inorganic reaction mechanisms-part ii

7/30/2019 Inorganic Reaction Mechanisms-Part II

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Rates of Water Exchange The rate constants of a given ion are approximately constant, no matter what the

nature of the entering group. (evidence for dissociative activation)

The substitution rates divide the metal ions into 4-distinct groups Class 1.

Exchange of water is very fast and is essentially diffusion controlled (k >= 108 sec-1) Ionsinclude Groups 1A , IIA (except Be2+, Mg2+), and IIB (except Zn2+), plus Cr2+, and Cu2+.

Class 2

Rate constants are in the range 104 - 108 sec-1. This includes most of the 1strow T.M divalent ions (except V2+, Cr2+, Cu2+) and Mg2+ and the lanthanideM3+ ions.

Class 3.

Rate constants are in the range 1 - 104 sec-1. This includes Be2+, Al3+, V2+,and some 1st row T.M. tri-valent ions.

Class 4.

Rate constants are in the range 10-3-10-6 sec-1. This includes Cr3+, Co3+, Rh3+,Ir3+, Pt2+

General Comments

Prof. Dr. Usama El-Ayaan


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For Class 1 .ionic size, and ionic charge are clearly the

important. As the ion becomes smaller the substitutionrate slows. Reflecting importance of orbital overlapbetween metal ion and departing ligand. Group IIA, andIIB show similar trend, (Be 2+ is anomalous due tocomplex hydrolysis reactions.

For T.M. metals the correlation of rate with size is notobeyed, e.g. Cr 2+ Ni 2+ and Cu 2+ have identical radii.

Cu 2+ (d (9 and Cr 2+ (d4 ( are structurally distorted by theJahn-Teller effect, with bond to the axial ligands longerand weaker than bonds to the equatorial groups.

Therefore the ground state structures are not farremoved from the transition state structures.

To explain the behavior of the other M 2+ ions we need tolook at the effects of d-electron configuration.



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:

I:

)1-sec810(k >=

IAIIA(Be2+, Mg2+)(IIB( )Zn2+)

Cr2+.Cu2

IA

():

Li+ < Na+ < K+ < Cs+



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II:)1-sec810-410k ((104-108 sec-1)

(V2+ , Cr2+ Cu2+)Mg2+M3+.

III:)1-sec410-1(k(1-104 sec-1)

Be2+ , Al3+ , V2+.

VI:)1-sec6-10--310(k(10-3-10-6 sec-1)Cr3+ ,

Co3+ , Rh3+ ,Ir3+ , Pt2+.



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(Jahn-Teller)Cu2+(d9)Cr2+ (d4)

(Axial)(equatorial)

eg



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Two parallel pathways

L M XL

L

L

M

L

Y

X

L

L M Y

L

L

+ Y-X

k2

L

M

L

L

k1

slow

+ Y

very fast

Associative

Dissociative

Rate = (k1 + k2[Y])[ML3X]


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Answer


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Nature256, 636 - 638 (21 August 1975); doi:10.1038/256636a0

Mechanisms of reactions at square planar metal centres

F. R. HARTLEY* & J. J. PRI*Department of Chemistry, The University, Southampton SO9 5NH, UK

U.E.R. de Chimie Organique, Universit Paul Sabatier, 118 route de Narbonne, 31Toulouse, France

SQUARE-PLANAR platinum(II) complexes have in the past been found to react by associativemechanisms (see refs 1 and 2). This has been of some comfort to the proponents3 of the'16 and 18 electron rules', which state that all stable transition metal complexes and

intermediates in the reactions of such complexes should have either 16 or 18 electrons inthe transition metal's valence shell. We point out here that though substitution reactions atplatinum(II) occur by an associative route, reactions between ligands coordinated toplatinum(II)here called combination reactionsoccur by a dissociative route that violatesthe '16 and 18 electron rules'.


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.:


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L M X

L

L

L

M

L

Y

X

L L M Y

L

L

+ Y

-Xk2

L

M

L

L

k1

- X

X

S

+ S

L M S

L

L

L

M

L

L

Y

S

-S

+ Y

very fast

Parallel pathways

Rate = (k1(solvent)+ k2[Y])[ML3X]


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Sq-Pl

Factors Which Affect The Rate Of Substitution1. Role of the Entering Group2. The Role of The Leaving Group3. The Nature of the Other Ligands in the Complex4. Effect of the Metal Centre

Pl-Sq

1-2-3-4-


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(entering group)

(thermodynamic stability)

Pt(II)

Y = H2O, NH3, Cl, Py, CN, PPh3, .


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nPt scale is the relative index of nucleophilicity obtained

from the reaction of the standard complex trans-

[Pt(py)2Cl2] in methanol,


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The graph on the left is drawn for the

reactions of

trans-[Pt(pyridine)2

Cl2

] with a number of

different nucleophiles in methanol


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(Leaving group)

[Pt(dien)X]+ + py [Pt(dien)(py)]2+ + X -

25

:

X = H2O > Cl-> Br-> I- > N3

- > SCN- > NO2- > CN

-

MX


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1-

2--


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()


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(ground state destabilization)


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()


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cis-, trans-[PtCl2(py)I]-

[PtCl4]2-, py , I-:I- > Cl- > py


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(Trans influence)

The trans influence is purely a thermodynamic phenomenon. That is,

ligands can influence the ground state properties of groups to which they

are trans. Such properties include;

(i) Metal-Ligand bond lengths

(ii) Vibration frequency or force constants

(iii) NMR coupling constants

Trans influence is a change in the bond distance between the

metal and a ligand that is trans to the ligand exerting the

effect through the same d-orbital interaction.


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The trans influence series based on structural data, has been given as;

R- ~ H- >= PR3

> CO ~ C=C ~ Cl- ~ NH3

(Trans influence)


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(Trans influence)

()

()

Pt-Cl:Et3P < H2C=CH2 < Cl


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(Trans influence)

inorganic reaction mechanisms-part ii

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