Química Organometálica e Catálise · Organometallic fundamentals
Contagem de elétrons, estados de oxidação e ligantes
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O que você vai aprender
- Classify ligands as L, X or Z and give their hapticity and electron contribution.
- Assign the oxidation state and dⁿ count of a metal in a complex.
- Count valence electrons by the neutral or the ionic convention and apply the 18-electron rule and its 16-electron exceptions.
- Relate back-bonding to ν(CO) and use Tolman electronic parameters and cone angles to compare phosphines.
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Ligand classes: L, X and Z
An L ligand is a neutral two-electron donor (CO, PR₃, NH₃, alkenes). An X ligand forms a covalent bond and counts as one electron in the neutral convention or as an anion giving two in the ionic convention (H, Cl, CH₃). A Z ligand is a Lewis acid such as BCl₃ or AlCl₃ that takes two electrons from the metal and adds nothing to the count. Multi-site ligands combine these: η³-allyl is LX, η⁵-Cp is L₂X, η⁶-benzene is L₃, 2,2′-bipyridine is L₂. A linear nitrosyl gives 3 electrons in the neutral count; a bent nitrosyl gives 1.
Oxidation state and d count
Remove every X ligand as an anion and every L ligand as neutral; the charge left on the metal is its oxidation state. The d count is the group number minus the oxidation state. In ferrocene each Cp is Cp⁻, so iron is Fe(II), d⁶. In RhCl(PPh₃)₃ rhodium is Rh(I), d⁸. Hydrides are always H⁻ in this formalism, even in acidic hydrides. A Schrock alkylidene is removed as CR₂²⁻ (X₂). Redrawing a strongly back-bonded alkene as a metallacyclopropane (X₂ instead of L) raises the oxidation state by two but leaves the electron count unchanged.
Two ways to count, one answer
Neutral counting: metal group number plus 2 per L and 1 per X, minus the overall charge. Ionic counting: dⁿ plus 2 per L and 2 per X⁻. Ni(CO)₄ = 10 + 8 = 18. Ferrocene = 8 + 2 × 5 = 18 (neutral) or 6 + 2 × 6 = 18 (ionic). Never mix the conventions. A metal–metal bond adds one electron to each metal, which is why Mn(CO)₅ (17 e) dimerises to Mn₂(CO)₁₀.
The 18-electron rule and its exceptions
Many low-valent complexes of the middle and late metals reach 18 electrons. Square-planar d⁸ complexes (Pt(II), Pd(II), Rh(I), Ir(I)) stop at 16 because the strongly antibonding d(x²−y²) orbital stays empty. Early metals often stop short for steric reasons (Cp₂ZrCl₂, 16 e), and 19-electron cobaltocene is a ready one-electron reductant. Hapticity changes such as η⁵ → η³ ring slip let saturated complexes open a site; indenyl slips far more easily than Cp because its benzo ring regains full aromaticity.
Back-bonding and ν(CO)
CO donates a carbon lone pair and accepts metal d electrons into π*. Back-donation weakens the C–O bond, so ν(CO) falls from 2143 cm⁻¹ in free CO to ≈ 2000 cm⁻¹ in Cr(CO)₆, and further for anions ([V(CO)₆]⁻ ≈ 1860 cm⁻¹). Bridging CO absorbs lower still, ≈ 1860–1700 cm⁻¹.
Tolman parameters
The Tolman electronic parameter is the A₁ ν(CO) of Ni(CO)₃L: the lower it is, the more donating L is (P(tBu)₃ 2056 cm⁻¹, PPh₃ 2069 cm⁻¹, PF₃ 2111 cm⁻¹). Phosphines accept back-donation through P–X σ* orbitals. The cone angle measures bulk: PMe₃ 118°, PPh₃ 145°, P(tBu)₃ 182°.
What to carry forward
Pick one counting convention and apply it consistently: X ligands set the oxidation state, group number minus oxidation state gives dⁿ, and 18 electrons (16 for square-planar d⁸) marks a saturated complex. Read ν(CO) as a gauge of how electron-rich the metal is.
Problemas de prática (40)
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The 18-electron rule
- Predict the binary cobalt carbonylPredict the formula of a neutral binary carbonyl from the 18-electron rule.
Back-bonding and phosphine parameters
- Which ligand lowers ν(CO) most?Predict how a phosphorus ligand’s donor–acceptor balance shifts the CO stretch of a carbonyl complex.
Classificar 11
The 18-electron rule
- Count (η⁶-benzene)Cr(CO)₃Count an η⁶-arene chromium tricarbonyl complex.
