M.C. White. Chem 153 Mechanism -43- Week of September 24th, 2002 Ligand Exchange mechanisms Associative ligand substition: is often called square planar substition becausel6 e-, d8 square planar complexes generally undergo ligand substitution via an associative mechanism( the M-Nu bond is formed before the M-X bond breaks ). The intermediate is 18e-and therefore provides a lower energy route to the product than a 14e -intermediate formed via dissociative substitution(the M-X bond is fully broken before the M-Nu bond begins to form). Analogous in many ways to SN2 reactions empty, non-bonding p, orbital can act as an acceptor orbital for the e- density of the incoming nucleophile L X 16e 1 8 e- intermediate 16 M= Ni(D), Pd(il) Pt(D), Rh(), Ir Dissociative ligand substitution is most favored in coordinatively saturated xes(e.g. d tetrahedral, octahedral ) In the dissociative mechanism, the M-X bond is fully Nu broken before the M-nu bond form thereby avoiding an energetically unfavorable 20 L L reactions Note that in all ligand M= Ru(i), Co(lr) processes, there Is no Rh(im), Ir(lm) state change at the met
M.C. White, Chem 153 Mechanism -43- Week of September 24th, 2002 Ligand Exchange Mechanisms Note that in all ligand substition processes, there is no oxidation state change at the metal center. Nu M = Ni(II), Pd(II), Pt(II), Rh(I), Ir(I). M L L X L Nu L M X Nu L M L L Nu L X L M L L X empty, non-bonding pz orbital can act as an acceptor orbital for the e- density of the incoming nucleophile 18 e- intermediates L 16 eL M L L Nu 16 eAssociative ligand substition: is often called square planar substition because16 e-, d8 square planar complexes generally undergo ligand substitution via an associative mechanism (the M-Nu bond is formed before the M-X bond breaks). The intermediate is 18e- and therefore provides a lower energy route to the product than a 14e- intermediate formed via dissociative substitution (the M-X bond is fully broken before the M-Nu bond begins to form). Analogous in many ways to SN2 reactions. Dissociative ligand substitution is most favored in coordinatively saturated 18e- complexes (e.g. d10 tetrahedral, d6 octahedral). In the dissociative mechanism, the M-X bond is fully broken before the M-Nu bond forms thereby avoiding an energetically unfavorable 20e- intermediate. Analogous in many ways to SN1 reactions. L M L L X L L M = Ru(II), Co(III), Rh(III), Ir(III) L M L L L L 18eNu: X- 16eL M L L Nu L L 18e-
M C. White. Chem 153 Structure bonding-17- Week of september 17, 2002 MO Description of o bonding in ML square planar Metal valenceorbitals Linear Combinations of The metry is favored by (), Pd (ID), Pt(D), Ir (D), rh(i) a stable electronic configuration is achieved at LUMO When combining orbitals, the resulting repelled by magnetic fields) and may be MOs must be symmetrically dispersed readily characterized by nmr between bonding and antibonding Thus, combining 3 orbitals(i.e. algs) requires one of the orbitals to be non- bonding HOMO eg In a square planar ligand field the degenerate d orbitals split into 9 symmetries.The degenerate p orbitals split into orbitals of eu and dxy
MO Description of σ bonding in ML4 square planar M.C. White, Chem 153 Structure & Bonding -17- Week of September 17, 2002 L M L L L L L L L y x dz2 dx2-y2 dxy dxz dyz pz px py Linear Combinations of Ligand σ Donor Orbitals Metal ValenceOrbitals LUMO 16 e - Rule: The square planar geometry is favored by d8 metals (e.g. Ni (II), Pd (II), Pt(II), Ir (I), Rh(I)). A stable electronic configuration is achieved at 16 e-, where all bonding and non- bonding orbitals are filled. Spin-paired compounds display diamagnetic behavoir (i.e. weakly repelled by magnetic fields) and may be readily characterized by NMR. s b2g eg b1g a1g a1g eu a2u a1g eu b1g a2u eu a1g a1g b1g eg b2g In a square planar ligand field the degenerate d orbitals split into orbitals of a1g, b1g, eg, and b2g symmetries. The degenerate p orbitals split into orbitals of eu and a2u symmetries. When combining orbitals, the resulting MO's must be symmetrically dispersed between bonding and antibonding. Thus, combining 3 orbitals (i.e. a1g's) requires one of the orbitals to be nonbonding. eg b2g a2u σ* HOMO n n σ
