obitan lai ha. S ph ca 2 obitan lai ho ny vi 2 obitan 3p ca hai nguyn t clo to thnh 2 lin kt Be - Cl tng ng thng hng.

- Lai ho sp2 L s lai ho gia mt AO s vi 2 AO p to ra 3AO lai ho sp2 nm trong cng

mt mt phng, trc i xng ca chng to vi nhau mt gc l 1200 hng v 3 nh ca mt tam gic u.

40

AOs 2AOp 3AOsp2

s + p + p (sp2)3

(sp2)1

(sp2)2

Hnh 13. Gin lai ha Sp2

Lai ho sp2 c dng gii thch cu trc hnh hc ca cc phn t BH3, BF3, BCl3, SO2, SO3 v gii thch lin kt i ca cc nguyn t C trong cc hp cht hu c.

V d: Phn t BCl3 c gc ha tr xc nh bng thc nghim Cl B Cl l 1200. Mt obitan 2s v 2 obitan 2p ca B lai ho sp2. Ba electron ha tr ca B c rp vo 3 obitan lai ha ny vi 3 obitan p ca 2 nguyn t Cl cho 3 lin kt B Cl vi gc ha tr 1200. - Lai ho sp3

Mt obitan ns v ba obitan np lai ho vi nhau to thnh 4 obitan ging ht nhau sp3 hng ti bn nh ca t din u to thnh gc gia cc obitan lai ho bng 109,280 hoc gn vi gc .

s + 3p (sp3)3

(sp3)2

(sp3)1(sp3)4

Hnh 14. Gin lai ha Sp3

V d: Phn t t din u CH4 c gc ha tr H C H l 1290 28. Khi to thnh phn t mtan, mt obitan 2s v 3 obitan 2p ca C lai ho cho 4 obitan. S xen ph 4 obitan lai ha ny vi 4 obitan 1s ca 4 nguyn t hiro cho 4 lin kt tng ng vi gc ho tr 109028.

41

Hnh 15. Cu to phn t CH4

Khi hnh thnh NH3, nguyn t nit khng s dng 3 obitan 2p thun khit xen ph vi 3 obitan 1s ca hiro. Nguyn t N trng thi lai ho sp3, trong 5 electron ho tr ca N c 1 cp c rp v 1 obitan lai (gi l cp electron khng chia), 3 electron cn li ln lt c rp v 3 obitan lai ha kia, s xen ph ca 3 obitan lai ha ny vi 3 obitan 1s ca 3 nguyn t H cho 3 lin kt N H. Tng tc y ca cp electron khng chia vi 3 cp electron lin kt N H lm cho gc ho tr t 1290 28 gim i cn 1270 1. Kt qu, phn t NH3 c cu trc hnh thp vi gc ha tr (HNH) bng 1070 3.

Hnh 16. Cu to phn t NH3

* D on kiu lai ho v dng hnh hc ca phn t Xut pht t tng cc cp electron ho tr ca mt nguyn t lun y ln nhau, R.J. Gillespie a ra quy tc tin on s nh hng cc lin kt xung quanh mt nguyn t trung tm ca phn t hoc ion gi l thuyt s y cc cp electron ho tr, vit tt l VSEPR (t ting anh: Valence Shell Electronic Pair Repusions)

Lun im chnh: Mi cp electron lin kt v khng lin kt (cp electron t do) ca lp ngoi

cng u c tr thng k cng mt khong cch n ht nhn, trn b mt qu cu m ht nhn nm tm. Cc electron tng ng s v tr xa nhau nht lc y ca chng gim n cc tiu. Theo m hnh VSEPR: Xt phn t AXnEm.

A: Nguyn t trung tm c cc cp electron ha tr to lin kt ; X: Cp electron lin kt vi nguyn t trung tm A;

n: s cp electron to thnh lin kt ; E: Cp electron t do; m: S lng cp electron t do; n + m = q l tng s cp electron ha tr bao quanh A.

n + m = 2 Phn t thng A lai ho sp

n + m = 3 Phn t phng tam gic A lai ho sp2

n + m = 4 Phn t t din A lai ho sp3

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n + m = 5 Phn t thp i y tam gic A lai ho sp3d

n + m = 6 Phn t thp i y vung (bt din) A lai ho sp3d2

V d: Trong BeH2 c m + n = 2 Be c lai ho sp.

