d-block metal chemistry: general...

Dr. Said El-Kurdi 3/1/2015

Dr. Said El-Kurdi 1

Inorganic Chemistry B Inorganic Chemistry B

Chapter 20

1 Dr. Said El-Kurdi

d-Block metal chemistry: general considerations

2 Dr. Said El-Kurdi


Dr. Said El-Kurdi 2

3 Dr. Said El-Kurdi

The term ‘transition elements (metals)’ is also widely used. However, the group 12 metals (Zn, Cd and Hg) are not always classified as transition metals.

The elements in the f-block are sometimes called inner

transition elements.

Each group of d-block metals consists of three members and is called a triad. Metals of the second and third rows are sometimes

called the heavier d-block metals. Ru, Os, Rh, Ir, Pd and Pt are collectively known as the platinum-group metals.

4 Dr. Said El-Kurdi

Ground state electronic configurations

the ground state of chromium is rather than

M2+ and M3+ ions of the first row d-block metals all have electronic configurations of the general form [Ar]3dn, and so the comparative chemistry of these metals is largely concerned with the consequences of the successive filling of the 3d orbitals.


Dr. Said El-Kurdi 3

5 Dr. Said El-Kurdi

Physical properties

6 Dr. Said El-Kurdi

The metallic radii (rmetal) for 12-coordination (Table 6.2 and Figure 20.1) are much smaller that those of the s-block metals of comparable atomic number.

Figure 20.1 also illustrates that values of rmetal: show little variation across a given row of the d-block; are greater for second and third row metals than for first row metals: are similar for the second and third row metals in a given triad. (lanthanoid contraction)


Dr. Said El-Kurdi 4

7 Dr. Said El-Kurdi

Metals of the d-block are (with the exception of the group 12 metals) much harder and less volatile than those of the s-block.

8 Dr. Said El-Kurdi

Metals in the second and third rows generally possess higher enthalpies of atomization than the corresponding elements in the first row.

This is a substantial factor in accounting for the far greater occurrence of metal–metal bonding in compounds of the heavier d-block metals compared with their first row congeners


Dr. Said El-Kurdi 5

9 Dr. Said El-Kurdi

The first ionization energies (IE1) of the d-block metals in a given period are higher than those of the preceding s-block metals.

Across each of the periods K to Kr, Rb to Xe, and Cs to Rn, the

variation in values of IE1 is small across the d-block and far greater among the s- and p-block elements.

Within each period, the overall trend for the d-block metals

is for the ionization energies to increase, but many small variations occur.

10 Dr. Said El-Kurdi

All 3d metals have values of IE1 and IE2 larger than those of calcium, and all except zinc have higher values of aH

o these factors make the metals less reactive than calcium.

In the formation of species containing M2+ ions, all the 3d metals are thermodynamically less reactive than calcium, and this is consistent with the standard reduction potentials listed in Table 20.1

Formation of a coherent surface film of metal oxide often renders a metal less reactive than expected


Dr. Said El-Kurdi 6

11 Dr. Said El-Kurdi

The reactivity of the metals

In general, the metals are moderately reactive and combine to give binary compounds when heated with dioxygen, sulfur or the halogens

product stoichiometry depending, in part, on the available oxidation states

Dr. Said El-Kurdi 12

Characteristic properties: a general perspective

The colors of d-block metal compounds are a characteristic feature of species with ground state electronic configurations other than d0 and d10.

[Cr(OH2)6]2+ is sky blue, [Mn(OH2)6]2+ very pale pink, MnO4

intense purple salts of Sc(III) (d0) or Zn(II) (d10) are colorless.

The fact that many of the observed colors are of low intensity is consistent with the color originating from electronic ‘d–d’ transitions.


Dr. Said El-Kurdi 7


The pale colors indicate that the probability of a transition occurring is low.

