Understanding Coordination and Bonding in Transition Metals

1. CHM 4311

2. Coordination and back and pi-bonding Nature of coordinative bond Back-bonding Bonding of metals to p-systems

3. Valence bond and hybridization • Each bond treated independently from the environment • Resonance concept • Impossibility to explain molecular geometries • Hybridization concept • What about transition metals? • Coordinative bond • Hybridization involving d orbitals and molecular geometries

4. Valence Bond Theory

5. Crystal Field

8. Crystal Field and molecular geometry

9. Ligand field

13. Effect of pi-bonding on ligand field: spectrochemical series

14. Analogy Organic-Inorganic

15. Electronic Equivalence Organic versus Inorganic Reactivity Parallelism: the first idea (J. HalpernC&En Oct 31-1966 pag. 68) Saturated Organic Inorganic anti-bond anti-bond 6 el. non-bonding 8 bonding el. (octect) 12 el 18 el Non bonding e- coordination # Non bonding e- coordination # 0 4 6 6 SN2 and SN1 reactions ligand replacement reactions

16. – Electronic Equivalence (empirical approach) Organic Inorganic Non-bonding el. Coord. # Non-bonding el. Coord. # Saturated (substitutions) Radical (atom abstractions & addition) Carbene (cyclopropanation, oxidative addition) Carbanion (nucleophiles) Carbocation (electrophiles, low spin)

17. – Electronic Equivalence Concept 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).

18. – Electronic Equivalence: is that meaningful? Parallels between Cl and Co(CO)4:

19. – Isolobal Analogy Parallels between S and Fe(CO)4: Note: pK values in aq. solution at 25 °C.

20. – Electronic Equivalence Limitations: 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).

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).

22. – Isolobal Analogy

23. – Isolobal Analogy Starting-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.

24. – Isolobal Analogy

25. – Isolobal Analogy The removal of 2 L creates 2 new frontier orbitals and the removal of 3 L creates 3 new frontier orbitals, ψhy:

26. – Isolobal Analogy The frontier orbitalst2g 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:

27. – Isolobal Analogy The isolobal connection allows a joint consideration of inorganic, organic and organometallic structures, the relationships being based on the isolobal nature of the respective molecular fragments.

28. – Isolobal Analogy

29. – Isolobal Analogy

30. – Isolobal Analogy

31. – Isolobal Analogy The concept of isolobality focuses on partly occupied orbitals only

32. – Isolobal Analogy Even the molecules P4, As4 and Sb4 may be included into a larger family: and hence the following tetrahedranes are analogous:

33. – Isolobal Analogy Inclusion 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:

34. – Isolobal Analogy Another analogy is that between cyclopropane and μ-alkylidene complexes:

35. – Isolobal Analogy xxxx

36. – Isolobal Analogy

37. – Isolobal Analogy The realm of isolobal connections is considerably extended if the mutual replacement of σ-donor ligands and metal electron pairs is introduced:

38. – Isolobal Analogy

39. – Isolobal Analogy

40. – Isolobal Analogy

41. – Isolobal Analogy

42. – Isolobal Analogy Examples of isolobal fragments: