R K Sharma Coordination Chemistry 10-30

8/20/2019 R K Sharma Coordination Chemistry 10-30

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(d) If a double salt contains the anions in the same oxidation state, theanions are written alphabetically. For example PbCIF is called lead (II)

chloride fluoride.

(e) If a double salt contains the anions in different oxidation states, theanions are arranged in the order : !", #$%, other simple inorganic

anions with two elements, organic anions, $"&

'he number of ions is indicated by writing the prefix bis. iris etc. in

parentheses. For example aCI.aF.!a!* is called (hexa) sodium

chloride fluoride (bis) sulphate.

( 'he hydrolysed or basid: salts are also treated as double salts containing

more than one anion and hence are named by the same rules as used for

naming double salts. For example +i#Cl bismuth (III) oxide chloride-,

n(#$)CI tin (I) hydroxide chloride-, /rCl,.$, 0irconium (I)

oxide (di)chloride 12 hydrate-, CuCl!.3Cu(#$)! or Cu!(#$)!C4 (di)

copper (II) trihydroxide chloride-.

Note: It may be noted that for $! molecule we ha5e used hydrate and thenumber of $, molecules has been indicated by 6rabic numbers.

C##78I6'I# C#9P#8 ;I'$ 9##868

Sequence of central atom and ligand names : 'he ligands are listed in

alphabetical order, without regard to charge, before the names of the central atom.

umerical prefixes indicating the number of ligands are not considered in

determining that order.

1. 8ichloro diphenylphosphine (thiourea) platinum(ll)-

2. 8ibromobis trimethylphosphine platinum()-?.

Number of ligands in a coordination entity : Two @inds of numerical

prefix are a5ailable for indicating the number of each @ind of ligand within thename of the coordination entity, etc. 'he simple di2 , tri2, etc., deri5ed from

cardinal numerals, are generally recommended. 'he prefixes bis2, tris2, tetra@is2,

deri5ed from ordinals, are used with complex expressions and when reAuired to

a5oid ambiguityB for example, one would use diammine but bis (methylamine) to

ma@e a distinction from dimeth5lamine. ;hen the latter multiplicati5e prefixes

are used, enclosing mar@s are placed around the multiplicand.


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positi5e sign is used. ;hen necessary a negati5e sign is placed before the number.

6rabic 0ero indicates the 0ero oxidation number. o space is left between this

number and the rest of the name.6lternati5ely, the charge on a coordination entity may be indicated. 'he net

charge is written in arable numbers on the line, with the number preceding the

charge sign, and enclosed in parentheses. It follows the name of the central atom

without the inter5ention of a space.!

Some Examples:

?. E *Fe(C)- potassium hcxacyanoferrate($)

potassiumhexacyanoferrate(*2)

!. C#($3)-CI3 hexaamminccobalt(III) chloride

3. C#C?($3)GHC4! pentaamminechlorocobalt(!) chloride

*. C#CI(!)($3)*-C? tetraamminechloronitrito22eobalt (in) chloride

G. PtCl($!C$3)($3)!-Cl diamminechIoro(methylamine) platinum (??)

chloride

. CuCl!J K C($!)!L!- dichlorobis(urea)copper(lI)

D. E !PdCl*- potassium trachloropalladate(Il)

1. E !#C?! - potassium pentachloronitridoosmate(!2)

. aPt+rCl(!M$3)- 2

platinate(II)"*

sodium amminebromochloronitrito2

,'he boldface italic letters are those used in the alphabetical placement of ligands

names. #ther, nondetermining letters are mar@ed with %stri@e2 throughs%-

Terminations for names of coordination entities : 6$ anionic coordinationentities ta@e the ending2ate, whereas no distinguishing termination is used for

cationic or neutral coordination entities.

Uses of Enclosing ar!s

'he formula for the entire coordination entity, whether charged or not, is

enclosed in sAuare brac@ets. ;hen ligands are polyatomic, their formulae are

enclosed in parentheses. =igand abbre5iations are also enclosed in parentheses. In

the special case of coordination entities, the nesting order of enclosures is as

gi5en. 'here should be no space between representations of ionic species with in a

coordination formula.