The intense colors of species such as MnO4 have a different

origin, namely charge transfer absorptions or emissions. The latter are not subject to selection rule 20.4 and are always more intense than electronic transitions between different d orbitals.

electronic spectra electronic spectra


Paramagnetism

The occurrence of paramagnetic compounds of d-block metals is common and arises from the presence of unpaired electrons. This phenomenon can be investigated using electron paramagnetic resonance (EPR) spectroscopy


Dr. Said El-Kurdi 8


Complex formation

d-Block metal ions readily form complexes, with complex formation often being accompanied by a change in color and sometimes a change in the intensity of color.

A central metal atom bonded to a group of molecules or ions is a metal complex.

If it’s charged, it’s a complex ion. Compounds containing complexes are coordination

compounds.


Lewis acids and bases

A Lewis base is a molecule or ion that donates a lone pair of electrons to make a bond

A Lewis acid is a molecule of ion that accepts a lone pair of electrons to make a bond

Examples: NH3 OH2 Cl-

F-

Examples: H+

Co3+ Co

2+ Mn+

Electrons in the highest occupied orbital (HOMO) of a molecule or anion are the best Lewis bases

Molecules or ions with a low lying unoccupied orbital (LUMO) of a molecule or cation are the best Lewis acids


Dr. Said El-Kurdi 9


The molecules or ions coordinating to the metal are the ligands.

They are usually anions or polar molecules They must have lone pairs to interact with metal






Alfred Werner: the father of the structure of coordination

complexes

Alfred Werner

Switzerland

University of Zurich

Zurich, Switzerland

b. 1866

(in Mulhouse, then Germany)

d. 1919

The Nobel Prize in Chemistry 1913

"in recognition of his work on the

linkage of atoms in molecules by

which he has thrown new light on

earlier investigations and opened

up new fields of research especially

in inorganic chemistry"




Same metal, same ligands, different number of ions when dissolved

Same metal, same ligands, different number of ions when dissolved

How did Werner deduce the structure of coordination complexes?


Werner suggested in 1893 that metal ions have

primary and secondary valences.

Primary valence equal the metal’s oxidation number

Secondary valence is the number of atoms directly

bonded to the metal (coordination number)




Variable oxidation states

The occurrence of variable oxidation states and, often, the interconversion between them, is a characteristic of most d-block metals. Exceptions are in groups 3 and 12





Electroneutrality principle

Pauling’s electroneutrality principle is an approximate method of estimating the charge distribution in molecules and complex ions.

The distribution of charge in a molecule or ion is such that the charge on any single atom is within the range +1 to 1

(ideally close to zero)

The distribution of charge in a molecule or ion is such that the charge on any single atom is within the range +1 to 1

(ideally close to zero)


(a) a conventional diagram showing the donation of lone pairs of electrons from ligands to metal ion

the charge distribution that results from a 100% covalent model of the bonding




the charge distribution that results from a 100% ionic model of the bonding

the approximate charge distribution that results from applying the electroneutrality principle.


Coordination numbers and geometries

most examples in this section involve mononuclear complexes




Coordination environments are often described in terms of regular geometries such as those in Table 20.4, in practice they are often distorted

Detailed discussion of a particular geometry usually involves bond lengths and angles determined in the solid state and these may be affected by crystal packing forces

Small energy difference may also lead to the observation of different structures in the solid state


sterically demanding ligands favor low coordination numbers

at metal centers;

high coordination numbers are most likely to be attained

with small ligands and large metal ions;

the size of a metal ion decreases as the formal charge

increases, e.g. r(Fe3+) < r(Fe2+);

low coordination numbers will be favored by metals in

high oxidation states with -bonding ligands.




The Kepert model

VSEPR model in predicting the shapes of molecular species of the p-block elements

we might reasonably expect the structures of the complex ions

to vary as the electronic configuration of the metal ion changes. However, each of these species has an octahedral arrangement of ligands


The VSEPR model is not applicable to d-block metal complexes.

Kepert model, in which the metal lies at the center of a sphere and the ligands are free to move over the surface

of the sphere.

Kepert model, in which the metal lies at the center of a sphere and the ligands are free to move over the surface

of the sphere.

Kepert ignores non-bonding electrons

Independent of the ground state electronic configuration of the metal center





One of the most important classes of structure for which the Kepert model does not predict the correct answer is that of the square planar complex, and here electronic effects are usually the controlling factor, as we will discuss in Section 21.3.

