Nature of coordinative bond Backbonding Bonding of metals to p systems Valence bond and hybridization Each bond treated independently from the environment Resonance concept Impossibility to explain molecular geometries ID: 1035695
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1. CHM 4311
2. Coordination and back and pi-bondingNature of coordinative bondBack-bondingBonding of metals to p-systems
3. Valence bond and hybridizationEach bond treated independently from the environmentResonance conceptImpossibility to explain molecular geometriesHybridization conceptWhat about transition metals?Coordinative bondHybridization involving d orbitals and molecular geometries
4. Valence Bond Theory
5. Crystal Field
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8. Crystal Field and molecular geometry
9. Ligand field
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13. Effect of pi-bonding on ligand field: spectrochemical series
14. Analogy Organic-Inorganic14
15. Electronic EquivalenceOrganic versus Inorganic Reactivity Parallelism: the first idea(J. Halpern C&En Oct 31-1966 pag. 68)Saturated Organic Inorganic8 bonding el.(octect)Non bonding e- coordination #0 4 6 6anti-bondanti-bondnon-bonding12 el6 el.18 elNon bonding e- coordination #SN2 and SN1 reactions ligand replacement reactions 15
16. – Electronic Equivalence (empirical approach)04661375228423850365Saturated(substitutions)Radical(atom abstractions & addition)Carbene(cyclopropanation, oxidative addition)Carbanion(nucleophiles)Carbocation(electrophiles, low spin)Non-bonding el.Non-bonding el.Coord. #Coord. #OrganicInorganic16
17. – Electronic EquivalenceConcept of electronically equivalent species: a more general concept (see J.E. Ellis, J.Chem. Educ., 1976, 53, 2).Comparisons useful in cases where 18e rule holds (in transition metal chemistry this is organometallic chemistry).17
18. – Electronic Equivalence: is that meaningful?Parallels between Cl and Co(CO)4:18
19. – Isolobal AnalogyParallels between S and Fe(CO)4:Note: pK values in aq. solution at 25 °C.19
20. – Electronic EquivalenceLimitations: octet expansion (main group), reaction chemistry (organometallic).Concept made more rigorous in terms of isolobal analogy (see R. Hoffmann, Angew. Chem. Int. Ed. Engl., 1982, 21, 711).20
21. – Isolobal Analogy(H&S p 597; D,M&A, pp 605-606; S,A&L, pp 699-701; S&A, pp 559-560)Definition: two fragments are isolobal if the number, symmetry properties, approximate energies and shape of frontier orbitals, as well as the number of electrons occupying them, are similar (not identical).21
22. – Isolobal Analogy22
23. – Isolobal AnalogyStarting-point for the derivation of the lobal properties of organometallic fragments is the simplified MO scheme of an octahedral complex. The removal of one ligand L converts a bonding σ-MO of the complex ML6 into a nonbonding frontier orbital ψhy of the fragment ML5.23
24. – Isolobal Analogy24
25. – Isolobal AnalogyThe removal of 2 L creates 2 new frontier orbitals and the removal of 3 L creates 3 new frontier orbitals, ψhy:25
26. – Isolobal AnalogyThe frontier orbitals t2g and ψhy are filled by n electrons of the central metal (configuration dn). This provides Mn(CO)5 with one, Fe(CO)4 with two, and Co(CO)3 with three, singly occupied orbitals with distinct spatial orientation. These organometallic fragments are complementary to organic analogues:26
27. – Isolobal AnalogyThe isolobal connection allows a joint consideration of inorganic, organic and organometallic structures, the relationships being based on the isolobal nature of therespective molecular fragments.27
28. – Isolobal Analogy28
29. – Isolobal Analogy29
30. – Isolobal Analogy30
31. – Isolobal Analogy31The concept of isolobality focuses on partly occupied orbitals only
32. – Isolobal AnalogyEven the molecules P4, As4 and Sb4 may be included into a larger family:and hence the following tetrahedranes are analogous:32
33. – Isolobal AnalogyInclusion of the ligand η-C5H5 - which, as a donor of 3 π-electron pairs formally occupies 3 coordination sites, yields the analogies:The following molecules are related:33
34. – Isolobal AnalogyAnother analogy is that between cyclopropane and μ-alkylidene complexes:34
35. – Isolobal Analogy35xxxx
36. – Isolobal Analogy36
37. – Isolobal AnalogyThe realm of isolobal connections is considerably extended if the mutual replacement of σ-donor ligands and metal electron pairs is introduced:37
38. – Isolobal Analogy38
39. – Isolobal Analogy39
40. – Isolobal Analogy40
41. – Isolobal Analogy41
42. – Isolobal AnalogyExamples of isolobal fragments:42