Transition Metal Complexes I The structures, nomenclature and isomers of coordination compounds

Transition Metal Transition Metal Complexes IComplexes I

The structures, nomenclature The structures, nomenclature and isomers of coordination and isomers of coordination

compoundscompounds

Coordination CompoundsCoordination Compounds

Any compound containing a Any compound containing a metal atom or ion with one or more metal atom or ion with one or more ligands is called a ligands is called a coordination coordination compoundcompound or complex. The ligands or complex. The ligands donate electrons to the metal via donate electrons to the metal via coordinate covalent bonds.coordinate covalent bonds.


The structures of these compounds The structures of these compounds was not always evident. Ions or was not always evident. Ions or molecules might be directly bonded to molecules might be directly bonded to the metal, or serve as a counter ion for the metal, or serve as a counter ion for an ionic salt.an ionic salt.

[Mn(OH[Mn(OH22))66] SO] SO44 sulfate ion is sulfate ion is outer sphereouter sphere

[Mn(OH[Mn(OH22))5 5 SOSO44]]..HH22O sulfate ion is O sulfate ion is inner inner spheresphere

Coordination Coordination CompoundsCompounds

Early chemists approached transition Early chemists approached transition metal complexes using the concept of metal complexes using the concept of “valences” adapted from main group “valences” adapted from main group metals. Metals with a +3 charge, such as metals. Metals with a +3 charge, such as iron(III) or cobalt(III) were believed to iron(III) or cobalt(III) were believed to only make three bonds.only make three bonds.

A compound such as [Co(NHA compound such as [Co(NH33))66]Cl]Cl33 was thought to have three Co-Cl bonds, was thought to have three Co-Cl bonds, with no way to explain the bonding of with no way to explain the bonding of ammonia in the compound.ammonia in the compound.


One approach by Blomstrand One approach by Blomstrand proposed chains of linked ammonia proposed chains of linked ammonia molecules, with the nitrogens having five molecules, with the nitrogens having five bonds and connecting a chloride to the bonds and connecting a chloride to the metal.metal.

Alfred Werner proposed that the Alfred Werner proposed that the ammonia molecules could bond strongly ammonia molecules could bond strongly and directly to the metal, with chlorides and directly to the metal, with chlorides either directly bonded, or loosely bonded either directly bonded, or loosely bonded and ionic in solution. and ionic in solution.



Jorgensen supported Jorgensen supported Blomstrand’s approach, and Werner, Blomstrand’s approach, and Werner, in order to support his theory, in order to support his theory, synthesized new compounds and synthesized new compounds and studied their isomers. Eventually, in studied their isomers. Eventually, in 1907, Werner prevailed and proved 1907, Werner prevailed and proved the octahedral geometry of the octahedral geometry of coordination compounds.coordination compounds.


Alfred Werner (1866-1919) also Alfred Werner (1866-1919) also determined the formulas and determined the formulas and structures of many transition metal structures of many transition metal compounds by studying their compounds by studying their isomers. Due to the existence of a isomers. Due to the existence of a variety of structural isomers, he variety of structural isomers, he proposed that complexes must have proposed that complexes must have square planar and octahedral square planar and octahedral shapes.shapes.

Possible Structures for 6-Possible Structures for 6-Coordinate CobaltCoordinate Cobalt



Proposed Structures for Proposed Structures for 6-Coordinate Cobalt6-Coordinate Cobalt

Structure of CompoundsStructure of Compounds

CompositionComposition # ions# ions Modern formulationModern formulation

PtClPtCl22..4NH4NH33 3 3 [Pt(NH[Pt(NH33))44]]2+2+ 2Cl 2Cl--

PtClPtCl22..3NH3NH33 2 2 [PtCl(NH[PtCl(NH33))33]]++ClCl--

PtClPtCl22..2NH2NH33 0 0 [PtCl[PtCl22

(NH(NH33))22]]

(2 forms)(2 forms)

Nomenclature of Nomenclature of Coordination CompoundsCoordination Compounds

There is a separate system for There is a separate system for naming coordination compounds:naming coordination compounds:

prefix indicating + ligand name + metal + prefix indicating + ligand name + metal + (oxidation #(oxidation #

# of ligands# of ligands in roman in roman numerals)numerals)

oror

“ “ ““ “ “ + “ “ “+ “ “ “ + “ “ “ + (charge of + “ “ “ + (charge of complex)complex)


1. If ionic, the positive ion is named 1. If ionic, the positive ion is named first, then the negative ion.first, then the negative ion.

