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. Halpern C&En Oct 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 #
SN2 and SN1 reactions ligand replacement reactions

16. – Electronic Equivalence (empirical approach)
Organic
Inorganic 4 6 1 3 7 5 2 8
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. Parallels between S and Fe(CO)4:
– 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 ; S,A&L, pp ; S&A, pp )
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 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:

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. The following molecules are related:
– 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
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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: