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CHEM 340 TRANSITION METAL CHEMISTRY Lect-12

Preliminaries/Background Just as element names/symbols were memorized for FRO8 atoms

(Li, Be, B, C, N, O, F, Ne), now you are asked to memorize the FROTM

atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn). These elements provide most of the examplesfor (1) main group chemistry and (2) now TM chemistry, so we need to be very familiar withtheir names, symbols, and ground state and ion electron configurations, and term states.

FROTM have vse configurationsof

4 3

2

s d

n. The

4

2

selectrons are first lost, so all

TM elements have a common + 2 ox.st. (In the FROTM only copper has a + 1ox.st., and inlater rows silver and gold.) Many TM atoms also display +3and higher ox.sts., when 3delectrons are removed.

Angular Distribution Diagrams of datomic orbitals Reacquaint yourselves with names,shapes, locations and trigonometric signs of datomic orbitals. (d orbitals are gerade)d

xy four lobes in the XY plane and between X and Y axes

dxz

four lobes in the XZ plane and between X and Z axes

dyz

four lobes in the YZ plane and between Y and Z axes

dx y2 2

four lobes in the XY plane and are centered on X and Y axes

dz

2 seems to be different, but is really a combination of x2- z2 plus y2- z2 =

(2 z2

- x2

- y2

). Main ( + ) lobes centered on Z axis, ( - ) ring in XY plane.

TERM STATE SYMBOLS Note the pattern obtained for term state symbol

values upon incrementing the number of delectrons:dn d1 d2 d3 d4 d5 d6 d7 d8 d9 d10

term symbol 2D 3F 4F 5D 6 S 5D 4F 3F 2D 1 S

Observe that only S D F, , terms are met. Sterms are always associated with

filled and half-filled subshells, i.e., d5 and d10 cases. Each of the remaining eightcases has one of two terms, D or F. Note that several FROTM ions can haveidentical term states and dn configurations, i.e., Cr(II) and Mn(III) for d4 , Fe(II) andCo(III) ford6 . Work in TM chemistry is greatly facilitated by seamless transitions fromdn configuration notation to actual TM atom/ion symbol and charge.

Magnetic Susceptibilities (Moments) Paramagnetic substances (with unpaired electrons)are attracted to a magnetic field, whereas diamagnetic substances (all electrons paired)

are repelled. Presence of unpaired electrons is common for many TM atoms/ions sothey and their compounds are often paramagnetic because of this feature.

The magnetic character of a substance is determined from two mass measurements,firstin the absence of a magnetic field (i.e., a "normal" weighing procedure), andsecondlyin the presence of a magnetic field. (Refer to text, Figure 10-1, page 314.)Masses of paramagnetic substances are greater in a magnetic field.Masses of diamagnetic substances are less in a magnetic field.


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An expression developed to calculate the magnetic character ( ) involves theproperties of total spin angular momentum ( )S and total orbital angular momentum

( )L as shown: ( )

( ) ( ( ) /S L

g S S L L+

= + + +1 1 4 (Bohr Magnetons)where gis the gyromagnetic ratio (which has a value very close to 2), and

S ms

= and L m= l

Apply this expression to the Ni(II) = d8 case of Ni(II) ion, and compareto the experimental magnetic

moment of 2.83Bohr Magnetons

S=2/2 L =3 ( )S L+

= 4.5B.M.

This does not compare well with the experimental value. A modification of this

expression suggests that for lighter metals (like FROTM) the contribution of the spinterm is more important than the orbital term. This leads to a spin-only formula:

( )

( )spin only

S S

= +2 1 or equivalently ( )

( )so

n n= +2 wheren = number of unpaired electrons. For Ni(II) ( )so = 2.83 B.M. which agrees well with

the experimental value. Accordingly, we will use the ( )so

formula for FROTM.

Magnetic moment determinations are important because they afford a means of (1)

COUNTING unpaired electrons by using experimental ( )so

value and calculating n

from the above expression, and (2) determining the oxidation of the TM.