2. The inner coordination sphere is 2. The inner coordination sphere is indicated by square brackets. In the indicated by square brackets. In the formula, the metal is written first, formula, the metal is written first, followed by the ligands. In naming, followed by the ligands. In naming, the ligands are named first, then the the ligands are named first, then the metal.metal.


3. 3. PrefixesPrefixes: If the ligand itself contains a prefix : If the ligand itself contains a prefix in its name (ex. dimethyl amine), then the prefix in its name (ex. dimethyl amine), then the prefix to indicate the number of ligands changes, and to indicate the number of ligands changes, and the ligand name is placed in parenthesis.the ligand name is placed in parenthesis.

2 2 didi or or bisbis 5 5 pentapenta or or pentakispentakis 8 8 octaocta or or octakisoctakis

3 3 tritri or or tristris 6 6 hexahexa or or hexakishexakis 99 nona nona or or nonakisnonakis

44 tetra tetra or or tetrakistetrakis 7 7 heptahepta or or heptakisheptakis 10 10 decadeca or or decakisdecakis


4. Ligands are listed in alphabetical order 4. Ligands are listed in alphabetical order (ignoring any prefixes). Most ligands (ignoring any prefixes). Most ligands have special names, with all negatively have special names, with all negatively charged ligands ending in the letter“charged ligands ending in the letter“oo”. ”.

Most neutral ligands retain their Most neutral ligands retain their usual names, with the following common usual names, with the following common exceptions:exceptions:

NHNH33 ammineammine HH22O O aquaaquaCOCO carbonyl carbonyl

Names of Common Names of Common LigandsLigands

FormulaFormula NameNameBrBr-- bromobromoCOCO33

2-2- carbonatocarbonatoClCl-- chlorochloroCNCN-- cyanocyanoHH-- hydridohydridoOHOH-- hydroxohydroxoOO2-2- oxooxo

Linkage IsomerismLinkage Isomerism

FormulaFormula NameName

NNOO22-- nitritonitrito (via O) (via O)

NNOO22-- nitro nitro (via N)(via N)

Linkage IsomersLinkage IsomersLinkage isomers Linkage isomers

involve ligands that involve ligands that may bond via different may bond via different sites. In this example, sites. In this example, nitro bonds via nitro bonds via nitrogen nitrogen

((NNOO22--), and nitrito ), and nitrito

bonds via an oxygen bonds via an oxygen (N(NOO22

--). The ). The compounds have compounds have different properties different properties and colors.and colors.

Polydentate LigandsPolydentate Ligands

FormulaFormula NameName

NHNH22CHCH22CHCH22NHNH22

ethylenediamine (en)ethylenediamine (en)

Both amines of the ligand can attach at the metal forming a ring.

Polydentate LigandsPolydentate Ligands

ethylenediaminetetraacetate ethylenediaminetetraacetate (EDTA) (EDTA) EDTA is a hexadentate ligand.

EDTAEDTA

EDTA can EDTA can wrap around a wrap around a metal ion to metal ion to coordinate at 6 coordinate at 6 (octahedral) sites. (octahedral) sites. Ligands that Ligands that bind to more than bind to more than one site are one site are called called chelating chelating agents.agents.


5. 5. There are two systems for indicating the There are two systems for indicating the oxidation number of the metal. The more oxidation number of the metal. The more commonly used system indicates the oxidation commonly used system indicates the oxidation number in Roman numerals in parentheses after number in Roman numerals in parentheses after the name of the metal.the name of the metal.

The other system puts the charge of the The other system puts the charge of the coordination complex in Arabic numbers in coordination complex in Arabic numbers in parentheses after the metal.parentheses after the metal.

[Cr(H[Cr(H22O)O)55Cl]Cl]2+2+ is is pentaaquachlorochromium(III) pentaaquachlorochromium(III) oror, , pentaaquachlorochromium(2+).pentaaquachlorochromium(2+).


6. Using either system, if the transition metal 6. Using either system, if the transition metal complex is negative in charge, the name of complex is negative in charge, the name of the metal ends in the metal ends in ateate. .

