Arrow Lab

Química Geral · Nuclear and transition-metal chemistry

Compostos de coordenação e teoria do campo cristalino

Uma lição resolvida gratuita e 40 problemas de prática em que você mesmo desenha cada seta curva.

Travou em compostos de coordenação e teoria do campo cristalino? 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.

Escaneie sua lição de graça

O que você vai aprender

  • Identify ligands, their denticity and donor atoms, and give the coordination number and common geometry of a complex.
  • Find the oxidation state and d-electron count of the metal from the formula of a coordination compound.
  • Name simple complexes and write formulas from names using the IUPAC rules.
  • Recognize linkage, ionization, hydrate, geometric (cis/trans, fac/mer) and optical isomers of simple complexes.
  • Use crystal field splitting, the spectrochemical series and pairing energy to predict high or low spin, unpaired electrons, magnetism and colour.

Lição gratuita

O texto da lição resolvida. No Arrow Lab, cada etapa vem com as estruturas e as setas desenhadas.

Ler a aula gratuita (8 partes)

Transition metals and complex ions

Most d-block metals (the transition metals) have partly filled d subshells in their common ions; zinc (Zn²⁺, d¹⁰) and scandium (Sc³⁺, d⁰) are the first-row exceptions. Transition metals typically show several oxidation states (iron +2 and +3, manganese +2 to +7), form coloured compounds and are often paramagnetic. Their cations are Lewis acids: they accept electron pairs from Lewis bases called ligands. The metal and its directly bonded ligands form the coordination sphere, written in square brackets; ions outside the brackets are counter-ions. In [Co(NH₃)₆]Cl₃ six ammonia molecules are bonded to Co³⁺ and the three Cl⁻ ions are free in solution, so excess AgNO₃ precipitates 3 mol of AgCl per mole. In [Co(NH₃)₅Cl]Cl₂ one chloride is a ligand, held by cobalt(III), and only 2 mol of AgCl precipitate. Werner used exactly these silver chloride counts to deduce the structures of cobalt ammines.

Ligands, denticity and geometry

A ligand needs at least one lone pair on a donor atom. Monodentate ligands bond through one atom: H₂O, NH₃, Cl⁻, OH⁻, CN⁻ (through C) and CO (through C). Bidentate ligands bond through two atoms: ethylenediamine (en, H₂NCH₂CH₂NH₂, drawn) through both nitrogens, and oxalate (C₂O₄²⁻) through two oxygens. EDTA⁴⁻ is hexadentate: two nitrogens and four carboxylate oxygens. A ligand that bonds through two or more atoms forms rings with the metal and is called a chelate; en and oxalate each close a five-membered ring (metal and four ligand atoms). Chelates bind more strongly than the same number of monodentate donors, mainly for an entropy reason: three en molecules release six NH₃ molecules, so the number of free particles rises. Ambidentate ligands can bond through either of two different atoms: thiocyanate (S or N) and nitrite (N, nitro; or O, nitrito). The coordination number is the number of donor atoms bonded to the metal, not the number of ligands: [Co(en)₃]³⁺ has three ligands but coordination number 6. Coordination number 2 is linear ([Ag(NH₃)₂]⁺), 6 is octahedral, and 4 is tetrahedral ([Zn(NH₃)₄]²⁺, [CoCl₄]²⁻) or square planar, the usual geometry for d⁸ ions such as Pt²⁺, Pd²⁺ and Au³⁺ ([PtCl₄]²⁻) and for Ni²⁺ with strong-field ligands ([Ni(CN)₄]²⁻).

Oxidation state and d-electron count

The charge of the complex ion equals the metal's oxidation state plus the charges of the ligands. Neutral ligands: H₂O, NH₃, CO, en, pyridine. Anionic ligands: F⁻, Cl⁻, Br⁻, I⁻, OH⁻, CN⁻, SCN⁻, NO₂⁻ (−1), oxalate (−2), EDTA (−4). In K₃[Fe(CN)₆] three K⁺ make the complex 3−; six CN⁻ contribute −6, so x − 6 = −3 and iron is +3. For a metal in groups 3–12, the d-electron count of the ion is the group number minus the oxidation state, because a transition metal loses its 4s electrons before any 3d electrons: Fe is group 8, so Fe³⁺ is [Ar]3d⁵, d⁵, and Fe²⁺ is [Ar]3d⁶, d⁶. Cobalt(III) is d⁶, chromium(III) d³, nickel(II) d⁸, copper(II) d⁹, zinc(II) d¹⁰.

