Principal mechanisms of ligand exchange in octahedral complexes
Principal mechanisms of ligand exchange in octahedral complexes Dissociative ML5 X k1 slow ML5 + X +Y k2 fast r = k 1 [ML5X] ML5 Y Associative ML5 X + Y k1 slow ML5 XY -X k2 fast r = k 1 [ML5X][Y] ML5 Y Dissociative pathway
(5-coordinated intermediate) MOST COMMON Associative pathway (7-coordinated intermediate) Experimental evidence for dissociative mechanisms Rate is independent of the nature of L Experimental evidence for dissociative mechanisms Rate is dependent on the nature of L Inert and labile complexes Some common thermodynamic and kinetic profiles Exothermic (favored, large K) Large Ea, slow reaction Exothermic (favored, large K) Large Ea, slow reaction Stable intermediate Endothermic (disfavored, small K) Small Ea, fast reaction Labile or inert? L
L L M L L Ea L L L M L L M L L L X L X G LFAE = LFSE(sq pyr) - LFSE(oct)
L Why are some configurations inert and some are labile? Inert ! Substitution reactions in square-planar complexes the trans effect L X M T L +X, -Y L Y M T (the ability of T to labilize X) L Synthetic applications of the trans effect Cl- > NH3, py
Mechanisms of ligand exchange reactions in square planar complexes L L M X L S +S L M -X Y L M X L +Y
L L L -d[ML 3X]/dt = (k s + ky [Y]) [ML 3X] X L L M L S +Y Y L -X L L M L Y
L -S M L S Electron transfer (redox) reactions -1e (oxidation) M1(x+)Ln + M2(y+)Ln M1(x +1)+Ln + M2(y-1)+Ln +1e (reduction) Very fast reactions (much faster than ligand exchange) May involve ligand exchange or not Very important in biological processes (metalloenzymes) Outer sphere mechanism [Fe(CN)6]3- + [IrCl6]3- [Fe(CN)6]4- + [IrCl6]2- [Co(NH3)5Cl]+ + [Ru(NH3)6]3+ [Co(NH3)5Cl]2+ + [Ru(NH3)6]2+ Reactions ca. 100 times faster than ligand exchange (coordination spheres remain the same)
A B "solvent cage" r = k [A][B] Ea Tunneling mechanism A + B A' G + B' Inner sphere mechanism [Co(NH3)5Cl)]2+ + [Cr(H2O)6]2+ [Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+ [CoIII(NH3)5(-Cl)CrII(H2O)6]4+
[CoII(NH3)5(-Cl)CrIII(H2O)6]4+ [CoII(NH3)5(H2O)]2+ [Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+ [CoIII(NH3)5(-Cl)CrII(H2O)6]4+ [CoII(NH3)5(-Cl)CrIII(H2O)6]4+ [CoII(NH3)5(H2O)]2+ + [CrIII(H2O)5Cl]2+ [Co(H2O)6]2+ + 5NH4+ Inner sphere mechanism Ox-X + Red k1 Ox-X-Red k2 Reactions much faster than outer sphere electron transfer (bridging ligand often exchanged) k3 k4 Ox(H2O)- + Red-X+
Ox-X-Red Tunneling through bridge mechanism r = k [Ox-X][Red] k = (k1k3/k2 + k3) Ea Ox-X + Red Ox(H2O)- + Red-X+ G Brooklyn College Chem 76/76.1/710G Advanced Inorganic Chemistry (Spring 2008) Unit 6 Organometallic Chemistry Part 1 General Principles Suggested reading: Miessler/Tarr Chapters 13 and 14 Elements of organometallic chemistry Complexes containing M-C bonds
Complexes with -acceptor ligands Chemistry of lower oxidation states very important Soft-soft interactions very common Diamagnetic complexes dominant Catalytic applications The d-block transition metals Group 4 3d row Ti 4d row Zr 5d row Hf 5 6 7 8 9 10 11 V Nb Ta
Cr Mo W Mn Tc Re Fe Ru Os Co Rh Ir Ni Pd Pt Cu Ag Au 8 7 6 5 4 3 2 1
9 8 7 6 5 4 3 2 10 9 8 7 6 5 6 3 10 9 8 7 6 5 4 dn 0 I II III IV
V VI VII 4 3 2 1 0 5 4 3 2 1 0 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
Main types of common ligands Ligand F. C. #e (A) #e (B) # CS X L XL XX LL XLL LLL -1 0 -1 -2 0 -1 0 2 2 4 4