Examples:

!N 'his is the


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1. C#C$3)-C?3

2. C#C?($3)-GCI!

3. C#CI(!M$3)*-CI4. PtCI($!C$3)($3)!-Cl

5. E"PdCl*-wSf

6. Co(en)3-Cl3.

"onic charges and oxidation numbers: If the formula of a charged

coordination entity is to be written without that of the counterion, the charge is

indicated outside the sAuare brac@et as a right superscript, with the number before

the sign. 'he oxidation number of a central atom may be represented by a roman

numeral used as a right superscript on the element symbol.

Examples:

#$ %ptcy&'

2. CE$O#)3. Cr I(C)*($3)!r.

C(()*"N+T"(N CE"ST)- C(NCE.T

'he compounds contain a ceatral atom or ion. sually a metal, which is

chemically bonded to 5arious groups is called the acceptor and the attachedgroups are @nown as donor groups or ligands. For example, in the complexionion is the central metal ion or acceptor while CN~ ions are ligands.

/"0+N*

/igands or Coordinating 0roups and Central etallic +tom

'he neutral molecules or ions (usually anions) which are attached with the

central ion in complex compounds are called ligands or coordinating groups. For

example in the complex ion, Fe(C)f" the six ions which are attached with the

central Fe3Q ion as shown in liie margin act as ligands. In =ewis sense, in most of

the complex compounds the ligands act as =ewis bases (electron pair donors) and

the central metal ion acts as a =ewis acid (electron pair acceptor), i.e., in most of

the complex compounds the ligands donate one or more electron pairs to the

central metal ion.

32

C


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nc

"n

JV Fe3 C f &5%C

Fig. ?.?

9nR x = 2S 9=

Central metal =igand (=ewis Complex

ion(=ewis acid base or electron compound

or electron pair pair donor)

acceptor)

In some complex compounds the ligand acts both as donor and acceptor. For

example, in metallic carbonyls, C# molecules which act as ligands act both as

donor and acceptor (9 C#). In a ligand the atom which actually donates the

electron pair to the central metal ion is called donor or coordinating atom. 'heligands are attached with the central metal ion through their donor atom (or

atoms).

'he metallic atom with which the ligands are attached through coordinating

bonds is called central metallic atom. 'his metallic atom may be in 0ero, positi5e

or negati5e oxidation state.

Coordination Number of the Central etal +tom1ion

Coordination number o the central metal atom!ion in a gi"en comple# compound is e$ual to the total number o donor atoms which areactuall%

attached with the central metallic atom. In other words we can say that the

coordination number of the central metallic atom is eAual to the number of sites at

which the ligand(s) is attached with the central metallic atom.

In case of complex compounds which contain only monodentate ligands, the

coordination number of the central metallic atom is eAual to the number of

monodentate ligands coordinated to the metallic atom. 'his rule does not hold

good for the complexes containing polydentate (i.e.9 bidentate, tridentate,tetradentate etc.) ligands. Coordination number of the metallic atom predicts the

geometry of the complex compound.

'hus, for coordination number eAual to !,3,*,G and , the geometry of the

complex compound is linear, trigonal planar, tetrahedral (or square planar),

trigonal bipyramidal and octahedral respecti5ely. 'his discussion shows that the

coordination number gi5es us an idea about the way in which the ligands arearranged round the central metallic atom.


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Coordination Sphere and "onisation Sphere

;hile writting the structural formula of a gi5en complex compound the

central metal atom and the ligands attached with it are always written in a sAuare brac@et, -. 'his sAuare brac@et is called coordination (or inner) sphere. 'he portion outside the coordination sphere is called ionisation (or outer) sphere. 'hus

in Co($3)GClOC=,, the sAuare brac@et which contains the central metal ion (Co3Q

ion) and the ligands (fi5e $3 molecules and one CI ion) is coordination sphere

and the portion that contains two CF ions is ionisation sphere.

'he species written in ionisation sphere are ionisable and hence can be

precipitated by means of a suitable precipitating agent while those gi5en in the

coordination sphere (i.e. metallic atom and ligands) are non2 ionisable and hencecannot be precipitated. 'his is shown below for

Co($3)GCI-CI! Co2N3)GCI-!* !C?%

'wo CI ions present in ionisation sphere can be precipitated as 6gCl (white

ppt) bS adding.