BH3 c m + n = 3 B c lai ho sp2.

CH4 c m + n = 4 C c lai ho sp3.

NH3 c m + n = 4 N c lai ho sp3.

PCl5 c m + n = 5 P c lai ho sp3d.

Bng 3. Dng hnh hc mt s phn t AXnEm

n + m a din phi tr

Cng thc cu trc

Dng phn t

AXm

Phn t lin kt n

Phn t lin kt bi

2 on thng

AX2E0

Thng

BeH2, HgCl2BeCl2

CO2HCN

AX3E0Tam gic

u BH3, BF3

AlCl3

SO3, NO3-

HClO23

Tam gic u

AX2E1 gp khc SnCl2SO2, NO2,

NOCl

AX4E0 t din CH4

NH4+, BH4-SO42-, POCl3

AX3E1Thp y tam gic

NH3OH3+

SOBr2, ClO3-4 T din

AX2E2 gp khc OF2, SCl2NH2-, H2S

ClO2-

3.3.4. Thuyt obital phn t MO (Molecular Obital) a. Lun im c bn Theo c hc lng t, phng php MO da trn s m rng khi nim hm sng mt electron cho trng hp phn t. Trong phng php MO, ngi ta xem phn t nh mt khung m cc ht nhn v electron cu thnh. Trong trng hp ny khng th ni n tng trng thi ring l ca electron m ch c th ni n hm sng chnh xc ca h, song iu ny khng th thc hin c v mt ton hc. khc phc tng t nh trng hp

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nguyn t nhiu in t, ngi ta gi thit l trong phn t mi electron chuyn ng trong mt in trng hiu dng trung bnh no gy ra bi b khung ca electron v ht nhn cn li. Theo cch lp lun trn, ngi ta cho rng trng thi c m t bng hm sng gi l MO. Ni cch khc, MO biu th trng thi chuyn ng ca tng electron trong phn t. Nu trong nguyn t, cc electron chuyn ng trong trng hiu dng i xng xuyn tm, th trong phn t cc electron chuyn ng trong trng hiu dng i xng xuyn tm, c i xng phc tp hn ty tng loi phn t c th. Phng php MO hon ton tn trng cc nguyn l v quy tc p dng cho nguyn t. i vi phn t c 2 nguyn t th khi t hp cc AO s c cc MO nh c hai tm. Cn i vi phn t nhiu nguyn t khc nhau, s t hp cc AO s dn n cc MO khng nh c nhiu tm. b. Phng php t hp tuyn tnh (MO LCAO) (Molecular Orbital Linear Combination of Atomic Orbital) Trong phn t, khi mt electron chuyn ng gn mt ht nhn nguyn t no , th lc electron ny s chu tc dng ch yu ca in trng ht nhn , cn tng tc gia electron kho st i vi electron v ht nhn cn li l khng ng k. Khi mt cch gn ng v mt ton hc, ta c:

1 1 2 2 3 31

...n

n n i ii

C C C C C =

= + + + =

1 2, ,... l hm sng AO m t trng thi chuyn ng ca tng electron;

1 2, ,..C C . l h s ca hm AO hay phn ng gp ca tng electron vo qu trnh hnh thnh lin kt. * iu kin t hp c hiu qu cc AO.

- Cc AO phi c cng tnh cht i xng. - Nng lng cc AO phi xp x nhau. - Cc AO phi xen ph nhau r rt.

V mt nh tnh, bit c cc AO c cng tnh cht i xng hay khng, c th da vo s xen ph dng, m hoc bng khng ca cc AO (hnh di).