Naming complexes

Name the cation before the anion, as for any salt; counter-ions get no number prefixes. Inside a complex, name the ligands first in alphabetical order, then the metal. Prefixes di-, tri-, tetra-, penta-, hexa- count simple ligands; bis-, tris-, tetrakis- with parentheses count ligands whose names already contain a number or are long, such as bis(ethylenediamine). Multiplying prefixes are ignored when alphabetizing, so dichlorido (c) comes before bis(ethylenediamine) (e). Neutral ligands: aqua (H₂O), ammine (NH₃, with two m's), carbonyl (CO). Anionic ligands end in -o: chlorido, cyanido, hydroxido, oxalato (older texts and many labs still write chloro, cyano, hydroxo). Give the metal's oxidation state in Roman numerals, (0) for zero. If the complex is an anion, the metal name ends in -ate: cobaltate, nickelate, platinate, zincate, and the Latin stems ferrate (Fe), cuprate (Cu), argentate (Ag) and aurate (Au). Examples: [Co(NH₃)₅Cl]Cl₂ is pentaamminechloridocobalt(III) chloride, K₃[Fe(CN)₆] is potassium hexacyanidoferrate(III), and [Ni(CO)₄] is tetracarbonylnickel(0).

Isomers of complexes

Structural isomers differ in which atoms are bonded. Linkage isomers use a different donor atom of an ambidentate ligand: [Co(NH₃)₅(NO₂)]²⁺ (N-bonded) and [Co(NH₃)₅(ONO)]²⁺ (O-bonded). Ionization isomers swap a ligand with a counter-ion: [Co(NH₃)₅Br]SO₄ releases free SO₄²⁻, while [Co(NH₃)₅(SO₄)]Br releases free Br⁻. Hydrate isomers swap water between ligand and lattice positions, as in the three CrCl₃·6H₂O isomers. Stereoisomers have the same bonds in different spatial arrangements. Square-planar [MA₂B₂] has cis (like ligands 90° apart) and trans (180°) isomers, as in cisplatin and transplatin; octahedral [MA₄B₂] also has cis and trans forms, and [MA₃B₃] has fac (three A on one face) and mer (three A in a plane through the metal) forms. Tetrahedral [MA₂B₂] has no geometric isomers, because every pair of tetrahedral positions is adjacent. A complex is optically active (chiral) when it is not superimposable on its mirror image: [Co(en)₃]³⁺ and cis-[Co(en)₂Cl₂]⁺ exist as pairs of enantiomers, while trans-[Co(en)₂Cl₂]⁺ has a mirror plane and is achiral, so [Co(en)₂Cl₂]⁺ has three stereoisomers in all.

Crystal field splitting and spin state

Crystal field theory treats the ligands as negative charges or dipoles that repel the metal's d electrons. In an octahedron the ligands lie on the x, y and z axes, so d(x²−y²) and d(z²), which point at them, rise in energy (the eg set), and dxy, dxz and dyz, which point between them, fall (the t2g set). The gap is Δo. Relative to the average, each t2g electron is stabilized by 0.4Δo and each eg electron destabilized by 0.6Δo; the sum is the crystal field stabilization energy (CFSE). In a tetrahedron the pattern is inverted (e below t2) and the gap is small, Δt ≈ 4/9 Δo. Δo depends on the ligand, following the spectrochemical series I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < bipy < NO₂⁻ < CN⁻ < CO, and on the metal: it grows with oxidation state (Co³⁺ > Co²⁺) and down a group (4d and 5d metals > 3d). For d⁴–d⁷ ions there are two choices. If Δo is smaller than the pairing energy P, electrons occupy eg before pairing (high spin, weak-field ligands); if Δo is larger than P, they pair in t2g first (low spin, strong-field ligands). Iron(II), d⁶, is high spin (t2g⁴eg², four unpaired electrons) in [Fe(H₂O)₆]²⁺ and low spin (t2g⁶, none unpaired) in [Fe(CN)₆]⁴⁻. Tetrahedral complexes are almost always high spin because Δt is smaller than P.