!6g !CI2 !6gCI I

CI% ion written in coordination sphere is not ionisable and can, therefore, not be

precipated.

C/+SS"3"C+T"(N (3 /"0+N*S

8epending on the number of sites at which one molecule of a ligand is

coordinated to the central metallic atom, the ligands ha5e been classified as

monodentate (or unidentate) and polydentate (or multidentate) ligands.

1. Monodentate or unidentate ligands : 'he ligands which ha5e only one

donor atom or are co2ordinated through one electron pair are called

monodentate or unidentate ligands. uch ligands are coordinated to

the central metal ion at one site or by one

, metal2ligand bond only. 'hese ligands may be neutral molecules or inanionic form.

2. Polydentate or multidentate ligands : 'hese may be bidentate,

tridentate, tetradentate, pentadentate and hexadentate y if the number

of donor atoms present in one molecule of the ligand attached with

the central metallic atom is !, 3, *T G and respecti5ely. 'hus one

molecule of these ligands is coordinated to the central metallic atom

at !, 3,*, G and sites respecti5ely. In other words, we can say that

one molecule of these ligands ma@es !,3,*,G and metaluligand

coordinate bonds respecti5ely.

Any atom, ion or molecule hich is capable o! donating a pair o! electrons to the metal atom is called a co"ordinating group or ligand. #n a

ligand, the particular atom hich actually donates the electron pair is called the donor atom. For example, in the complex potassium ferrocyanide


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E *Fe(C)), the six (C)" ions are ligands and the nitrogen in (C) is the

donor atom. =igands may be classified as unidentate and polydentate ligands.

'he ligands ha5e been found to be arranged around the central metal ioninside the first sphere of attraction in preferred geometries. 'he common

geometries found in complexes are linear, eAuilateral, triangular, tetrahedral,

sAuare planar, trigonal bipyramidal, sAuare pyramidal and octahedral.

Types of /igands

=igands ha5e been classified in two ways :

4$ Classification based on donor and acceptor properties of the

ligands : uch ligands ha5e been further classified as follows :

(a) $igands ha%ing one (or more) lone pair &or pairs) o! electrons. 'hese

ligands are of the following two types :

(i) 'irst type includes such ligands hich ha%e %acant p"type orbital

that can recei%e hac donated p"electrons !rom the metal ion inlo oxidation state. he main examples o! such ligands are*+,*N, isocyanides, #, 7.P, 7,6s, a, a2 dipyridyl, o"

phenanthroline and unsaturated organic molecules. uring the !ormation o! complexes, these ligands as ell as metal atoms act both as donors and acceptors

or

(9 U S =), he reason is that these ligands ha%e !illed donor

orbitals in addition to %acant p"type acceptor orb it a #s.

(ii) -econd type includes such ligands hich ha%e no %acant orbitals

to recei%e bac donated electrons !rom the metal. xamples o! this

type are .,, N5, F% etc.

(b) $igands ha%ing no lone"pairs o! electrons but ha%ing p2 bonding

electrons.


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uch ligands are generally good =ewis bases. 'hey belong to the first

short period of the periodic table.


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2a9 Normal Complexes : hese are such complexes hich are re%ersiblydissociated in solution into their constituent species. 3or examples, Cd(C)*-!

and Co($3)-

!

are the complexes of this type.

Cd (C)*-!% U22222222S Cd! *C%

Co($3)-! U222222 22S C#!Q $3

'hus, the complex ions such as Cd(C)*r and Co($3)r constitute normal

complexes because in solution sufficient Cd%" and Co !Q ions will exist and can be

detected with suitable reagents and tests.V

'he normal complexes are characterised by relati5ely wea@ bonds between

the central atom and the donor groups. 9agnetic susceptibility measurements of

normal complexes re5eal that these complexes do nott a

ha5e any deep2seated electronic arrangements.

ometimes the normal complexes are also referred to as ionic complexes.

(b) Penetration Complexes: 'hese are the coordination compounds which

ha5e sufficient stabilities to retain their identity in solution, i.e., they are not

re5ersibly dissociated in solution li@e normal complexes FeC) -*" Cu(C)*-

3"

and Co($3)-3 are examples of penetration complexes.