Ch c s xen ph dng mi c th to lin kt v trong trng hp ny cc AO mi c cng tnh cht i xng, ngha l chng mi t hp c vi nhau. Tuy nhin vic t hp c hiu qu hay khng cn ph thuc vo hai iu kin cn li.

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Hnh 17. S xen ph ca cc obitan

* Cc c trng ca lin kt cng ho tr trong phng php MO: - Ch s lin kt hay bi lin kt (N).

1 ( *2

N n n= )

n - s electron nm MO lin kt; n* - s electron nm MO phn lin kt. - di lin kt bng khong cch gia tm ca hai ht nhn, di lin kt

cng nh khi ch s lin kt cng ln. - Nng lng lin kt cng ln khi lin kt cng bn. c. Bi ton ion phn t H2+

V hai nguyn t trong phn t ging nhau nn cc AO ho tr ca chng u ging nhau, ngha l chng c cng tnh cht i xng.

Trong trng hp ny s t hp n AO s cho ta 2n MO lin kt c nng lng

thp hn nng lng AO em t hp v cho 2n MO phn lin kt (k hiu MO*) c

nng lng cao hn cc AO trong t hp.

V d: H + H+ H2+

1s1(A) 1s(B)

= C1 1s(A) + C2 1s(B); gii phng trnh Schroinger vi hm ta thu c:

1 ( ) 1 ( )1 ( )2 s A s B

=

= C1 1s(A) + C2 1s(B) (MOlk , k hiu s, c nng lng E-)

1 ( ) 1 ( )1 ( )2 s A s B+

= +

(MO*, k hiu s*, c nng lng E+)

Hnh 18. Gin nng lng cc MO ca ion + 2

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Trong gin trn:

s l MO lin kt;

s* l MO lin kt dng ;

l lin kt H H+. s l lin kt to thnh t AOs.

Electron duy nht ca H2+ tt nhin s chim MO c nng lng thp, nn cu hnh electron ca ion H2+: s1 (c 1 electron trn MO lin kt s). T gin trn ta thy rng khi to ion H2+ t H va H+, nng lng gim xung mt gi tr a ngha l h H2+ bn hn h H v H+ ring bit. Phng php VB khng gii thch c s tn ti ca ion H2+ ny. Gin nng lng cc MO ca h H2, He2+, He2, tng t gin trn.

- Phn t H2 c 2e, Cu hnh electron H2: s2

- Phn t He2+ c 3e, Cu hnh electron He2+: s2 s*1

- Phn t He2 c 4e, Cu hnh electron He2: s2 s*2

Phn t He2 khng tn ti v 2e chim s lm nng lng ca h gim xung khng b cho nng lng ca h tng ln do 2e chim MO phn lin kt tng ln s*.

c. Thuyt MO cho phn t hai nguyn t ng hch thuc chu k 2

Bao gm: Li2, Be2, B2, C2, N2, O2, F2, Ne2. * Nguyn tc chung - MO c thit lp l do s t hp tuyn tnh cc AO ho tr (cc electron phn lp ngoi cng). S t hp ca nAO s to thnh nMO. - Cc AO xen ph ln nhau theo cc iu kin: + C mc nng lng xp x nhau; + C kh nng xen ph nhau mt mc ng k; + C cng tnh i xng vi trc lin kt; - Ty theo hng xen ph cc AO, ta s c:

+ Nu 2AO xen ph dc trc lin kt, ta c cc MO-;

+ Nu 2AO xen ph hai bn sn lin kt, ta c cc MO-. - S hnh thnh MO lin kt, MO phn lin kt ph thuc vo du ca cc AO tham gia t hp tuyn tnh.

* Gin MO - Cc nguyn t chu k 2 u c 4 AO ho tr khi to thnh phn t A2 bng cc t hp sau: (hnh 19)

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Hnh 19. Hai gin nng lng MO ca cc phn t A2 thuc chu k 2

- Gin a) l ca cc phn t cui chu k : O2, F2 v Ne2. Trong trng hp ny, hiu ng nng lng Enp Ens cc nguyn t ln nn khng c s tng tc s - z.