4 6 6 1 2 3 2 4 5 6 1 1 2 2 2 3 3 A simple classification of the most important ligands X L L2 L2 X L3 Counting electrons Method A
Method B Determine formal oxidation state of metal Deduce number of d electrons Ignore formal oxidation state of metal Count number of d electrons for M(0) Add d electrons + ligand electrons (A) Add d electrons + ligand electrons (B) The end result will be the same Why is this relevant? Stable mononuclear diamagnetic complexes generally contain 18 or 16 electrons The reactions of such complexes generally proceed through 18- or 16-electron intermediates Although many exceptions can be found, these are very useful practical rules for predicting structural and reactivity properties C. A. Tollman, Chem. Soc. Rev. 1972, 1, 337. Why 18 electrons? antibonding Organometallic complexes 18-e most stable
16-e stable (preferred for Rh(I), Ir(I), Pt(II), Pd(II)) <16-e OK but usually very reactive > 18-e possible but rare generally unstable A closer look at some important ligands Typical -donor ligands Hydride: M-H (terminal) is a -1, 2e (1e) ligand Halide: M-Cl (terminal) is a -1, 2e (1e) ligand Alkyl: M-CH3 (terminal) is a -1, 2e (1e) ligand Alkoxide: M-OR (terminal) is a -1, 2e (1e) ligand Thiolate: M-SR (terminal) is a -1, 2e (1e) ligand Amide: M-NR (terminal) is a -1, 2e (1e) ligand Phosphide: M-PR2 (terminal) is a -1, 2e (1e) ligand H
M M Cl M M H3 C M M R O M M R S M M R N M M R2
P M M (-bridging) is a -1, 2e (1e) ligand (-bridging) is a -1, 4e (3e) ligand (-bridging) is a -1, 2e (1e) ligand (-bridging) is a -1, 4e (3e) ligand (-bridging) is a -1, 4e (3e) ligand (-bridging) is a -1, 4e (3e) ligand (-bridging) is a -1, 4e (3e) ligand Other important C-donor ligands M M M terminal, 1-aryl, alkenyl, alkynyl, -1, 2e (1e) M M
M' M' bridging, 2-alkenyl, alkynyl, -1, 4e (3e) M or 1-allyl -1, 2e (1e) M or 3-allyl -1, 4e (3e) Other important ligands M M M M 2- (2e) 4-diene, 4e 4- (4e) 6- (6e) arene M M
1-Cp -1, 2e (1e) 5- Cp -1, 6e (5e) O M M M M C 2-alkene or alkyne, 2e N O C M M N C 2- / side-bonded and 1- / end-bonded aldehyde/ketone, 2e imine, 2e
C Other important ligands M CR2 M Fischer carbene, 2e (2e) Schrock carbene, -2, 4e(2e) M O M C O M Fischer carbyne, 4e (3e) Schrock carbyne, -3, 6e(3e) M Oxo, -2, 4e (2e) NR M
imido, -2, 4e (2e) N N CR M N nitrido, -3, 6e (3e) N O M N O carbonyl, 2e M NR3 amine, 2e dinitrogen, 2e
M PR3 phosphine, 2e linear nitrosyl +1, 2e (3e) M AsR3 arsine, 2e bent nitrosyl -1, 2e (1e) M SbR3 stibine, 2e The M-L-X game Group 4 5 6
7 8 9 10 3d row Ti V Cr Mn Fe Co Ni 4d row 5d row Zr Hf Nb Ta
Mo W Tc Re Ru Os Rh Ir Pd Pt Neutral stable compounds 0 I II III IV V ML7 ML6 MXL6 MX2L6 MXL5 MX2L5
MX3L4 (16e) MX4L4 (16e) ML5 ML4 MXL3 (16e) MX2L4 MX3L4 MX4L3 (16e) MX2L2 (16e) MX3L3 MX4L3 MX5L2 (16e) Each X will increase the oxidation number of metal by +1. Each L and X will supply 2 electrons to the electron count. MX4L2 Group 4 5 6 7
8 9 10 3d row Ti V Cr Mn Fe Co Ni 4d row 5d row Zr Hf Nb Ta Mo
W Tc Re Ru Os Rh Ir Pd Pt Stable monocationic compounds 0 I II III IV V Now looking at compounds having a charge of +1 to obey 18 e rule. Elec count: 4 (M) +2 (NO) +12 (L6) = 18 Group 4 5 6 7 8
9 10 3d row Ti V Cr Mn Fe Co Ni 4d row Zr 5d row Hf Nb Ta Mo W Tc Re Ru Os
Rh Ir Pd Pt + ML4 Stable monocationic compounds + 0 + [M(NO)L6] [M(NO)L5] + I + [ML6] (16e) + II +
[ML6] [ML4] (16e) + [MXL7] + [MXL6] + III IV [M(NO)L4] + [MX2L5] (16e) + [MX3L5,6] [MXL5] + [MX2L5] +