Some Examples of Science "in Action"

First, accumulate sets of information, and

then analyze for trends and/or anomalies.Consider several examples:

a. Ionization Energy IE vs. at.no.

yields a graph with a maxima ateach noble gas. Note that

mini reversals are met after

configurations of ns2 and np3,

This information is analyzedand intepreted to show very stable electron configurations at (2), (2)+(2+6),(2)+(2+6)+(2+6), and (2)+(2+6)+(2+6+10)+(2+6) electrons.

b. Radii for TM free atoms (decreasing size is almost linear), but

Radii for TM IONS (shows double peaked curve) minimum at Fe(II) and Co(III). Why?

c. Lattice energies (double peaked curve) minimum at Mn(II). Why?

d. Enthalpy of hydration(double peaked curve) with minimum at Mn(II) and Fe(III).

maximum at Cr(III) & Co(III). Why?

The questions are posed. Let's begin looking for answers (and finding understanding).

ml= 2 0 -1 -21


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Descriptive Ligand Field Theory

CHEMICAL BONDS (1) The "chemical glue" binding two atoms together has itsorigin in vs electrons shared by the atoms, and (2) remember that electrons reside in orbitals.

Accordingly, knowledge of vs orbitals informs where electrons are to be found, and that'swhere bonds are expected to form. Accordingly, knowledge of TM vs orbitalshapes, locations, and names/labels are critical to developments that follow.

Six-coordinate Complexes in an Oh Field C.N. = 6 and pt. gp. symmetry = Oh

transformation properties of dorbitals in Oh pt. gp. symmetry (a char.table look-up)d

z2 and dx y2 2 transform as the egtwo-dimensional irreducible representation.

d d dxy xz yz

, , transform as the t

g2three-dimensional irreducible representation

Consider six ligands positioned at Oh sites asthey approach a TM atom/ion to form an

Oh coordination complex, as shown to the right:

Now superimpose on this diagram the five

dorbitals. Note that orientation of thed

z2 and dx y2 2 pair allows for greater

interaction with ligand orbitals , then does

the d d dxy xz yz

, , set, which has lobes oriented between incoming ligands. Based on

TM-ligand e-e repulsions, TM electrons in the former set will be perturbed more by ligand

electrons and this leads to a differentiation (splitting) of energies for dorbitals in the complex.

(1) Sigma-bonded Oh TM Complexes

Transformation properties of sigma-bondingp-type atomic orbitals for six ligands

Using methods met and used in Chapter 5, ligand -bonding orbitals transform as

( )bonds

= a e tg g u1 1

+ + The only TM - ligand bonding interaction involvesthe e

gsymmetry orbitals, i.e.,

dz

2 and dx y2 2 of the TM.

The TM d d dxy xz yz

, , arenon-bonding in this case. The MO

diagram for a sigma bonded Oh TMcomplex is shown as Figure 10-5 on

page 321. Isolation of TM dorbitals from this diagram shows the above pattern. Theenergy difference between e

gand t

g2TM orbitals in Oh complexes is labeled o

and is represented by the arrow in the above diagram. The MAGNITUDE of the

energy difference o varies, depending on (a) inherent bonding strength of theligand, and (b) charge of the TM ion.

eg

t2g

+y+x

+z

L3

L1

L6

L5

L2

L4

TM


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Energy is conserved in the splitting, so the net destabilization of the eg

set (two

orbitals) is offset by the stabilization of the tg2

set (three orbitals). Or,

2 3

53

2

50x destabiliz x stabiliz( ) ( )

+

=

Applying factors of (3 / 5) and ( -2 / 5) to electrons in egand t g2 sets respectively,gives the Ligand Field Stabilization Energy. LFSE can be used to show which

particular dn configurations will be more (or less) favored for Oh complex formation.Refer to text Table 10-5 on page 325, and the following development

d1 [as in Ti(III)] LFSE = (3/5)[ 0 ] + (-2/5)[ 1 ] = -2/5

d2 [as in Ti(II), LFSE = (3/5)[ 0 ] + (-2/5)[ 2 ] = -4/5

V(III) ]

d3 [as in V(II), LFSE = (3/5)[ 0 ] + (-2/5)[ 3 ] = -6/5

Cr(III) ]

Two situations arise for d4 through d7 depending on the MAGNITUDE of o .When energy of o is small, relative to energy required for electron pairing, then aWEAK FIELD (with respect to ligand bonding strength) or HIGH SPIN (with respect to

number of unpaired electrons) condition prevails and so the maximum number ofunpaired electrons will be present.