Another factor that may lead to a breakdown of the Kepert model is the inherent constraint of a ligand. For example:




Chelate Effect


Chelate Effect




The oxidation +2 state is common for almost all the

transition metals. Suggest an explanation.

No compounds are known in which scandium is in the +2

oxidation state. Suggest an explanation.

How many electrons are in the valence d orbitals in these

transition-metal ions? (a) Co3+ , (b) Cu+, (c) Cd2+ , (d) Os3+.

Why can the NH3 molecule serve as a ligand but the BH3

molecule cannot?

Would you expect ligands that are positively charged to be

common?


Some, but not all, of these ligand arrangements are in accord with the Kepert model.

For example, the coordination sphere in [Cu(CN)3]2 is predicted by the Kepert model to be trigonal planar.

Indeed, this is what is found experimentally.

The other option in Table 20.4 is trigonal pyramidal, but this does not minimize interligand repulsions.




the four nitrogen donor atoms of a porphyrin ligand are confined to a square planar array


tripodal ligands such as 20.3 have limited flexibility which means that the donor atoms are not necessarily free to adopt the positions predicted by Kepert;

macrocyclic ligands are less lexible than open chain ligands.

polyether 18-crown-6




Coordination numbers in the solid state

molecular formula can be misleading in terms of coordination number

For example in CdI2, each Cd center is octahedrally sited, and molecular halides or pseudohalides (e.g. [CN]) may contain MXM bridges and exist as oligomers, e.g. -PdCl2 is polymeric


when the bonding mode of a ligand can be described in more than one way. This often happens in organometallic chemistry, for example with cyclopentadienyl ligands




Coordination number 2

Examples of coordination number 2 are uncommon, being generally restricted to Cu(I), Ag(I), Au(I) and Hg(II), all d10 ions.

Examples include [CuCl2], [Ag(NH3)2]2+, [Au(CN)2], (R3P)AuCl, [Au(PR3)2]+ (R = alkyl or aryl) and Hg(CN)2, in each of which the metal center is in a linear environment.


Bulky amido ligands, are often associated with low coordination numbers.

3-coordinate





3-Coordinate complexes are not common. Usually, trigonal planar structures are observed, and examples involving d10 metal centers include:






4-Coordinate complexes are extremely common, with a tetrahedral arrangement of donor atoms being the most frequently observed.

The tetrahedron is sometimes ‘flattened’, distortions being attributed to steric or crystal packing effects or, in some cases, electronic effects.

Tetrahedral complexes for d3 ions are rare


Tetrahedral complexes for d4 ions have been stabilized only with bulky amido ligands

for M = Hf or Zr




Square planar complexes are rarer than tetrahedral, and are often associated with d8 configurations where electronic factors strongly favor a square planar arrangement

the steric demands of the ligands distort the structure from the square planar structure expected for this d8 metal centre



The limiting structures for 5-coordination are the trigonal

bipyramid and square-based pyramid.

The energy differences between trigonal bipyramidal and

square-based pyramidal structures are often small

trigonal bipyramidal




square-based pyramidal



For many years after Werner’s proof from stereochemical studies that many 6-coordinate complexes of chromium and cobalt had octahedral structures

The regular or nearly regular octahedral coordination sphere is found for all electronic configurations from d0 to d10,

Jahn–Teller distortion

low-spin and high-spin complexes




there is a small group of d0 or d1 metal complexes in which the metal center is in a trigonal prismatic or distorted trigonal prismatic environment. The octahedron and trigonal prism are closely related, and can be described in terms of two triangles which are staggered (20.9) or eclipsed (20.10).


contain regular trigonal prismatic (D3h) metal centres

the coordination environment is distorted trigonal prismatic (C3v)

The common feature of the ligands in these complexes is that they are -donors, with no -donating or -accepting properties




For a regular trigonal prism, angle in 20.13 is 0o



High coordination numbers (7) are observed most frequently for ions of the early second and third row d-block metals and for the lanthanoids and actinoids, i.e. rcation must be relatively large




capped trigonal prismatic [ZrF7]3

capped octahedral [TaCl4(PMe3)3]

pentagonal bipyramidal


capped octahedral





Specifying the counter-ion is important since the energy difference between 8-coordinate structures tends to be small with the result that the preference between two structures may be altered by crystal packing forces in two different salts.

which possess square antiprismatic or dodecahedral structures depending on the cation.