Fe(C)- 2222222222S Fe!f C%

Cu(C)*-3% 222222222W Cu *C"

C#($3)-3Q 2222222222W Co3 $3

'hus, the ions li@e Fe(C)-*& Cu(C)*-

3 and Co($3)-3 are penetration

complexes because these can be detected as such and there is hardly any e5idence

of the existence of free Fe!& Cu and Co3 ions respecti5ely.

'he penetration complexes are characterised by a short bond distance

between the central ion and donor groups, deep2seated electronic arrangements

and are not readily and re5ersibly dissociated either in the solid or in solution

state.

ometimes the penetration complexes are also referred to as co%alent

complexes.

+lit0 recognised that most coordination compounds lie in between normal

and penetration complexes. +lit0Rs classification is of more con5enience than of

any fundamental importance.

&$ Second ethod of Classification : 'he coordination compounds may be

di5ided into two groups:


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2a9 .erfect complexes : hese are those coordination compounds hichretain their complex character in solid as ell as in solution state. 6 numberless

complex compounds such as E * Fe (C)-, Co($3)-C=!, C($3)**-, E 3FC(C)-, etc., fall under this class of coordination

compounds.

2b9 "mperfect Complexes : #mper!ect complex compounds are those co"ordination compounds hich remain as complexes either in solution state hut not in the solid phase or hich exist as complexes in the solid state hut brea uphen dissol%ed in the sol%ent.

'he imperfect complexes which exist only in solution are E Cd(C) *-,

C($3)!-CE E !CCIH, E !i(C)*-, 64 ($!)-3& etc.

'he examples of imperfect complexes which exist only in the solid state are

E !CoCl*, Cu!Cl!.!C#, ($*)! FeC4* etc.

3. 'hird 9ethod of Classification : 6 more general, precise and con5incing

classification, has also been gi5en.(a) 'XP< I: Complex compounds belonging to this class are those

compounds which contain complex cations or are formed by the union of metal

ions (cations) with inorganic molecules such as $! and $3.

Complexes which contain such complex cations as the ammoniates are

/n($3)*f& Cu($3)-!, i ($3)-

!, Cd ($YHQ 6g ($3),-Q and hydrated

complex ions li@e +e($!)*-!, Cr($!)-

3, 6? ($!)-3f etc. 'he extent to

which an ammoniated or hydrated complex is formed with a cation has been

found to depend upon the following two factors :•

(i) 'he concentration of ammonia, and

(ii) 'he stability of the resulting complex.,TFor instance, FeR and 6l3Q ions, when treated with ammonia, are always

precipitated as hydroxides because their ammonia complexes are unstable.(b) 'XP


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2d9 T-.E " : 'his is the largest class of co2ordination compounds. uch

complexes are formed by the metal ions with organic anions and organic

molecules. 'he compounds of this class are electrolytes as well as electrolytes. 6great maOority of the co2ordination compounds of this class contain one or more

rings in their molecules. Cienerally the complexes containing fi5e or six

membered rings are 5ery stable. 'hey are @nown as chelates. ic@el complex with

dimethylglyoxime is the most familiar example of this class of compounds.

$ere nic@el atom has a coordination number of * and is attached to two

molecules of dimethyl2Iglyoxime by two co5alent and two coordinate bonds. 6lso

Fe(lll) on treatment with oxalate ions yields complex ion Fe(C,*)3-3".

f c o 2 o o 2 c o f %I FeZ I C#2 o o 2 C #

3 * G

+ C

6? i P

UZ&, I I C# [C#

;$ 3ourth ethod of Classification : 6 fourth method by which one may

classify complexes is according to the electronic configuration of the metal atom

or ion in Auestion.

2a9 Category " : 'his includes complexes of all metal ions which possess a

5alence shell with inert gas configuration, i.e.. Is! or ns! np where n has 5alues

from ! to . 'hese ions are all spherically symmetrical with the element being in

the highest possible oxidation state. 'he elements in the first category are shown

in below

1+ 2+

a

9g 3 * G D 1

E Ca c 'i Cr 9n

7b r X /r b 9o 'c 7u

#$ #

I 'C$S2 CK KC 2 C$i

I i I

C$32 CK Z K C 2 C $, i I# #$

ic@el dimethygloximate 5R Ferric oxalate

Fig. ?.!