MX2L2 (16e) [MX2L4] + [MX3L4] (16e) [MX3L4] + V [MX4L3] (16e) NO+ is isoelectronic to CO X increases O N by 1 Elec Count: 4 (M) + 4 (L2) + 10 (L5) MX4L2 Actors and spectators Actor ligands are those that dissociate or undergo a chemical transformation (where the chemistry takes place!) Spectator ligands remain unchanged during chemical transformations They provide solubility, stability, electronic and steric influence (where ligand design is !) Organometallic Chemistry
1.2 Fundamental Reactions Fundamental reaction of organo-transition metal complexes Reaction (FOS) (CN)) (N)VE) Association-Dissociation of Lewis acids 0 1 0 Association-Dissociation of Lewis bases 0 1 2 Oxidative addition-Reductive elimination 2 2 2 0
0 0 Insertion-deinsertion FOS: Formal Oxidation State; CN: Coordination Number NVE: Number of valence electrons Association-Dissociation of Lewis acids (FOS) = 0; (CN)) = 1; (N)VE) = 0 Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2 H H + BF3 W: H BF3 W H This shows that a metal complex may act as a Lewis base The resulting bonds are weak and these complexes are called adducts Association-Dissociation of Lewis bases (FOS) = 0; (CN)) = 1; (N)VE) = 2 A Lewis base is a neutral, 2e ligand L (CO, PR3, H2O, NH3, C2H4,)
in this case the metal is the Lewis acid HCo(CO)4 HCo(CO)3 + CO Crucial step in many ligand exchange reactions For 18-e complexes, only dissociation is possible For <18-e complexes both dissociation and association are possible but the more unsaturated a complex is, the less it will tend to dissociate a ligand Oxidative addition-reductive elimination (FOS) = 2; (CN)) = 2; (N)VE) = 2 Mn+ + X-Y M(n+2)+ X Y H Ph3P Cl Ir I CO PPh3 Vaskas compound
+ H2 Ph3P Cl IrIII CO Very important in activation of hydrogen H PPh3 Oxidative addition-reductive elimination H becomes H- Concerted reaction H Ph3P Cl CO IrI PPh3 Vaskas compound + H2 Cl
IrI CO PPh3 IrIII Cl H H M PPh3 via H CO Ir: Group 9 cis addition CH3+ has become CH3- SN2 displacement Ph3P Ph3P
+ CH3 + CH3I Ph3P Cl IrIII I- CO PPh3 CH3 Ph3P Cl IrIII CO PPh3 I trans addition Also radical mechanisms possible Oxidative addition-reductive elimination Mn+ + X-Y
M(n+2)+ X Y Not always reversible Mn+ + R-X Mn+ + R-H M(n+2)+ X R M(n+2)+ H R Insertion-deinsertion (FOS) = 0; (CN)) = 0; (N)VE) = 0 M-X + L (CO)5Mn-CH3 + CO M-L-X O (CO)5Mn-C-CH3 Mn: Group 7 Very important in catalytic C-C bond forming reactions
(polymerization, hydroformylation) Also known as migratory insertion for mechanistic reasons Migratory Insertion CH3 OC CO CO Mn OC OC + CO O C Mn CO OC CO CH3 CO
CO k1 k2 O OC C Mn OC + CO CH3 CO CO Also promoted by including bulky ligands in initial complex Insertion-deinsertion The special case of 1,2-addition/-H elimination R2C LnM CR'2 H LnM
R2 C H C R'2 A key step in catalytic isomerization & hydrogenation of alkenes or in decomposition of metal-alkyls Also an initiation step in polymerization Attack on coordinated ligands Nu- Favored for electron-poor complexes (cationic, e-withdrawing ligands) M L E+ Favored for electron-rich complexes (anionic, low O.S., good donor ligands) Very important in catalytic applications and organic synthesis Some examples of attack on coordinated ligands Electrophilic addition Nucleophilic addition Cl Pt Cl
py Pt Cl py Cl py O O + N Et Et3O+ + Fe(CO)3 Fe(CO)3 Electrophilic abstraction Nucleophilic abstraction Cp
Cp + Ta CH3 CH3 Cp Me3PCH2 + Me4P+ Ta Cp Cp CH2 CH3 Fe OC OC OH- OH Cp Fe OC OC
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