Colour and magnetism

A d–d transition promotes an electron from t2g to eg by absorbing a photon of energy close to Δo, and the solution shows the complementary colour of the light absorbed (red–green, orange–blue and yellow–violet are complementary pairs). [Ti(H₂O)₆]³⁺, d¹, absorbs at 493 nm, so Δo = NA·hc/λ = (6.022 × 10²³ mol⁻¹)(6.626 × 10⁻³⁴ J s)(2.998 × 10⁸ m s⁻¹)/(493 × 10⁻⁹ m) = 2.43 × 10⁵ J/mol = 243 kJ/mol, and the solution looks purple. Stronger-field ligands raise Δo and shift the absorption to shorter wavelength: green [Ni(H₂O)₆]²⁺ turns violet when ethylenediamine replaces the water. d⁰ and d¹⁰ ions (Ti⁴⁺, Zn²⁺) have no d–d transition and are colourless unless another kind of absorption occurs. Unpaired electrons make a complex paramagnetic (drawn into a magnetic field); a complex with all electrons paired is diamagnetic. The spin-only magnetic moment is μ = √(n(n + 2)) μB for n unpaired electrons: 1.73 μB for n = 1, 2.83 for 2, 3.87 for 3, 4.90 for 4 and 5.92 for 5. Measuring μ therefore counts the unpaired electrons and tells high spin from low spin.

What to carry forward

Read a complex from the outside in: counter-ions fix the charge of the complex ion, ligand charges then fix the metal's oxidation state, and group number minus oxidation state gives the d count. Coordination number counts donor atoms. Name ligands alphabetically before the metal and add -ate for anionic complexes. Look for cis/trans, fac/mer and mirror-image isomers. Then split the d orbitals (t2g below eg in an octahedron), place the electrons by comparing Δo with the pairing energy, count the unpaired electrons for the magnetism, and use Δo = NA·hc/λ and complementary colours for the colour.

Problemas de prática (40)

Praticar é grátis com uma conta.

Preveja o produto 1

  • What colour is the solution?Predict the observed colour of a complex from the wavelength it absorbs using complementary colours.

Classificar 12

Ligands and the coordination sphere

  • Spot the ambidentate ligandDistinguish an ambidentate ligand, which can bond through either of two different atoms, from chelating and monodentate ligands.
  • Which ligand is bidentate?Identify a bidentate ligand from its structure by finding two donor atoms that can reach the same metal ion.
  • Tetrahedral or square planar?Use the d-electron count to pick the tetrahedral complex among four-coordinate ions.

Naming coordination compounds

  • Name [Co(en)₂Cl₂]ClApply alphabetical ordering, bis/tris prefixes and the oxidation state when naming a complex with mixed ligands.
  • Name [Co(NH₃)₆]Cl₃Name a cationic ammine complex salt, with the correct ligand spelling, metal name and oxidation state.
  • Name [Ni(CO)₄]Name a neutral metal carbonyl, including a zero oxidation state.
  • Name K₂[PtCl₄]Name a salt with an anionic complex, using the -ate ending and the oxidation state from the charge balance.

Isomers of complexes

  • Which complex is chiral?Identify a chiral octahedral complex by checking for a mirror plane.
  • Which complex has cis and trans isomers?Recognize which formula and geometry allow cis and trans geometric isomers.
  • Nitro or nitrito?Classify two complexes that differ only in the donor atom of an ambidentate ligand.

Crystal fields, colour and magnetism

  • The higher-energy d orbitals in an octahedronIdentify the d orbitals raised in energy by an octahedral crystal field from where their lobes point.

Oxidation state and d-electron count

  • Find the metal in the +2 stateAssign oxidation states to the metals in several coordination compounds and pick the one that is +2.

Comparar 3

Oxidation state and d-electron count

  • Rank complexes by d-electron countOrder five complex ions by the d-electron count of the metal, working out each oxidation state first.