=i +e


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Cs +a =a $f 'a ; 7e #s

Fr 7a 6c 'h Pa

'he stereochemistry of the complexes formed by these metal atoms (shownin abo5e) is in general that predicted by e 6s

7u 7h Pd 6g Cd In n b

#s Ir Pt 6u $g '? Pb +i

(c) Category I I I : 'his includes the complexes of such metal atoms which

ha5e pseudo2inert gas plus two configuration, i.e., (n 2 I) d / 0 , ns!, where n is * Gor . 'hese complexes possess certain geometries (shown in below).

? 2+ 3 * G N

>a >e 6s e +r

In n b 'e ? $e

'? Pb +i Po 6t 7n

For example : (e,'e) M* compounds ha5e geometries based upon the lone

pair occupying a stereochemical site, and the same is true of compounds of +r(),

I(), Me(I), etc., e5en though these are not generally considered to be central

metal atoms.

2d9 Category I : 'his includes complexes of metal atoms which possess

incompletely filled d orbital, (n 2 I) d l to g where n is *, G or . 'his group of central atoms (shown in below) is by far the largest and most di5erse since it

includes all of the transition metals in all of their many oxidation states except

those which would place them in categories I and II. 'he complexes ha5e

perfectly regular structures predicted by


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'h Pa

3inal )emar! on Classification : 6fter examining the foregoing four

methods by which we might classify complexes, we see that no one methodstands out clearly as best and none of them is totally satisfactory.

$owe5er, the mere attempt to find a suitable classification system has

hopefully led the reader to a greater appreciation of the broad scope of the field

and many facets of it to be explored.

(N('NUC/E+) C(()*"N+T"(N C(.(UN*S


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$Otca triethanolamine !,!Q. !%2nitrilotriethanol

$!dea diethanolamine !,!2iminodiethanol

$ydrocarbons

cod cycloctadiene ?,G2cyclooctadiene

cot cyclooctatetraene ?,3,G,D2cyclooctatetraene

Cp cyclopentadienyl cyclopentadienyl

Cy cyclohexyl cyclohexyl

6c acetyl •acetyl

+u butyl butyl

+0l ben0yl ben0yl


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$*edta ethylenediaminetetraacetic acid (?.!2ethanedi5ldinitrilo) tctraacetic acid

$s dpta . , & R& %2diethylene2

triamine pentaacetic acid

(carboxymethyl) iniino- bis(ethaned iy ? n itri lo )tetraacet ic acid

$3 nta nitrilotriacetic acid

$* cdta trans"?,!cyclohexanediaminetettraacctic acid

trans A 1

cyclohexanediyldinitrilo) tetraacetic

acid

$, ida iminodiacetic acid iminodiacetic acid

dien diethylenetriamine "!2(aminoethyl) ?,!2

ethanediamine

en ethylencdiamine ?,!2ethanediamine

pn prop5lenediamine ?.! propanediamine

tmen N,N.N2N2 N,N.N2N2 2 tetramcthyl

2tetranrethylethylenediamine [?,!2ethanediamine

tn trimethylenediamine ?,32propanediamine

tren tris(!2aminoethyl )amine

ethanediamine

.2b i s( !2 am inoethS ? y ?,!2

trien triethyleneteramine .. 6R2bis(!2aminoethS iS2 ?,!2

ethanedianiine

chxn ?,!2diaminocyclohexane ?,!2cyclohexanediamine

hmta hexamethylenctetraminc \3.3.?.t.GDldecane

?,3,G,D2tetraa0atricyclo

? Ithsc thioscmicarba0ide hydra0inecarbothioamide

dope ?.!2bis(diethylphosphino) ?.!2cthanediylbis ethane

(diethylphosphine)

$. 2salgly salicylidcneglycine methylene-

glycine

2(!2hydroxyphenyl)

$, 2saltri bis(salicylidene)2l, 3

2diaminopropane

!.!2 ?,32propanecliylbis

(nitrilometh5lidyne)- diphenol

$52sa)dien his(salicy)idcnc) dicth5lenetriamine !,!R2iminobts( ?,!2)

ethanediylnitriolmethylidyn)


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$, tsalen bis( !2mercaptoben0yl idcne)

ethyienediamine

]