When energy of o is large, relative to energy required for electron pairing, then aSTRONG FIELD or LOW SPIN condition prevails and the t

g2set is filled before the e

g.

Consider the following diagrams and LFSE's

WEAK FIELD STRONG FIELD

d4 [as in Cr(II),

Mn(III) ]

LFSE = (3/5)[ 1 ] + ( -2/5)[ 4 ] = -3/5

LFSE = (3/5)[ 0 ] + (-2/5)[ 4 ] = -8/5

o (weak field) < o (strong field)


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Notice that the d4 weak field case results in FOUR unpaired electrons, but the strong fieldcase has only TWO unpaired electrons. So measurements of magnetic moments can beused to identify and distinguish between strong field and weak field ligands and complexes.

Convince yourself that the following results would be obtained for d d d5 6 7, , cases

[ WEAK FIELD ] LFSE n [ STRONG FIELD ]LFSE n d5 0 5 -10/5 1d6 -2/5 4 -12/5 0d7 -4/5 3 -9/5 1

For 8 and 9 dvse, only weak field cases are possible so they would be as indicated below.d8 -6/5 2 and d9 -3/5 1LFSE are largest for d6 in a strong field, and zero for d5 in a weak field. This information canbe used to account for some observations in the accumulated data presented at the beginning

of the hour: Co(III) has the largest hydration energy , and its aqua complex is a d6 strongfield case. Mn(II) and Fe(III) have lowest hydration energies, and they both are d5 weak fieldcases having LFSE = 0.

Magnetic moments are very informative in identifying ox.sts. and field strengths of complexes.

Consider some information about iron complexes. Octahedral complexes of Fe(II), a d6 case,can be strong field/low spin with n=0, or weak field/high spin with n=4. Octahedralcomplexes of Fe(III), a d5 case, can be strong field/low spin with n=1, or weak field/high spinwith n=5. Magnetic moments for the four situations would bediamagnetic, ( )so = 4.9, ( )so = 1.8, and ( )so = 5.9 B.M. respectively.

(2) Pi-bonded Oh TM Complexes

Carbon Monoxide, a pi-acceptor ligand.Consider the mo diagram for CO, a ten

electron system.

The HOMO is associated more withcarbon, as is the empty pi* LUMO .

The sigma bond from CO to the TMinvolves the HOMO, and is why the

bond is TM - C instead of TM - O.

Energy of the empty pi* LUMO is

similar to that of the TM dvse, and moreimportantly, they are symmetry matched, so

bonding between them is possible. In this instance however, the TM furnisheselectrons to the empty pi* LUMO, in a manner known as "back-bonding".

OC 2s

2p

2p

2s


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The combination of a sigma bond (donation from C in CO to the TM) and pi-bond (back-

bonding from TM to empty pi* LUMO in CO) makes for exceptionally strong netbonding. This is the reason for the toxicity of CO; it binds so strongly to heme iron in

blood that oxygen can no longer be processed. Cyanide anion is isoelectronic with COand is also an excellent pi-acceptor ligand. Pi-acceptor ligands are strong field ligands.

Compare CO and CN1 -to chloride anion, Cl1 -. Chloride anion has a complete octet

of vse; all of its vs are filled so it cannot accept (pi) electrons from TM. However, it candonate pi electrons to the TM. But the TM in a complex is already in enough troublew/r to electrons and doesn't need any more. Consequently, pi-donor ligands likechloride anion are weak field ligands; they present too much electron density to the TM.

(3) Relative Bonding Strengths of Ligands - The Spectrochemical Series

When the bonding effects ( i.e., o values) of a series of ligands are compared forsimilar TM's, and then arranged in order of magnitudes of o , a qualitative sequenceof ligands results that is called the Spectrochemical Series. A simple example follows:

STRONG FIELD MODERATE FIELD WEAK FIELD

CO CN1 - o-phen NO21 -

en NH3 H2O F1 -

RCO21 -

OH1 -

Cl1 -

Br1 -

I1 -

pi-acceptor ligands sigma donor ligands pi-donor ligands

The effect of larger o values can be demonstrated by the color of coordinationcomplexes. Consider two complexes of Ni(II), the hexaaqua and hexaammine

complexes. As will become apparent soon, Ni(II), a d8 system, displays a three-bandabsorption spectrum. The hexaaqua complex is green and the hexaammine blue.These colors are observed b/c the energies they represent are not absorbed, as

shown in the following diagram of the visible range of energies

Visible region of theelectromagneticspectrum

[ Ni ( H2O )6]2 +

complex, absorbsenergies (in black)and appears green

in color.

