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Valence Bond Description of the CO Ligand

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Valence Bond Description of the CO Ligand Valence Bond description of the CO ligand • In the CO molecule both the C and the O atoms are sp hybridized . • The singly occupied sp and pz orbitals on each atom form a σ and a π bond, respectively. • This leaves the C py orbital empty, and the O py orbital doubly occupied, and so the second π bond is formed only after we have formed a dative bond by transfer of the lone pair of O py electrons into the empty C py orbital. δ− δ+ • This transfer leads to a C −−−O p p polarization of the molecule, which is almost exactly canceled out by a partial Cδ+−−−Oδ− polarization of all three sp C sp sp O sp bonding orbitals because of the higher p dative p electronegativity of oxygen. • The free CO molecule therefore has a net dipole moment very close to zero. 1 MO correlation diagram for CO The CO LUMO orbitals are anti bonding (π*). These are empty orbitals and accept electron density from the metal centre via π-backbonding with the C metal dxy, dxz and dyz orbitals. C O O LUMO HOMO 2p 2p The CO HOMO orbital is a bonding orbital of σ symmetry with significant electron density on the 2s carbon. This orbital forms a σ bond with metal p , dz2 and dx2–y2 orbitals. This is a filled orbital and 2s donates electron density to the metal centre. bond order = (8 - 2)/2 = 3 2 3 x T2g LUMO ∆∆∆O HOMO dz2 dx2-y2 3 4 • The net result is that C becomes more positive on coordination, and O becomes more negative. This translates into a polarization of the CO on binding. • This metal-induced polarization chemically activates the CO ligand making C more sensitive to nucleophilic attack and O more sensitive to electrophilic attack. • The polarization will be modulated by the effect of the other ligands on the metal and by the net charge on the complex. + In LnM(CO), the CO carbon becomes particularly δ in character if the L groups are good π + acids or if the complex is cationic, e.g. Mo(CO) 6 or [Mn(CO) 6] , because the CO-to-metal σ- donor electron transfer will be enhanced at the expense of the metal to CO back donation. 2− If the L groups are good donors or the complex is anionic, e.g. Cp 2W(CO) or [W(CO) 5] , back donation will be encouraged, the CO carbon will lose its pronounced δ+ charge, but the CO oxygen will become significantly δ−. • The range can be represented in valence bond terms the extreme in which CO acts as a pure σ ∗ ∗ donor, through to the extreme in which both the π x and π y are both fully engaged in back bonding. 5 π-acceptor effects on reactivity Semmelhack JACS 1980 (102) 5926 CO's render the electron rich Cr metal electrophilic via strong π-backbonding. Complexation of benzene with the electrophilic Cr(CO)3 fragment withdraws electon density from the aromatic ring activating it towards nucleophilic attack. Yamamoto JACS 1971 (93)3350. Acrolein is thought to act as a π-acid, withdrawing electron density from the Ni(II) complex via π-backbonding and promoting elimination of the diethyl fragment to reduce the metal. 6 Dewar-Chatt-Duncanson model Explain the trend in CO vibrations for the following series of compounds v(CO) cm -1 2- [Ti(CO) 6] 1748 - [V(CO) 6] 1859 Cr(CO) 6 2000 + [Mn(CO) 6 ] 2100 2+ [Fe(CO) 6 ] 2204 7 M-CO πππ-Backbonding and the trans-effect The increase in electron density at the nickel from PR 3 σ- donation is dispersed through the M-L π system via π- backbonding . Much of the electron density is passed onto the CO π* and is reflected in decreased v(CO) stretching frequencies which corresponds to weaker CO bonds. CO stretching frequencies Recall: Band position in IR is governed by : measured for Ni(CO) 3L where L are 1. force constant of the bond ( f) and PR ligands of different σ-donor 3 2. individual masses of the atoms (M and M ). abilities. [v(CO) =2143 cm -1] x y Stronger bonds have larger force constants than weaker bonds. Tolman Chem. Rev. 1977 (77) 313 8 MO description for L→M πππ-donor system in an Oh complex • In the L→M π-donor system filled p ligand orbitals exist slightly lower in energy than the metal t2g set with which overlap occurs. • The bonding t2g MO’s formed are again lower in energy than the initial metal t2g set, however, the corresponding t2g* anti-bonding MO’s are lower in energy than the eg* σ anti-bonding orbitals. • Additionally, as the ligand t2g p orbitals are typically of lower energy than the metal t2g set, the bonding t2g MO’s formed are predominantly ligand based, with the corresponding t2g* anti -bonding MO’s being predominantly metal based . • Thus the LFSE ∆o is decreased relative to the σ only system. The t2g bonding MO’s are stabilized which is countered by occupation of the t2g* anti-bonding orbitals. • Overall this combined σ and π donation from ligand to metal results in an increased M-L bond order and a stronger bond, however, the metal now becomes more electron rich which can decrease the bond strengths of the remaining ligand set making ligand-metal π bonding a less favorable interaction. 