9acrocycles

!.!R ?,!2ethanediylbis (nitrilo

methylidyn)ditben0enethiol

?12cro&5n2 ?.*.D, ?. ?3, ?2hcxaoxacy2

clooctadecane

?.*,D,?,?3,?2hcxaoxacy2

clooctadecane

bcn0o2?G2 !,3 ben0o2?.*.D,?,?32 !.3,G,,1,,??,?!2octahydro2

cro55 n2G pcntaoxacyclopentadec2!2ene ?,*,D,?.?3 ben0opentaoxac2

yclopentadccene

cryptand !!! *. D, ?3. ?, !?, !*2hexaoxa2 ?,?2

dia0abicyclo 1.1.1.- hexacosanc

*.D,?3,?,!?,!*2hexaoxa2

l,42dia0abicyclo1.1.1-

hexacosanc

cryptand !? ? *.D,?3.?12tetraoxa2?.?

dia0abicyclo1.G.G-icosane

*,D,?3,?1[tetraoxa2?,?[

dia0abicyclo 1.G.G Oicosane

?!-anc*•

?,*,D,?#2tetrathiacyclododecane ?,*,D,? #2tetrathiacyclododecane

$! pc phthalocyanine phthalocyanine

$!tpp tetrapheny I porphyrin G, ?, ?G, !2 tetraphenyloporphyrin

$oep octaethylporphyrin

octaethylporphyrin

!, 3. D, 1, ?!. ?3, ?D. ?12

pp4M protoporphyrin IM 3. D, ?!, ?D2tetramethyl2g, ?32

di5inylporp2hyrin2!,?12 dipropanoic

acid

?1-aneP*! ?,?2dioxa2*,D.?3,?2 tetraphos2 phacyclooctacRecane

?.?2d ioxa2*,D,?3.?2letrapiiosphacyclo2octadecane

4*Hane* ?,*.1.? l2tetraa0acyclcietra2 decane ?.*,1,? 9etraa0acyc2 lotetradecane

?*H?.32 tetraa0acyciotciradeca2 ?,32diene ?,*,1,?? 2tetraa0acyclotetradeca 2l.32

diene

.(/-*ENT+TE /"0+N*S : 3/E6"*ENT+TE C+)+CTE)

Polydentate ligands are said to ha5e flexidentate character if they do not use

all its donor atoms to get coordinated to the metal ion. 6n interesting example of

such polydentate ligands is ethylene diamine tetraacetic acid. 'his ligand

generally acts as a hexodentate ligand but it acts as a pent a dent ate ligand

3e.g., Cr ?

% (#$) ($


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and bidentate 3e.g., Com ($,)3 *- respecti5ely. 'his has been confirmed byinfrared spectroscopy.

;hen the infrared spectrum of (Co???

($3)G #HQ is recorded, it shows sixseparate absorption bonds due to 2# 5ibrations. 'his re5eals that an oxygen

atom of the sulphate group gets co5alentaly bonded to Co3 Fig. ?.3(a)-

;hen the infrared spectrum of Co??? (en)! *-B is recorded, it shows eight

bands due to # 5ibrations. 'his re5eals that the sulphate group acts as a

bidentate group Fig. ?.3(b).

+mbidentate ligands : Certain ligands are @nown which possess two or

more donor atoms but in forming complexes they use only one donor atom to

attach themsel5es to the metal ion at a gi5en time.

I

NH3

5No

H3N

3ig$ 4$5 : Structure of fCo"U2en&S?;. and "Co@ANAS(B exhibiting

the flexidentate character of S?;&D ion$

uch ligands are @nown as ambidentate ligands. ome examples of

ambidentate ligands are gi5en in the 'able ?.?.

In certain cases ambidentate ligands result in lin@age isomerism (for

definition, see isomerism).

or e#ample* the comple# +Co(N,.)5N-C/2 has been ound to e#ist in two lin0age isomers* i.e., +Co(N,3)3 -N-C/* (nitrito isomer)and +Co(N,3)5 N2C/2 (nitro isomer)

6RZ C o QZ 2 $3

9h3

2b9

o R. Cor / (a)


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.

!* extboo o! *oordin

Table 4$&

Amhidentate ligands Metal" ligand bond

T#O ion 'hiosulphato2(9_3)

'hiosulphato2(9#!

