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 18-electron rule (diamagnetic complexes) Most stable complexes contain 18 or 16 electrons in their valence shells Most comon reactions take place through 16 or 18 electron intermediates

A simple classification of the most important ligands X LX L L2L2 L2XL2X L3L3

Counting electrons Method A Determine formal oxidation state of metal Deduce number of d electrons Add d electrons + ligand electrons (A) Ignore formal oxidation state of metal Count number of d electrons for M(0) Add d electrons + ligand electrons (B) Method B The end result will be the same

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

Organometallic Chemistry Fundamental Reactions

Reaction  (FOS)  (CN)  (NVE) Association-Dissociation of Lewis acids 0±10 Association-Dissociation of Lewis bases 0±1 Oxidative addition-Reductive elimination ±2 Insertion-deinsertion 000 Fundamental reaction of organo-transition metal complexes

 (FOS) = 0;  (CN) = ± 1;  (NVE) = 0 Lewis acids are electron acceptors, e.g. BF 3, AlX 3, ZnX 2 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 acids

 (FOS) = 0;  (CN) = ± 1;  (NVE) = ±2 Association-Dissociation of Lewis bases A Lewis base is a neutral, 2e ligand “L” (CO, PR 3, H 2 O, NH 3, C 2 H 4,…) in this case the metal is the Lewis acid For 18-e complexes, dissociative mechanisms only For <18-e complexes dissociative and associative mechanisms are possible

 (FOS) = ±2;  (CN) = ± 2;  (NVE) = ±2 Oxidative addition-reductive elimination Very important in activation of hydrogen Vaska’s compound

 (FOS) = 0;  (CN) = 0;  (NVE) = 0 Insertion-deinsertion Very important in catalytic C-C bond forming reactions (polymerization, hydroformylation) Also known as migratory insertion for mechanistic reasons

Metal Carbonyl Complexes CO is an inert molecule that becomes activated by complexation to metals CO as a ligand strong  donor, strong π-acceptor strong trans effect small steric effect

Frontier orbitals Larger homo lobe on C “C-like MO’s”

6CO ligands x 2  e each 12  bonding e “ligand character” “18 electrons” non bonding anti bonding “metal character” Mo(CO) 6

6  ligands x 2e each 12  bonding e “ligand character” non bonding anti bonding “metal character” Mo(CO) 6  -only bonding The bonding orbitals will not be further modified

t 2g egeg Mo(CO) 6  -only Mo(CO) 6  + π Energy gain (empty π-orbitals) oo ’o’o  ’o >  o π-bonding may be introduced as a perturbation of the t 2g /e g set: Case 1: CO empty π-orbitals on the ligands M  L π-bonding (π-back bonding) t 2g (π) t 2g (π*) egeg

Metal carbonyls may be mononuclear or polynuclear

Synthesis of metal carbonyls

Characterization of metal carbonyls IR spectroscopy (C-O bond stretching modes)

Effect of charge Effect of other ligands

The number of active bands as determined by group theory

13 C NMR spectroscopy 13 C is a S = 1/2 nucleus of natural abundance 1.108% For metal carbonyl complexes  ppm (diagnostic signals) Very long T 1 (use relaxation agents like Cr(acac) 3 and/or enriched samples)

Typical reactions of metal carbonyls Ligand substitution: Always dissociative for 18-e complexes, may be associative for <18-e complexes Migratory insertion:

Metal complexes of phosphines PR 3 as a ligand Generally strong  donors, may be π-acceptor strong trans effect Electronic and steric properties may be controlled Huge number of phosphines available

Tolman’s electronic and steric parameters of phosphines

Typical reactions of metal-phosphine complexes Ligand substitution: Very important in catalysis Mechanism depends on electron count

Metal hydride and metal-dihydrogen complexes Terminal hydride (X ligand) Bridging hydride (  -H ligand, 2e-3c) Coordinated dihydrogen (  2 -H 2 ligand) Hydride ligand is a strong  donor and the smallest ligand available

Synthesis of metal hydride complexes

Characterization of metal hydride complexes 1 H NMR spectroscopy High field chemical shifts (  0 to -25 ppm usual, up to -70 ppm possible) Coupling to metal nuclei ( 101 Rh, 183 W, 195 Pt) J(M-H) = Hz Coupling between inequivalent hydrides J(H-H) = 1-10 Hz Coupling to 31 P of phosphines J(H-P) = Hz cis; Hz trans IR spectroscopy (M-H) = cm -1 (terminal); cm -1 bridging (M-H)/ (M-D) = √2 Weak bands, not very reliable

Some typical reactions of metal hydride complexes Transfer of H - Transfer of H + A strong acid !! Insertion A key step in catalytic hydrogenation and related reactions

Bridging metal hydrides 2-e ligand 4-e ligand bonding Non-bonding Anti-bonding

Metal dihydrogen complexes If back-donation is strong, then the H-H bond is broken (oxidative addition) Very polarized  +,  - Characterized by NMR (T 1 measurements)

NMR characterization of organometallic complexes If X = CO 1 (CO) band 2 (CO) bands 1 H NMR

Metal-olefin complexes 2 extreme structures metallacyclopropane π-bonded only sp 3 sp 2 Zeise’s salt

Effects of coordination on the C=C bond CompoundC-C (Å)M-C (Å) C2H4C2H (2) C 2 (CN) (2) C2F4C2F4 1.31(2) K[PtCl 3 (C 2 H 4 )]1.354(2)2.139(10) Pt(PPh 3 ) 2 (C 2 H 4 )1.43(1)2.11(1) Pt(PPh 3 ) 2 (C 2 (CN) 4 )1.49(5)2.11(3) Pt(PPh 3 ) 2 (C 2 Cl 4 )1.62(3)2.04(3) Fe(CO) 4 (C 2 H 4 )1.46(6) CpRh(PMe 3 )(C 2 H 4 )1.408(16)2.093(10) C=C bond is weakened (activated) by coordination

Characterization of metal-olefin complexes NMR 1 H and 13 C,  < free ligand X-rays C=C and M-C bond lengths indicate strength of bond IR (C=C) ~ 1500 cm -1 (w)

[PtCl 4 ] C 2 H 4  [PtCl 4 (C 2 H 4 )] - + Cl - Synthesis of metal-olefin complexes RhCl 3.3H 2 O + C 2 H 4 + EtOH  [(C 2 H 4 ) 2 Rh(  -Cl) 2 ] 2

Reactions of metal-olefin complexes

Metal cyclopentadienyl complexes Metallocenes (“sandwich compounds”) Bent metallocenes “2- or 3-legged piano stools”

Cp is a very useful stabilizing ligand Introducing substituents allows modulation of electronic and steric effects

Metal alkyl, carbene and carbyne complexes

Main group metal-alkyls known since old times (Et 2 Zn, Frankland 1857; R-Mg-X, Grignard, 1903)) Transition-metal alkyls mainly from the 1960’s onward W(CH 3 ) 6 Ti(CH 3 ) 6 PtH(C  CH)L 2 Cp(CO) 2 Fe(CH 2 CH 3 ) 6 [Cr(H 2 O) 5 (CH 2 CH 3 ) 6 ] 2+ Why were they so elusive? Kinetically unstable (although thermodynamically stable)

Reactions of transition-metal alkyls