For example, [Pt(NHFor example, [Pt(NH33))22ClCl44]]2-2- is named is named diamminetetrachloroplatinate(II).diamminetetrachloroplatinate(II).

For metals with Latin names, the For metals with Latin names, the negatively charge complex uses:negatively charge complex uses:

ferrate (for Fe)ferrate (for Fe)argentate (for Ag)argentate (for Ag)

plumbate (for Pb)plumbate (for Pb) stannate (for Sn)stannate (for Sn)

aurate (for Au)aurate (for Au) cuprate (for Cu)cuprate (for Cu)


7. The complete name of the complex must 7. The complete name of the complex must also indicate the presence of geometric also indicate the presence of geometric isomers. Prefixes such as isomers. Prefixes such as ciscis, , transtrans, , mermer, , and and facfac are used to indicate the relative are used to indicate the relative positions of similar ligands.positions of similar ligands.

In addition, stereoisomers are also In addition, stereoisomers are also possible with tetrahedral and octahedral possible with tetrahedral and octahedral geometries, and optical isomers are geometries, and optical isomers are indicated with the prefixes ∆ and indicated with the prefixes ∆ and ΛΛ..


8. Bridging ligands between two metal 8. Bridging ligands between two metal atoms have the prefix atoms have the prefix μμ..

IsomerismIsomerism

StereoisomerismStereoisomerism

Stereoisomers have the same Stereoisomers have the same connectivities but different spatial connectivities but different spatial arrangements.arrangements.

In In geometric isomersgeometric isomers, the ligands , the ligands have different spatial arrangements have different spatial arrangements about the metal ion.about the metal ion.

Optical isomersOptical isomers are compounds are compounds with non-superimposable mirror images.with non-superimposable mirror images.

Geometric IsomerismGeometric IsomerismGeometric isomers differ in the Geometric isomers differ in the

geometric arrangement of the ligands geometric arrangement of the ligands around the central metal.around the central metal.

Common examples are square planar Common examples are square planar complexes such as [Pt(NHcomplexes such as [Pt(NH33))22ClCl22].].

Geometric IsomerismGeometric IsomerismIn octahedral complexes, the prefixes In octahedral complexes, the prefixes

ciscis and and transtrans are used for complexes of are used for complexes of the form [MXthe form [MX44YY22]]

Octahedral ComplexesOctahedral Complexes

For complexes with the formula For complexes with the formula [MX[MX33YY33], there are two spatial ], there are two spatial arrangements of the ligands.arrangements of the ligands.

Octahedral ComplexesOctahedral Complexes

facfac stands for facial, and stands for facial, and mermer stands for meridian.stands for meridian.

ChiralityChirality

Both four-coordinate and six-Both four-coordinate and six-coordinate complexes exhibit chirality. coordinate complexes exhibit chirality. Chiral molecules have either no Chiral molecules have either no symmetry elements (other than identity), symmetry elements (other than identity), or only a Cor only a Cnn axis. axis.

Tetrahedral complexes can be chiral Tetrahedral complexes can be chiral in the same way that organic compounds in the same way that organic compounds are: they may have four different are: they may have four different ligands. They may also have ligands. They may also have unsymmetrical chelating ligands.unsymmetrical chelating ligands.

ChiralityChirality

Square-Square-planar planar complexes complexes can also be can also be chiral, as chiral, as seen in these seen in these compounds of compounds of platinum(II) platinum(II) and and palladium(II).palladium(II).

Optical IsomersOptical Isomers

Octahedral complexes Octahedral complexes containing polydentate ligands can containing polydentate ligands can form optical isomers. Complexes form optical isomers. Complexes with three rings, such as [Co(en)with three rings, such as [Co(en)33]]3+3+, , can be viewed like a propeller with can be viewed like a propeller with three blades. The structure can be three blades. The structure can be either left or right handed, with non-either left or right handed, with non-superimposable mirror images.superimposable mirror images.

Optical Optical IsomersIsomers

The upper isomer is right handed, and the lower one is left handed.







The right-The right-handed isomer handed isomer requires going requires going clockwise to get clockwise to get from the upper from the upper triangle to the triangle to the lower one. The lower one. The prefix for this prefix for this isomer is ∆.isomer is ∆.