Isomerism in d-block metal complexes

Stereoisomers possess the same connectivity of atoms, but differ in the spatial arrangement of atoms or groups.

If the stereoisomers are not mirror images of one another, they are called diastereoisomers.

Stereoisomers that are mirror images of one another are called enantiomers.





Self-study exercises P627 All the answers can be found by reading Section 2.9. Self-study exercises P627 All the answers can be found by reading Section 2.9.

Structural isomerism: ionization isomers

Ionization isomers result from the interchange of an anionic ligand within the first coordination sphere with an anion outside the coordination sphere.




Structural isomerism: hydration isomers

Hydration isomers result from the interchange of H2O and another ligand between the first coordination sphere and the ligands outside it.

When this is dissolved in water, the chloride ions in the complex are slowly replaced by water to give blue-green

blue-green violet


Structural isomerism: coordination isomerism

Coordination isomers are possible only for salts in which both cation and anion are complex ions; the isomers arise from interchange of ligands between the two metal centers.




Structural isomerism: linkage isomerism

Linkage isomers may arise when one or more of the ligands can coordinate to the metal ion in more than one way, e.g. in [SCN] , both the N and S atoms are potential donor sites. Such a ligand is ambidentate.

Linkage isomers may arise when one or more of the ligands can coordinate to the metal ion in more than one way, e.g. in [SCN] , both the N and S atoms are potential donor sites. Such a ligand is ambidentate.

For the O-bonded ligand, characteristic absorption bands at 1065 and 1470 cm1 are observed,

N-bonded ligand, the correspondin vibrational wavenumbers are 1310 and 1430 cm1.


The DMSO ligand (dimethylsulfoxide) can coordinate to metal ions through either the S- or O-donor atom. These modes can be distinguished by using IR spectroscopy:




An example of the interconversion of linkage isomers involving the DMSO ligand is


Indicate the coordination number of the metal and the oxidation number of the metal as well as the number and type of each donor atom of the ligands for each of the following complexes:




Square planar species

In a square planar species such as [PtCl4]2 , the four Cl atoms are equivalent. Similarly, in [PtCl3(PMe3)] , there is only one possible arrangement of the groups around the square planar Pt(II) center.


The introduction of two PMe3 groups to give [PtCl2(PMe3)2] leads to the possibility of two stereoisomers




Trigonal bipyramidal species

axial position

equatorial position


Octahedral species

In EX2Y4 the X groups may be mutually cis or trans

[SnF4Me2]2




If an octahedral species has the general formula EX3Y3, then the X groups (and also the Y groups) may be arranged so as to define one face of the octahedron or may lie in a plane that also contains the central atom E


Indicate the likely coordination number of the metal in each of the following complexes:

what would you predict for the magnitude of the equilibrium constant? Explain your answer.




Is the following ligand a chelating one? Explain.


Distinguishing between cis- and trans-isomers of a square planar complex or between mer- and fac-isomers of an octahedral complex is most unambiguously confirmed by structural determinations using single-crystal X-ray diffraction. Vibrational spectroscopy may also be of assistance.

The selection rule for an IR active vibration is that it must lead to a change in molecular dipole moment

The selection rule for an IR active vibration is that it must lead to a change in molecular dipole moment




[Pt(NH)2Cl2]


Stereoisomerism: enantiomers

A pair of enantiomers consists of two molecular species which are non-superposable mirror images of each other

The occurrence of enantiomers (optical isomerism) is concerned with chirality

The occurrence of enantiomers (optical isomerism) is concerned with chirality




[Cr(acac)3], an octahedral tris-chelate complex

A molecule is chiral if it is non-superposable on its mirror image A molecule is chiral if it is non-superposable on its mirror image

Enantiomers are distinguished by using the labels and

Chiral molecules rotate the plane of polarized light. This property is known as optical activity. Enantiomers rotate the light to equal extents, but in opposite directions, the dextrorotatory (d) enantiomer to the right and the laevorotatory (l) enantiomer to the left.