Crystal fields, colour and magnetism

  • Rank cobalt complexes by ΔoRank complexes by crystal field splitting using both the metal oxidation state and the spectrochemical series.
  • Spectrochemical seriesRank five common ligands by the crystal field splitting they produce, using the spectrochemical series.

Calcular 13

Isomers of complexes

  • Counting stereoisomers of [Co(en)₂Cl₂]⁺Count all stereoisomers, geometric and optical, of an octahedral complex with two chelating ligands.
  • Isomers of a square-planar MABCD complexCount the geometric isomers of a square-planar complex with four different monodentate ligands.

Crystal fields, colour and magnetism

  • CFSE of low-spin [Fe(CN)₆]³⁻Calculate the crystal field stabilization energy of a low-spin octahedral complex in units of Δo.
  • Unpaired electrons in [Fe(H₂O)₆]³⁺Place the d electrons of a high-spin octahedral ion in t2g and eg and count the unpaired electrons.
  • Spin-only moment of [CoF₆]³⁻Use magnetic behaviour to choose between high and low spin and then calculate the spin-only magnetic moment.
  • Δo of [Ti(H₂O)₆]³⁺ from its absorptionConvert the wavelength of a d–d absorption into the crystal field splitting energy per mole.

Ligands and the coordination sphere

  • Coordination number with a chelating ligandFind the coordination number of a complex that mixes bidentate and monodentate ligands by counting donor atoms.
  • Chelate rings formed by EDTACount the chelate rings formed when a hexadentate ligand wraps around one metal ion.
  • Silver chloride from a cobalt ammineCalculate the mass of AgCl precipitated from a complex by counting only the chloride ions outside the coordination sphere.

Oxidation state and d-electron count

  • d count in a chromium oxalate complexFind the d-electron count of a metal bound to a dianionic chelating ligand and a neutral ligand.
  • Oxidation state of iron in K₃[Fe(CN)₆]Find the oxidation state of a metal from the counter-ions and the charges of anionic ligands.
  • d electrons in hexaamminecobalt(III)Count the d electrons of a cobalt ion in a complex, removing the 4s electrons first.
  • The iron inside Prussian blueUse the charges of counter-ions in a mixed-valence compound to deduce the oxidation state and d count of the complexed metal.

Encontre o erro 2

Oxidation state and d-electron count

  • Where did the d electrons go?Find the error in a d-electron count that removes 3d electrons before 4s electrons.

Naming coordination compounds

  • What is wrong with this name?Find the error in the name of a salt that contains an anionic complex.

Interprete as evidências 1

  • Match the magnetic data to the iron(II) saltsInterpret measured magnetic moments to assign high- and low-spin iron(II) complexes to their ligands.

Trabalhe de trás para frente 3

Isomers of complexes

  • Which hydrate isomer of CrCl₃·6H₂O?Deduce which hydrate isomer a sample is from the moles of AgCl it precipitates.

Naming coordination compounds

  • Formula from a nameWrite the formula of a coordination compound from its name, balancing the complex ion with its counter-ion.

Oxidation state and d-electron count

  • Which metal fits the spin data?Work back from the unpaired-electron count of a high-spin octahedral ion to its d count and then to the metal.

Planeje uma síntese 1

  • Shift the absorption to shorter wavelengthChoose a ligand substitution that increases Δo and so moves a d–d absorption to shorter wavelength.

Explique por quê 4

Ligands and the coordination sphere

  • Why chelates bind more tightlyExplain the chelate effect in terms of the change in the number of free particles and the entropy of reaction.

Crystal fields, colour and magnetism

  • Why tetrahedral complexes are high spinExplain why tetrahedral complexes are almost always high spin by comparing Δt with the pairing energy.
  • Why zinc(II) solutions are colourlessExplain the absence of colour in a d¹⁰ complex in terms of d–d transitions.

Isomers of complexes

  • Why no cis/trans for tetrahedral complexes?Explain from the geometry of a tetrahedron why tetrahedral [MA₂B₂] complexes have no geometric isomers.

Travou em compostos de coordenação e teoria do campo cristalino? 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.

Escaneie sua lição de graça

Antes deste tópico