M.C. White. Chem 153 Mechanism -44- Week of september 24th, 2002 Associative substitution: the nucleophile Rate=-d[PtCl]=k/[PtCl,]+k,[Nul[PtChl dt Clv Meoh rt k: first order rate constant that arises from substition of leaving group by solvent k,: second-order rate constant for bi-molecular 16e attack of Nu on metal complex relative rate NI relative rate Meoh CHiCO Basicity of the incoming ligand F <158 (nucleophile) plays only role in its reactivity for soft 2754 metal centers. In general, the softest (i.e. most polarizable) nucleophiles react fastest with Br- soft metals like Pt(Il)via CHO 15.000 associative substitution. Steric 1175 29X10 ChUc (i.e. picoline vs pyridine)can retard the rate of substition 1349 (CH3O)3P Ph3P 85x10 1096 Et P 9.8x108 NH 1175
M.C. White, Chem 153 Mechanism -44- Week of September 24th, 2002 Nu relative rate 1 <100 <100 <158 <250 1175 MeOH CH3CO2- CO F- CH3O- (Et)3N N N H ClNH3 Nu relative rate N BrIC6H11CN (CH3O)3P PhSPh3P Et3P 158 1349 1096 1175 1549 N N H 2754 15,000 2.9 x 105 2.2 x 106 1.7 x 107 1.5 x 107 8.5 x 108 9.8 x 108 PtII Cl N Cl N Nu 16 ePtII Cl N Nu N 16 e- + Cl MeOH, rt - Associative Substitution:the nucleophile Basicity of the incoming ligand (nucleophile) plays only a minor role in its reactivity for soft metal centers. In general, the softest (i.e. most polarizable) nucleophiles react fastest with soft metals like Pt(II) via associative substitution. Steric hinderance at the nucleophile (i.e. picoline vs pyridine) can retard the rate of substition. Rate = -d [PtCl2] = k1[PtCl2] + k2[Nu][PtCl2] dt k1: first order rate constant that arises from substition of leaving group by solvent. k2: second-order rate constant for bi-molecular attack of Nu on metal complex
M.C. White. Chem 153 Mechanism -45- Week of september 24th, 2002 Associative substitution sterics Sterically shielding the positions above and below the plane of the square planar complex can lead to significant decreases in the rates of associative substition Et3 PIr, rt Eta k EtaP Py 100.000M-I sec EtaN Eta EtP EtaP EtaP IM-Isec Pearson Chem Soc. 1961 220
M.C. White, Chem 153 Mechanism -45- Week of September 24th, 2002 Associative Substitution: Sterics Pearson J. Chem. Soc. 1961 2207. k2 = 100,000 M-1 sec-1 + ClPtII Et3P Et3P Cl k2 = 200 M-1 sec-1 + ClPt Et3P II Et Cl 3P k2 = 1 M-1 sec-1 , rt + Cl- Pt Et3P II Et3P Py N N , rt Pt Et3 II P Et3P Py N , rt PtII Et3P Et3P Cl PtII Et3P Et3P Py Sterically shielding the positions above and below the plane of the square planar complex can lead to significant decreases in the rates of associative substition
M. C. White. Chem 153 Mechanism -46- Week of September 24, 2002 Associative substitution sterics as the steric bulk of the im ine backbone increases the associative second order rate constants fo aryl groups become more rigidly locked perpendicula ethylene exchange were examined by HNMR to the square plane making their ortho substituents more effective at blocking the axial sites above and in cdch at -85C (BAr4) (BAr4 k= too fast to measure k=8100 L/mol/sec k=45 L/mol/sec even at-1000C Brookhart JACS 1995(117)6414 (BAr4) CH.R Ruffo om1998(17)2646
M.C. White, Chem 153 Mechanism -46- Week of September 24, 2002 = Brookhart JACS 1995 (117) 6414. N N Pt CH3 R + (BAr'4)- N N Pd CH3 N N Pd CH3 N N Pd CH3 k = too fast to measure even at -100oC. + (BAr'4)- + (BAr'4)- k = 8100 L/mol/sec + (BAr'4)- k = 45 L/mol/sec as the steric bulk of the imine backbone increases, the aryl groups become more rigidly locked perpendicular to the square plane making their ortho substituents more effective at blocking the axial sites above and below the plane. associative second order rate constants for ethylene exchange were examined by 1HNMR in CDCl2 at -85oC Associative Substitution: Sterics Ruffo OM 1998 (17) 2646