A mixture of equal amounts of two enantiomers is called a racemate.

A mixture of equal amounts of two enantiomers is called a racemate.

The rotation, , may be measured in an instrument called a polarimeter (Figure 20.14). In practice, the amount of rotation depends upon the wavelength of the light, temperature and the concentration of compound present in solution.

The specific rotation, [],




(+) and () prefixes: the specific rotation of enantiomers is equal and opposite, and a useful means of distinguishing between enantiomers is to denote the sign of []D. Thus, if two enantiomers of a compound A have []D values of +12o and 12o, they are labeled (+)-A and ()-A.

d and l prefixes: sometimes (+) and () are denoted by dextro- and laevo- (derived from the Latin for right and left) and these refer to right- and left-handed rotation of the plane of polarized light respectively; dextro and laevo are generally abbreviated to d and l.


The +/ or d/l notation is not a direct descriptor of the absolute configuration of an enantiomer (the arrangement of the substituents or ligands) for which the following prefixes are used.

R and S prefixes: the convention for labeling chiral carbon atoms (tetrahedral with four different groups attached) uses sequence rules

This notation is used for chiral organic ligands, and also for tetrahedral complexes.




and prefixes: enantiomers of octahedral complexes containing three equivalent bidentate ligands (tris-chelate complexes) are among those that are distinguished using (delta) and (lambda) prefixes.

The octahedron is viewed down a 3-fold axis, and the chelates then define either a right- or a left-handed helix. The enantiomer with right-handedness is labeled , and that with left-handedness is .


The complexes [Cr(acac)3]




[Co(en)2Cl2]+


Nomenclature

Ligands are frequently named using older trivial names rather than the International Union of Pure and Applied Chemistry (IUPAC) names

Common Name IUPAC Name Formula

hydrido hydrido H

fluoro fluoro F

chloro chloro Cl

bromo bromo Br

iodo iodo I

nitrido nitrido N3

azido azido N3

oxo oxido O2

cano cano CN

thiocyano thiocyanato-S(S-bonded) SCN

isothiocyano isothiocyanato-N(N-bonded) NCS

hydroxo hydroxo OH

aqua aqua H2O

carbonyl carbonyl CO

thiocarbonyl thiocarbonyl CS

nitrosyl nitrosyl NO+ nitro nitrito -N (N-bonded) NO2

nitrito nitrito- O (O-bonded) ONO

methylisocyanide methylisocyanide CH3NC

phosphine phosphane PR3

pyridine pyridine (abbrev. py) C5H5N

ammine ammine NH3

methylamine methylamine MeNH2

amido azanido NH2

imido azanediido NH2




Common Name IUPAC Name Formula

hydrido hydrido H

fluoro fluoro F

chloro chloro Cl

bromo bromo Br

iodo iodo I

nitrido nitrido N3

azido azido N3

oxo oxido O2

cano cano CN

thiocyano thiocyanato-S(S-bonded) SCN

isothiocyano isothiocyanato-N(N-bonded) NCS

hydroxo hydroxo OH

aqua aqua H2O

carbonyl carbonyl CO

thiocarbonyl thiocarbonyl CS

nitrosyl nitrosyl NO+ nitro nitrito -N (N-bonded) NO2

nitrito nitrito- O (O-bonded) ONO

methylisocyanide methylisocyanide CH3NC

phosphine phosphane PR3

pyridine pyridine (abbrev. py) C5H5N

ammine ammine NH3

methylamine methylamine MeNH2

amido azanido NH2

imido azanediido NH2






Nomenclature Rules

1. The cation comes first, followed by the anion.

2. The inner coordination sphere is enclosed in square brackets. Although the metal is provided first within the brackets, the ligands within the coordination sphere are written before the metal in the formula name.