9 3- Cl CrIII 6Cl- Cl Cl Cr Cl Cl Cl * t1u 3 x T2g 4p (t1u) * a1g 4s (a1g) * eg * t2g 3d (t2g , eg) dz2 dx2-y2 t2g t2g eg t1u eg a1g t1u 10 a1g MO description for L→M πππ-donor system in an O h complex The energy of the HOMO is directly affected by M-L π-bonding. Ligand to metal π- donation increases the energy of the HOMO making the metal more basic . π-donor ligands stabilize electron poor, high oxidation state metals. Very prevalent for early TM complexes (low d electron count) and less so for late TM (high d electron count). 11 • Alkoxy, amido and halido ligands are sp 2 hybridized so only one set of p electrons are available for π bonding 12 Summary of πππ-bonding in a O h complexes πππ donor ligands result in L→M πππ bonding, a smaller ∆∆∆o favoring high spin configurations and a decreased stability. πππ acceptor ligands result in M→L πππ bonding, a larger ∆∆∆o favoring low spin configurations with an increased stability. 13 Types of bonding σ π δ • Bond polarity is evaluated by the difference in electronegativities of neighboring atoms 14 M-L σ bond polarization Ionic bonding • Greater when elements of high and opposite charge interact. • Differences in charge are paralleled in differences in electronegativities. • Large differences in electronegativity favor strong ionic bonding. • M-L σ-bonding in early metals has significant ionic character. Covalent bonding • Greater when orbitals of similar energies interact. • The energy of atomic orbitals is inversely proportional to the element's electronegativity (i.e. the orbital energy of an electronegative element is lower than that of a electropositive element). • Small differences in electronegativity favor strong covalent bonding. • M-L σ-bonding in late metals has a high degree of covalent bonding. 15 Ionic vs. covalent model • Both the covalent model and the ionic model differ only in the way the electrons are considered as coming from the metal or from the ligands - emphasize model …not a true representation of metal charge!!! • Each model is often invoked without any warning in the literature therefore it is important to be able to identify their use. • The ionic model is most commonly used for traditional M−−−L inorganic coordination compounds therefore coordinating ligands are treated equally in both models . • The ionic model is more appropriate for high-valent metals with N, O or Cl ligands. • In the ionic model the M−X bond is considered as arising from a cationic M+ and an anionic X− (heterolytic) • The covalent model is sometimes preferred for organometallic species with low- valent metals where the metal and ligand oxidation states cannot be unambiguously defined. • In the covalent model the M−X bond is considered as arising from a neutral metal and ligand radical X• (homolytic). 16 Ionic model covalent model ηηη1 (hapticity = 1) # of e- # of e- charge donated charge donated -1 2 0 1 Hydride -1 2 0 1 Alkyl (e.g. methyl) -1 2 0 1 Alkenyl -1 2 0 1 Alkynyl -1 2 0 1 Allyl -1 2 0 1 Aryl -1 2 0 1 Cyclopentadienyl -2 4 0 2 Carbene -3 6 0 3 Carbyne Acyl -1 2 0 1 0 2 0 1 Carbon monoxide -1 2 0 1 Nitrile -1 2 0 1 Fulminate 17 Ionic model covalent model ηηη1 (hapticity = 1) # of e- # of e- charge donated charge donated 0 2 +1 1 Aqua -1 2 0 1 Hydroxyl -2 4 0 2 Oxo -1 2 0 1 Alkoxide 0 2 +1 1 Ether (Sulfide/Tioether O = S) -1 2 0 1 Carboxylate -1 2 0 1 Carbonate -1 2 0 1 Cyanate (Thiocyanate O = S) Isocyanate (Isothiocyanate O = S) -1 2 0 1 Sulfoxide 0 2 +1 1 0 2 +1 1 18 Ionic model covalent model ηηη1 (hapticity = 1) # of e- # of e- charge donated charge donated Isocyanate -1 2 0 1 Nitrogen 0 2 +1 1 Amine 0 2 +1 1 0 2 +1 1 Pyridyl Imine 0 2 +1 1 Amide -1 2 0 1 Nitrile 0 2 +1 1 0 2 +1 1 Isonitrile -1 2 0 1 Nitrosyls 0 2 +1 1 -1 2 0 1 Nitro Nitrito -1 2 0 1 0 2 +1 1 Phosphine -1 2 0 1 Phosphide -1 2 0 1 Halide (e.g.
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