[ Ni ( NH3)6]2 +

complex, absorbsenergies (in black)

and appears bluein color.

In both spectra the color is due to the three-band absorption system of the nickel(II) ionHowever, ammonia is a stronger field ligand than water so the 3 bands absorb at higher

energies. This shifts the transparent "window" between the center and right absorptionbands to higher energies and imparts a blue color to the ammine complex.


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Water is classified as a ligand of moderate/weak donating ability, so it would be

anticipated that aqua complexes are WEAK FIELD / HIGH SPIN cases. Indeed that isfound to be the case , except for one FROTM ion. The one TM cation forming a strong

field complex with water as ligand is Co(III), a d6 system. Recall that d6 strong fieldcase has the highest LFSE . Note that complexes of d6 strong field are diamagnetic.

Four-coordinate Complexes in a Square-Planar Field C.N. = 4 and pt. gp. symmetry = D h4

transformation properties of dorbitals in Dh4

pt. gp. symmetry (a char.table look-up)

dz

2 transforms as the a g1 irreducible representation

dx y2 2

transforms as the bg1

irreducible representation.

dxy

transforms as the bg2

irreducible representation, and

d dxz yz

, transform as the eg

two-dimensional irreducible representation

Consider four ligands in a square-planar

arrangement approaching on the x andy axes. Clearly the d

x y2 2would be

most directly in-line with the ligands and

has the highest energy. Remaining dorbitals would be affected in the following

order dxy

then dz

2 and lastly the d dxz yz, pair. This leads to the splitting pattern

as shown to the right. The numbers indicate theordering of entering electrons into this pattern.

Ni(II), a d8 system, forms square-planar complexes.

Introducing 8 electrons into the sq-planar patternresults in a diamagnetic condition. This feature

allows identification and differentiation of sq-planarNi(II) complexes from Oh Ni(II) complexes because

the Oh compounds will be paramagnetic with

( )so

=2.83 B.M.

All available information is used when characterizing coordination compounds, including

chemical analysis, magnetic moments, conductances, and (from the next chapter)absorption spectra. So while it may be possible to differentiate sq-planar Ni(II) with four

donor atoms, from Oh Ni(II) with six donor atoms by chemical analysis alone, allinformation available is collected and applied to the problem. Differences in magneticmoments for sq-planar and Oh Ni(II) offers additional reliable information for making

the determination.

TM

L1

L2

L3L4

+x

+y

+z

eg

a1g

b2g

b1g

1,3 2,4

5,6

7,8


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Four-coordinate Complexes in a Tetrahedral Field C.N. = 4 and pt. gp. symmetry = Td

transformation properties of dorbitals in Td pt. gp. symmetry (a char.table look-up)d

z2 and dx y2 2 transform as the e two-dimensional irreducible representation.

d d dxy xz yz

, , transform as the t2

three-dimensional irreducible representation

Note in Td symmetry there are no gor u subscripts. This is b/c in Td symmetry there is noinversion center.

In a Td symmetry field, the splitting pattern

for the five dorbitals is as shown to the right.Note that it is the inverse of the Oh splittingpattern.

The energy difference between the two levels is labeled tand is represented by the arrowin the diagram. A Td field is less than an Oh field because there are only four donor atoms

instead of six. It can be shown that t o4

9. Consequently, Td complexes are

always WEAK FIELD / HIGH SPIN cases.

Ni(II) forms Td as well as Oh and sq-planar complexes, and even five -coordinate complexes.Its coordination number and geometry depend very much on the ligand itself. One of the manyfun parts of coordination chemistry is met in the design and subsequent preparation of specific

ligand types to cause formation of a complex with a particular geometrical arrangement.

Chemical analysis and magnetic moment determinations of nickel(II) complexes often allowsdifferentiation between coordination numbers of six, five and four, and in the latter case,between tetrahedral and square-planar geometries.

e

t2

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