Cu hnh electron ca cc phn t O2, F2, Ne2 c dng:

(KK) s s*z (x = y ) (x* = y* ) z* ( hnh a) ) - Gin b) l ca cc phn t u chu k : Li2 , Be2, B2, C2, N2 . Trong trng hp ny, hiu ng nng lng Enp Ens tng i nh nn khi to thnh phn t A2 c s tng tc s - z lm cho mc nng lng cc MO z v x , y thay i so vi gin a). Cu hnh electron ca cc phn t Li2 , Be2, B2, C2, N2 c dng:

(KK) s s*(x = y )z (x* = y* ) z* ( hnh b) ) K hiu (KK) y ch rng c 4e (1s2 ) ca hai nguyn t ca lp K khng tham

gia to thnh MO.

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V d:

- Phn t Li2 c 6e, Cu hnh electron Li2: s2 N = 1 l = 2,67 A0

E = 105 kj/mol

- Phn t Be2 c 8e, Cu hnh electron Be2: s2 s*2 ( N = 0, nn phn t khng tn ti ).

- Phn t B2 c 10e, Cu hnh electron B2: s2 s*2 x1 = y1

N = 1, nn phn t bn. l = 1,59 A0; E = - 287 kj/mol Phn t c 2e c thn nn c tnh cht thun t.

- Phn t N2 c 14e, Cu hnh electron N2: s2 s*2 x2 = y2z2 N = 3, lin kt 3: N N l = 1,10 A0; E = -942 kj/mol

- Phn t O2 c 16e, Cu hnh electron O2: s2 s*2z2 x2 = y2 x*1 = y*1

N = 2 l = 1,21 A0; E = -494 kj/mol Phn t c 2e c thn nn c tnh cht thun t.

- Phn t F2 c 18e, Cu hnh electron F2: s2 s*2s2 x2 = y2 x*2 = y*2

N = 1, l = 1,42A0; E = -155 kj/mol - Phn t Ne2: khng tn ti. d. Thuyt MO cho phn t c hai ht nhn khc nhau AB V nguyn tc cc xy dng cc MO i vi cc phn t dng AB hon ton ging vi phn t dng A2. Do trong phn t AB, cc AO c tnh i xng khc nhau, nn phn xen ph to thnh MO khc nhau. V vy ch c cc AO c cng tnh i xng mi tham gia to tnh lin kt. Trong hai nguyn t A v B, nguyn t no c m in ln hn s c mc nng lng AO bn hn v c biu din thp hn trn bn nng lng (hnh 20):

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Hnh 20. Gin nng lng MO ca cc phn t AB

3.4. Lin kt phi tr L lin kt cng ho tr m cp electron dng chung ch do mt nguyn t a

ra, nguyn t ny gi l nguyn t cho. y c s chuyn cp electron t do ca nguyn t cho v orbitanl trng ca nguyn t nhn. Lin kt phi tr c k hiu bng mt mi tn hng t nguyn t cho sang nguyn t nhn.

c th to lin kt cho - nhn, mt nguyn t (hay ion) phi c cp electron ho tr cha tham gia lin kt, cn nguyn t (hay ion) kia cn obitan ho tr trng. V d:

H H H N : + H+ H N H

H H

+

F

B + : F :

F

F

F

B F

F

F. .

. .

_

Trong thc t, khng th phn bit lin kt phi tr v lin kt cng ho tr

thng thng. V d: trong NH4+, bn lin kt ca N vi H hon ton ging nhau v di lin kt v nng lng lin kt. Lin kt phi tr thng gp trong cc phc cht. Lin kt cho nhn c biu din bng mi tn hng t nguyn t cho cp e sang nguyn t nhn cp e . Lin kt cho nhn trong NH4+ v BF4- c gi tr tng ng vi 3 lin kt cn li trong chng, v cc lin kt ny u c cng bn cht chung cp e. Trong phn t CO, lin kt gia C v O l lin kt 3, trong 2 LK c to thnh do s gp chung 2e c thn ca 2 nguyn t, cn LK th 3 l lin kt cho nhn c to thnh bng cp e ha tr cha tham gia lin kt ca O v obitan ha tr