7 :# ion 2bonded. #2bonded

eC% ion 9eC79Ce

C# ion 9#C.9C#

C" ion 'hiocyanato, 9C

Isothiocyanato, 9C

C9 % ion Cyano(9C) Isocyano (9

#, ]! 2 ion

itro (92!)

itrito (9 2 #2 K )

Einds of the ligand atoms : #n the basis of formation of complexes with different

atoms, Pearson classified ligands as well as

the metals into hard and soft ones, i.e.,


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(a) 'he metal ions with almost empty or

completely filled d2 subshell cannot be used for the

formation of n bond. 'hese metal ions are called the

hard acids or class (4) metals. a, In,

n, Pb, 'i, /r, $f, X.c, =a, , etc. 'he ligands that

form stronger complexes with metals or hard acids

are called the hard bases or class (a) bases.

(b) 'he metal ions with nearly filled d2subshell

can form n bonds with ligands which can accept

these d orbitals electrons in their empty d orbitals or

suitable rc2orbitals (5acant).

'hese metal ions are called the so!t acids or

class (b) metals while the ligands are called so!t

bases or class (b) bases.


20/23

can he arranged or coordinated around a central

metal ion.

It is usually abbre5iated as C.. In E 3Fe(C).

six cyanide ligands are coordinated to Fe3U and hence

coordination number of Fe3R is . imilarly, in

Pt($3)*CI!- coordination number of Pt!R is * and in

C#($3)-CI3 coordination number of Co3 is six.

Coordination number of metal 5aries from ! to

?, but the most common coordination numbers are

* and , but may be ! or 1 or an odd number in rare

cases.

Metal ionC.N. Metal ion C.N. Met

6g 2 C! *. O

6u

!,* /n

* I

'I ! Pb! * 6

Cu !.* Pt! * P

!Q c3S P

Fe!Q CrQ

Co! *, Fc3 9

i! *. *o56 $

Table 4B


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'he coordination number is pre5iously

considered to be a fixed number for a particular

metal but many complexes are @nown in which the

metal ion has more than one coordination number.

ome examples are tabulated abo5e :

'he maximum coordination number of

elements in the second row of elementT of the

periodic table is four, for the elements in the third

and fourth rows ii iQ for the rlenients in

the fifth or sixth row,

six or eight are more commonly seen and in some

cases ll is ten. For the se5enth row of the periodic

table there seems to be some possibility of

coordination number of twel5e.

3actors affecting the coordination number

and geometry of the complex: 'he coordinationnumber of a metal ion depends on its nature, its

oxidation state and on the ligands which are arranged

around it. 'he coordination number is also

influenced by the en5ironmental factors such as

temperature, pressure or sol5ent.

'he geometry of the complex depends upon thecoordination number of its central metal ion. If its

coordination number is . the ligands are usually

directed toward the corners of an octahedron and the

shape ol the complex is octahedral. 'hus, it means


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that the ligands are coordinated to the central metal

ion in a fixed geometryR. ame is true for other

coordination numbers.

(C) Complex "on : #t is an electrically

charged species hich is !ormed by the union o! a

simple cation ith one or more neutral molecules

or one or more simple onions.

For example, i($3)-!Q ion is obtained by

.the union of six molecules of ammonia with one i!% ion. imilarly, 6g(C!)- is obtained by the

union of two cyanide ions with one 6g: ion.

It is important to mention here that the charge

carried by a complex ion is equal to the algebraic

sum o! the charges carried by the central ion and

the ligands attached to it.

For example, the complex ferrocyanide ion,

Fe(C)-* has a charge of * because the ferrous ion

carries a charge of ! while six (C)" ions carry a

charge of 2 . In the case of complex ion 6g(C!)-"

has a charge of 2 ? because 6gR ion has charge of ?

and two cyanide ions ha5e a charge of 2 !. Complex

ions are generally written inside the sAuare brac@ets.

(D) Coordination Sphere : he central metal

atom and ligands hich are directly attached to ii


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are enclosed in square bracets and are collecti%ely

called as the coordination sphere.

'he ligands and metal atom inside the sAuare

brac@ets beha5e as a single constituent unit.

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R K Sharma Coordination Chemistry 10-30