The left-The left-handed isomer handed isomer requires going requires going counterclockwise counterclockwise to get from the to get from the upper triangle to upper triangle to the lower one. The the lower one. The prefix for this prefix for this isomer is isomer is ΛΛ..


Octahedral complexes with two Octahedral complexes with two chelating ligands and two non-chelating ligands and two non-chelating ligands can also be chelating ligands can also be optically active. optically active.

IsomersIsomers

Co(III) and ethylenediamine react to Co(III) and ethylenediamine react to form several products. form several products. ciscis[CoCl[CoCl22(en)(en)22]]+ + is violet, and the is violet, and the trans isomer is green. The reaction trans isomer is green. The reaction also forms a yellow product, also forms a yellow product, [Co(en)[Co(en)33]]3+3+. Determine the number . Determine the number of isomers of each of the products. of isomers of each of the products. Label any enantiomers with the Label any enantiomers with the proper prefix (∆ or proper prefix (∆ or ΛΛ).).

Isomer ProblemIsomer Problem

The yellow product is The yellow product is [Co(en)[Co(en)33]]+3+3. It exists as an . It exists as an enantiomeric pair.enantiomeric pair.

Isomer ProblemIsomer Problem

The violet product consists of a pair The violet product consists of a pair of optical isomers. The green product is of optical isomers. The green product is not optically active, as it has a mirror not optically active, as it has a mirror plane.plane.

Common Coordination Common Coordination Numbers & StructuresNumbers & Structures

Factors Affecting Factors Affecting Coordination NumberCoordination Number

1. The size of the central atom or ion.1. The size of the central atom or ion.

2. Steric interactions between bulky 2. Steric interactions between bulky ligands such as P(Cligands such as P(C66HH55))33..

3. The electronic structure of the metal 3. The electronic structure of the metal atom or ion. If the oxidation number is atom or ion. If the oxidation number is high, the metal can accept more high, the metal can accept more electrons from the (Lewis base) electrons from the (Lewis base) ligands. Metals with many ligands. Metals with many dd electrons electrons will have lower coordination numbers.will have lower coordination numbers.

Coordination NumbersCoordination Numbers

Coordination numbers of 1, 2 Coordination numbers of 1, 2 and 3 are relatively rare. and 3 are relatively rare. Coordination number 4 leads to two Coordination number 4 leads to two common structures: tetrahedral or common structures: tetrahedral or square planar geometry. Square square planar geometry. Square planar structures are most planar structures are most commonly seen with dcommonly seen with d88 metals such metals such as Ni(II), Pd(II) or Pt(II), or when the as Ni(II), Pd(II) or Pt(II), or when the ligand is planar.ligand is planar.

5 Coordinate Complexes5 Coordinate Complexes

5 coordinate complexes with 5 coordinate complexes with monodentate ligands are often monodentate ligands are often highly fluxional, so that axial and highly fluxional, so that axial and equatorial positions interchange. equatorial positions interchange. Once such method of interchange of Once such method of interchange of these positions is via a these positions is via a Berry Berry pseudorotationpseudorotation. .

Berry PseudorotationBerry Pseudorotation

A

A

A

A

A

A

5 Coordinate Complexes5 Coordinate Complexes

A planar A planar polydentate polydentate ligand, such as a ligand, such as a porphyrin, may porphyrin, may result in square result in square pyramidal pyramidal structures, when structures, when a fifth ligand is a fifth ligand is added, such as in added, such as in myoglobin.myoglobin.

6-Coordinate Complexes6-Coordinate Complexes

6-coordinate 6-coordinate complexes are complexes are usually octahedral. usually octahedral. Sometimes Sometimes distortion from distortion from octahedral octahedral geometry occurs if geometry occurs if a bidentate ligand a bidentate ligand has a small has a small bite bite angleangle. .

6-Coordinate Complexes6-Coordinate Complexes

In extreme In extreme cases, the cases, the “staggered” “staggered” configuration of configuration of the octahedron the octahedron can be distorted can be distorted into a trigonal into a trigonal prism.prism.

7- Coordinate Complexes7- Coordinate ComplexesThese These

are more are more common for common for ff-block -block elements, elements, and include and include pentagonal pentagonal bipyramids, bipyramids, capped capped octahedrons octahedrons and capped and capped trigonal trigonal prisms.prisms.

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Transition Metal Complexes I The structures, nomenclature and isomers of coordination compounds