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trng 2p ca C:

C O Lin kt cho nhn c to thnh i khi do s sp xp li e ha tr ca phn t nhn to ra obitan ha tr trng. V d:

CH3

N O

CH3

CH3

ON

OH

O y, trong nguyn t O c s sp xp li cc e ha tr to ra mt obitan ha tr trng to iu kin cho vic nhn cp e lin kt:

O :1s2 2s2 2p4 1s2 2s2 2p4

Nng lng cn cho s sp xp li ny c b bng nng lng ca phn ng ha hc. 3.5. Tng tc gia cc phn t

Ngoi cc lin kt chnh xt nh lin kt ion, lin kt cng ho tr cn c cc tng tc gia cc phn t nh lin kt hydr, lc Vanderwaals. 3.5.1. Lin kt hydro

Nguyn t hydro ngoi kh nng tham gia lin kt cng ho tr thng thng, cn c kh nng to mt mi lin kt th hai vi mt nguyn t khc c m in ln v kch thc nh. Mi lin kt th hai ny km bn hn nhiu so vi lin kt cng ho tr thng thng v c gi l lin kt hydro.

V d: - Lin kt hydro lin phn t: . . . H O . . . H O . . . H O . . .

C2H5 H C2H5 in kt hir trong nc: - S hnh thnh l

ca ClO4- vi nc: - S hnh thnh lin kt hir

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http://vi.wikipedia.org/wiki/T%E1%BA%ADp_tin:Wasserstoffbr%C3%BCckenbindungen-Wasser.svg

Cc phn t nc tng tc ln nhau thng qua lin kt hir v nh vy c lc ht phn t ln. y khng phi l mt lin kt bn vng. Lin kt ca cc phn t nc thng qua lin kt hir ch tn ti trong mt phn nh ca mt giy, sau cc phn t nc tch ra khi lin kt ny v lin kt vi cc phn t nc khc.

Trong phn t HF, nguyn t H sau khi lin kt vi nguyn t F bng mt lin kt cng ho tr cn hnh thnh mi lin kt th hai (lin kt hydr) vi nguyn t F ca phn t HF khc to nn phn t lin hp (HF)n.

- Lin kt hydro ni phn t. * Nguyn nhn xut hin lin kt hydro

Do c im cu to nguyn t ca hydro l ch c mt electron duy nht nn khi nguyn t hydro lin kt cng ho tr vi mt nguyn t ca nguyn t c m in ln th my electron ca hydro b ht lch mnh v pha nguyn t v lm nguyn t hydro b bin thnh ht tch in dng mc khc cn do kch thc ca hydro rt nh nn ion hydro d dng tin gn n cc nguyn t hay ion khc, thm ch thm nhp vo lp v electron ca cc nguyn t hay ion khc hnh thnh nn mi lin kt hydro.

Nng lng ca lin kt hydro rt b (khong 2 - 10 kcal/mol) nn lin kt hydro km bn hn lin kt ho hc thng thng (khong hng trm kcal/mol). * nh hng ca lin kt hydro n tnh cht ca cc cht

- Lm tng nhit nng chy, nhit si ca cc cht do lin kt hydro to nn lc ht gia cc phn t, gy nn s trng hp phn t lm phn t lng trung bnh ca cc cht tng nn nhit nng chy v nhit d si ca cc cht tng.

V d:

Cht M T0nc (0C) T0s (0C)

HF 20 -83 19,5

HCl 36,5 -112 -84,9

- Lm gim in li, gim tnh axit ca cc cht. V d 1: Trong dy HX cc axit HCl, HBr, HI l cc axit mnh nhng do trong

cc phn t HF c lin kt hydro lin phn t nn HF l cht in li yu v c tnh axit trung bnh.

- Lm tng tan ca cc cht: lin kt hydro gia cc phn t cht tan v dung mi lm tan ca cc cht tng do d hnh thnh solvat.