Examples: diamminesilver(I) chloride, [Ag(NH3)2]Cl

potassium hexacyanoferrate(III), K3[Fe(CN)6]

Examples: tetraamminecopper(II) sulfate, [Cu(NH3)4]SO4

hexaamminecobalt(III) chloride, [Co(NH3)6]Cl3




2 di

bis

3 tri

tris

4 tetra tetrakis

5 penta pentakis 6 hexa hexakis 7 hepta heptakis 8 octa octakis 9 nona nonakis 10 deca decakis

3. The number of ligands of each kind is indicated by prefixes (Table 3). In simple cases, the prefixes in the second column are used. If the ligand name already includes these prefixes or is complicated, it is set off in parentheses, and prefixes in the third column (ending in –kis) are used.

dichlorobis(ethylenediamine)cobalt(III),

[Co(NH2CH2CH2NH2)2Cl2]+

tris(2,2-bipyridine)iron(II),

[Fe(C10H8N2)3]2+


4. Ligands are generally written in alphabetical order according to

the ligand name, not the prefix.

Examples: tetraamminedichlorocobalt(III), [Co(NH3)4Cl2]+

amminebromochloromethylamineplatinum(II), Pt(NH3)BrCl(CH3NH2)

5. Anionic ligands are given an o suffix. Neutral ligands retain their usual name. Coordinated water is called aqua and coordinated ammonia is called ammine .




6. Two systems exist for designating charge or oxidation number:

a. The Stock system puts the calculated oxidation number of the metal as a Roman numeral in parentheses after the metal name.

b. The Ewing-Bassett system puts the charge on the coordination sphere in parentheses after the name of the metal.

In either case, if the charge is negative, the suffix -ate is added

to the name.

tetraammineplatinum(II) or tetraammineplatinum(2+), [Pt(NH3)4]2+

tetrachloroplatinate(II) or tetrachloroplatinate(2–), [PtCl4]2

hexachloroplatinate(IV) or hexachloroplatinate(2–), [PtCl6]2


7. Prefixes designate adjacent (cis -) and opposite (trans -) geometric locations.

cis - and trans -diamminedichloroplatinum(II), [PtCl2(NH3)2]

8. Bridging ligands between two metal ions (Figures 1) have the prefix m-.

tris(tetrammine-m-dihydroxocobalt)cobalt(6+), [Co(Co(NH3)4(OH)2)3]6+ m-amido-m-hydroxobis(tetramminecobalt)(4+),

[(NH3)4Co(OH)(NH2)Co(NH3)4]4+




9. When the complex is negatively charged, the names for these metals are

derived from the sources of their symbols:

iron (Fe) ferrate lead (Pb) plumbate silver (Ag) argentate

tin(Sn) stannate gold (Au) aurate copper (Cu) cuprate

Examples: tetrachloroferrate(III) or tetrachloroferrate(1–), [FeCl4]

dicyanoaurate(I) or dicyanoaurate(1–), [Au(CN)2]


a. Triamminetrichlorochromium(III) b. Dichloroethylenediamineplatinum(II) c. Bis(oxalato)platinate(II) or bis(oxalato)platinate(2-) d. Pentaaquabromochromium(III) or pentaaquabromochromium(2+) e. Tetrachloroethylenediaminecuprate(II) or tetrachloroethylenediaminecuprate(2-) f. Tetrahydroxoferrate(III) or tetrahydroxoferrate(1-)

Name these coordination complexes:

a. Cr(NH3)3Cl3

b. Pt(en)Cl2

c. [Pt(ox)2]2

d. [Cr(H2O)5Br]2+

e. [Cu(NH2CH2CH2NH2)Cl4]2

f. [Fe(OH)4]




Give the structures of these coordination complexes: a. Tris(acetylacetonato)iron(III) b. Hexabromoplatinate(2–) c. Potassium diamminetetrabromocobaltate(III) d. Tris(ethylenediamine)copper(II) sulfate e. Hexacarbonylmanganese(I) perchlorate f. Ammonium tetrachlororuthenate(1–)

a


b

c




d

e f

d-block metal chemistry: general...

Documents