Presentation on theme: "Chapter 13 Lecture 1 Organometallic Ligands and Bonding"— Presentation transcript:
1 Chapter 13 Lecture 1 Organometallic Ligands and Bonding Organometallic BasicsAn Organometallic Complex contains at least one M—C bondIncludes ligands: CO, NO, N2, PR3, H2Doesn’t include CN- (classical coordination chemistry ligand)Both s and p bonding between M and C occurHistoryZeise’s Salt synthesized in 1827 = K[Pt(C2H4)Cl3] • H2OConfirmed to have H2C=CH2 as a ligand in 1868Structure not fully known until 1975Ni(CO)4 synthesized in 1890Grignard Reagents (XMgR) synthesized about 1900Accidentally produced while trying to make other compoundsUtility to Organic Synthesis recognized early on
2 4) Ferrocene synthesized in 1951 a) Modern Organometallic Chemistry begins with this discoveryb) Many new ligands and reactions produced ever since5) Organometallic Chemistry has really been around for millions of yearsa) Naturally occurring Cobalimins contain Co—C bondsb) Vitamin B12
3 Ligands and Nomenclature Common Organic LigandsBinding ModesBridging is possible with organometallic ligands
4 Different numbers of atoms of the organometallic ligand may be involved in bond and is called the “Hapticity” of the ligandThe 18-electron RuleCounting ElectronsThe octet rule governs organic and simple ionic compounds: s + 3p orbitalThe 18-electron rule governs organometallics (with many exceptions)s + 3p + 3d orbitalsDonor ligands provide the electrons other than the d-electrons
5 3) The “Donor Pair” method of electron counting (Method A in your book) a) Common organometallic ligands are assigned an electron count and chargeb) The charge on ligands helps determine d-electron count of metalc) Add up all electrons from Metal d orbitals and ligands to find total e- count(isonitrile or isocyanide)(oxo, sulfido)(nitrido)
6 Examples of Electron Counting Cr(CO)6Total charge on ligands = 0, so charge on Cr = 0, so Cr = d66 CO ligands x 2 electrons each = 12 electronsTotal of 18 electrons(h5-C5H5)Fe(CO)2ClTotal charge on ligands = 2-, so Fe2+ = d6(h5-C5H5- = 6) + (2CO x 2 = 4) + (Cl- = 2) = 12 electronsCharged complex: [Mn(CO)6]+Total ligand charge = 0, so Mn+ = d612 electrons from 6 CO ligands gives a total of 18 electronsM—M Bond: (CO)5Mn—Mn(CO)5Each bond between metals counts 1 electron per metal: Mn—Mn = 1 e-Total ligand charge = 0, so Mn0 = d75 CO ligands per metal = 10 electrons for a total of 18 electrons per Mn
7 Justification for and exceptions to the 18-electron Rule MO Theory predicts that 18 electrons fill bonding orbitalsThis number is more stable than more (filling antibonding orbitals) or lessDo
8 When is the 18-electron rule most valid? a) With octahedral complexes of large Do.Ligands are good s-donors and good p-acceptors (CO)Exceptions to the 18-electron rule are commonWeak field ligands with small Do make filling eg* possible ( > 18e-)p-donor ligands can make t2g antibonding ( < 18 e-)
9 5). Square Planar Complexes (d8) follow a 16-electron rule 5) Square Planar Complexes (d8) follow a 16-electron rule. 18 electrons would destabilized the complexes by filling the high energy dx2-y2 orbital.
10 Carbonyl Complexes (CO) BondingReview of CO Molecular OrbitalsHOMO resides mostly on C = s-donationLUMO resides mostly on C = p-acceptanceReinforce each other and provide strong bondingBonding of CO to a Metal
11 Characteristics of CO complexes Infrared SpectroscopyFree CO stretch n = 2143 cm-1Cr(CO)6 CO stretch n = 2000 cm-1 because p-back donation from metal weakens the CO bond by adding e- to antibonding p* orbitalNegative charge on complex further weakens CO bond:[V(CO)6]- n = 1858 cm [Mn(CO)6]+ n = 2095 cm-1d) Bridging CO further weakened by extra p-back donation (e- count = 1/M)X-Ray CrystallographyFree CO bond length = pmM—CO carbonyl bond length = 115 pm
12 Synthesis and Reactions of CO Complexes Carbonyl complexes of most metals exist.Most obey the 18-electron ruleBridging decreases down the periodic table as d-orbitals become larger.SynthesisDirect reaction: Ni CO Ni(CO)4 Toxic, used to purify NiReductive Carbonylation: CrCl3 + CO + Al Cr(CO)6 + AlCl3Thermal/Photochemical: 2 Fe(CO)5 + hn Fe2(CO)9 + COReactions: useful for the synthesis of other compounds by substitution of COCr(CO) PPh Cr(CO)5(PPh3) CORe(CO)5Br en Re(CO)3(en)Br CO
14 Ligands Similar to COCN- (cyanide) is isolectric to COStronger s-donor and slightly weaker p-acceptor than COMore stable with M+ due to –1 charge than M0 (which favors CO)Considered a classical ligand rather than organometallic for this reasonNN (dinitrogen) is isoelectric to COWeaker s-donor and weaker p-acceptor than CO, so doesn’t bind wellVery important in Nitrogen Fixation, so much research centers on complexesNO+ (nitrosyl) is isoelectric to COSimilar to CO in s-donor and p-acceptor propertiesElectron counting scheme considers linear NO complexes as 2 e- NO+Electron counting scheme considers bent NO complexes as 2 e- NO-
15 Hydride and Dihydrogen Complexes Hydride ComplexesM—H bonding is s-donation only from H- (2 electron, -1 charge)SynthesisReaction with H2: Co2(CO) H HCo(CO)4Reduction of carbonyl complex followed by addition of H+Co2(CO) Na Na[Co(CO)4]2 Na[Co(CO)4] + H HCo(CO)4Dihydrogen ComplexesFirst Complex characterized in Mo(CO)3(PR3)(H2)Bondings-donation from H2 s molecular orbitalp-acceptance from H2 s* molecular orbitalH—H bond is weaker and longer than free H2 (82-90 pm vs. 74 pm)Electron-rich metals can completely rupture the H2 bond by p-back donationOther good p-acceptor ligands on the metal helps stabilize the H2—M complex
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