V d 2: Gia ru v nc c lin kt hydro nn ru tan v hn trong nc, trong khi te (ROR) khng tan trong nc do gia te v nc khng c lin kt hidro. S hnh thnh lin kt hydro lin phn t cho php gii thch tnh d ho tan ca cc hp cht c nhm phn cc trong nhng dung mi phn cc nh nc, ru

Ngoi ra lin kt hydro lin phn t cn lm gim tnh baz ca cc cht v gy nn s bin i bt thng v khi lng ring.

* Lin kt hydro mi phn t lm thu gn cc phn t li ng thi lm gim kh nng to lin kt hydro gia cht vi dung mi do lm gim nhit nng

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chy, nhit si v tan ca cc cht, lin kt ny cn lm gim tnh axit ca cc cht. 3.5.2. Lc Vanderwaals

L lc tng tc gia cc phn t, n c bn cht tnh in. Lc Vanderwaals cng ln th mmen lng cc ca phn t, kch thc v khi lng ca phn t cng ln. Lc vanderwaals b hn vi nng lng ca lin kt ion v lin kt cng ho tr nn n ch c nh hng ch yu n mt s tnh cht l hc ca cc cht. V d:

Theo dy F2, Cl2, Br2, I2 th kch thc v khi lng tng t u n cui dy nn lc vaderwaals tng dn do nhit si v nhit nng chy ca cc cht ny tng dn.

Lc Vardenwaals bao gm cc thnh phn sau: * Lc nh hng: khi cc phn t phn cc gn nhau chng tng tc vi

nhau bng lc tnh in lm cho cc phn t xoay hng sp xp mt cch c trt t nhm lm gim nng lng ca h.

* Lc cm ng: cc phn t phn cc tng tc vi cc phn t khng phn cc lm xut hin lng cc gia chng.

* Lc khuych tn: cc phn t khng phn cc gn nhau do chuyn ng ca cc electron lm cho mt electron lun lun bin i nn lm dch chuyn trng tm in tch dng v m nn lm xut hin cc lng cc tm thi v chng ht nhau bng lc ht tnh in.

Lc Vanderwaals gm E = E1 + E2 + E3 Lc Vanderwaals tuy nh nhng cng lm cho cc phn t ny buc nhau nn

thng nh hng n tnh cht vt l ca cc cht nh nhau, nhit nng chy, nhit si

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Cu hi v bi tp 1. Theo quy tc bt t, hy biu din lin kt trong cc phn t v ion sau y: H2O, NH3, NH4+, SO2, SO3, N2O5, HNO2, HNO3, H3PO3, H3PO4. 2. S phn loi cc lin kt da vo m in. Cho v d, nu cc iu kin v gii thch qu trnh hnh thnh lin kt ion, lin kt cng ha tr, lin kt cho nhn. 3. Hy nu nhng lun im c bn ca thuyt lin kt ha tr (VB). 4. Hy gii thch ha tr ca cc nguyn t sau: N (Z = 7); O (Z = 8); F ( Z = 9); S (Z = 16); Cl (Z = 17).

5. Cho v d, gii thch c im ca lin kt v lin kt . So snh, gii thch s hnh thnh v bn ca 2 loi lin kt ny. 6. Khi nim v lai ha. c im ca cc m my lai ha sp, sp2, sp3. Cho cc v d gii thch v nhng nguyn t c s lai ho ny. 7. Dng hnh hc phn t ca cc m my electron trong cc phn t: H2, O2, N2, BeH2, CH4, SO2, H2O? Trong cc phn t th Be, C, S, O c kiu lai ha g. 8. V gin nng lng ca cc phn t H2, H2+, A2 (thuc chu k 2)? Vit cu hnh electron dng MO. 9. Gii thch bn lin kt ca mt s cht sau y: a. HF, HCl, HBr, HI. b. BeH2, MgH2. c. NH3, PH3, AsH3. 10. Cho v d v s hnh thnh lin kt hiro. ng dng ca lin kt hiro trong vic gii thch tnh tan, nhit si ca mt s hp cht.

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