- Count [Co(CO)₄]⁻ (ionic)Count a metal carbonyl anion using the ionic convention.
- Is Fe(CO)₅ an 18-electron complex?Apply the 18-electron rule to iron pentacarbonyl.
Ligand types and hapticity
- Spot the L₂ chelateIdentify a neutral bidentate L₂ ligand among common ligands.
- Classify η⁵-cyclopentadienylAssign the covalent ligand class of an η⁵-cyclopentadienyl ligand.
- Spot the L-type ligandRecognise an L-type ligand as a neutral two-electron donor.
- Spot the Z-type ligandRecognise a Z-type ligand as a Lewis acid that accepts electron density from the metal.
Oxidation state and d-electron count
- d count in ferroceneDerive the d-electron count of iron in ferrocene and link it to the 18-electron total.
- Oxidation state in ferroceneAssign the oxidation state of iron in ferrocene by treating each Cp as Cp⁻.
- d count in zirconocene dichlorideRecognise zirconocene dichloride as a d⁰ Zr(IV) complex.
Back-bonding and phosphine parameters
- Spot the strongest π-acceptorIdentify carbon monoxide as a strong π-acceptor among common ligands.
Comparar 3
Back-bonding and phosphine parameters
- Rank phosphines by cone angleOrder phosphine ligands by Tolman cone angle.
- Rank ν(CO) in an isoelectronic seriesOrder isoelectronic hexacarbonyls by ν(CO) using charge and back-donation.
Oxidation state and d-electron count
- Rank metal oxidation statesOrder five complexes by the oxidation state of the metal.
Calcular 15
Ligand types and hapticity
- η³ → η¹ allyl slipCalculate how a change in allyl hapticity changes the metal electron count.
- Electrons from η⁵-Cp (neutral count)Count the electrons donated by an η⁵-cyclopentadienyl ligand in the neutral convention.
- Count Co(CO)₃(NO)Count a nitrosyl complex treating linear NO as a three-electron donor.
Oxidation state and d-electron count
- Oxidation state of Pt in cisplatinAssign the oxidation state of platinum in cisplatin from its X and L ligands.
- d count of Pt(II)Compute the d-electron count of a Pt(II) centre from group number and oxidation state.
- Oxidation state in a Schrock alkylideneAssign the oxidation state of a Schrock alkylidene complex by counting the alkylidene as X₂.
- d count after oxidative additionCompute the d-electron count of iridium after H₂ oxidatively adds to Vaska’s complex.
- d count in Wilkinson’s catalystCompute the d-electron count of rhodium in Wilkinson’s catalyst.
The 18-electron rule
- Count cobaltoceneCount cobaltocene and connect a 19-electron count to its reducing power.
- Count ferroceneCount the valence electrons of ferrocene in the neutral convention.
- Count Mn₂(CO)₁₀ per metalCount a metal–metal-bonded dimer per metal centre, including the M–M bond.
- Count Ni(CO)₄Count the valence electrons of nickel tetracarbonyl.
- Count after oxidative additionCount the six-coordinate product of H₂ oxidative addition to Vaska’s complex.
- Count Wilkinson’s catalystCount Wilkinson’s catalyst and recognise a 16-electron square-planar complex.
- Count zirconocene dichlorideCount an early-metal metallocene that falls short of 18 electrons.
Encontre o erro 1
- Find the error: H₂Fe(CO)₄Diagnose an incorrect oxidation-state assignment for a metal hydride.
Interprete as evidências 2
Back-bonding and phosphine parameters
- Interpret a low ν(CO) bandAssign a bridging carbonyl from its IR stretching frequency.
- Read Tolman electronic parametersUse Tolman electronic parameters to identify the most electron-donating phosphine.
Trabalhe de trás para frente 1
- Which metal fits M(CO)₅?Work backwards from an 18-electron formula to the metal group.
Explique por quê 5
Ligand types and hapticity
- Alkene: π complex or metallacyclopropane?Explain how the L and X₂ descriptions of a bound alkene affect oxidation state but not the electron count.
- The indenyl effectExplain why η⁵-indenyl complexes undergo associative substitution much faster than η⁵-Cp analogues.
Back-bonding and phosphine parameters
- Why ν(CO) drops on bindingExplain the lower C–O stretching frequency of metal carbonyls compared with free CO.
- Why PF₃ is a π-acceptorIdentify the orbital through which phosphines accept π back-donation.
The 18-electron rule
- Why d⁸ square-planar stops at 16Explain why square-planar d⁸ complexes are stable with 16 electrons.
Travou em contagem de elétrons, estados de oxidação e ligantes? Escaneie o problema.
Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.
Depois, Scan Standard: /mês