CHM4311$ Coordinaon$and$back$and$pi5 bonding$ · 2013-01-07 · 2/1/12 1 CHM4311$...
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2/1/12 1 CHM 4311 Coordina0on and back and pi bonding Nature of coordina0ve bond Backbonding Bonding of metals to πsystems Valence bond and hybridiza0on Valence Bond Theory
Transcript of CHM4311$ Coordinaon$and$back$and$pi5 bonding$ · 2013-01-07 · 2/1/12 1 CHM4311$...
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CHM 4311
Coordina0on and back and pi-‐bonding
Nature of coordina0ve bond Back-‐bonding
Bonding of metals to π-‐systems
Valence bond and hybridiza0on Valence Bond Theory
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Crystal Field
Crystal Field and molecular geometry
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Ligand field
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Effect of pi-‐bonding on ligand field: spectrochemical series
Analogy Organic-‐Inorganic
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Electronic Equivalence Organic versus Inorganic Reac1vity Parallelism: the first idea
(J. Halpern C&En Oct 31-‐1966 pag. 68)
Saturated Organic Inorganic
8 bonding el. (octect)
Non bonding e-‐ coordina0on # 0 4 6 6
an0-‐bond an0-‐bond
non-‐bonding
12 el
6 el.
18 el Non bonding e-‐ coordina0on #
SN2 and SN1 reac1ons ligand replacement reac1ons 15
– Electronic Equivalence (empirical approach)
0 4 6 6
1 3 7 5
2 2 8 4
2 3 8 5
0 3 6 5
Saturated (subs0tu0ons)
Radical (atom abstrac0ons & addi0on)
Carbene (cyclopropana0on, oxida0ve addi0on)
Carbanion (nucleophiles)
Carboca0on (electrophiles, low spin)
Non-‐bonding el.
Non-‐bonding el.
Coord. # Coord. #
Organic Inorganic
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– 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 transi1on metal chemistry this is organometallic chemistry).
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– Electronic Equivalence: is that meaningful? Parallels between Cl and Co(CO)4:
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– Isolobal Analogy Parallels between S and Fe(CO)4:
Note: pK values in aq. solu@on at 25 °C.
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– Electronic Equivalence
Limita1ons: octet expansion (main group), reac1on chemistry (organometallic). Concept made more rigorous in terms of isolobal analogy (see R. Hoffmann, Angew. Chem. Int. Ed. Engl., 1982, 21, 711).
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– Isolobal Analogy (H&S p 597; D,M&A, pp 605-‐606; S,A&L, pp 699-‐701; S&A, pp 559-‐560) Defini1on: two fragments are isolobal if the number, symmetry proper1es, approximate energies and shape of fron1er orbitals, as well as the number of electrons occupying them, are similar (not iden1cal).
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– Isolobal Analogy
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– Isolobal Analogy Star1ng-‐point for the deriva1on of the lobal proper1es 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 fron1er orbital ψhy of the fragment ML5.
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– Isolobal Analogy
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– Isolobal Analogy The removal of 2 L creates 2 new fron1er orbitals and the removal of 3 L creates 3 new fron1er orbitals, ψhy:
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– Isolobal Analogy
The fron0er orbitals t2g and ψhy are filled by n electrons of the central metal (configura0on dn). This provides Mn(CO)5 with one, Fe(CO)4 with two, and Co(CO)3 with three, singly occupied orbitals with dis0nct spa0al orienta0on. These organometallic fragments are complementary to organic analogues:
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– Isolobal Analogy The isolobal connec0on allows a joint considera0on of inorganic, organic and organometallic structures, the rela0onships being based on the isolobal nature of the respec0ve molecular fragments.
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– Isolobal Analogy
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– Isolobal Analogy
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– Isolobal Analogy
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– Isolobal Analogy
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The concept of isolobality focuses on partly occupied orbitals only
– Isolobal Analogy Even the molecules P4, As4 and Sb4 may be included into a larger family: and hence the following tetrahedranes are analogous:
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– Isolobal Analogy Inclusion of the ligand η-‐C5H5
-‐ which, as a donor of 3 π-‐electron pairs formally occupies 3 coordina1on sites, yields the analogies:
The following molecules are related:
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– Isolobal Analogy
Another analogy is that between cyclopropane and μ-‐alkylidene complexes:
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– Isolobal Analogy
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– Isolobal Analogy
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– Isolobal Analogy The realm of isolobal connec1ons is considerably extended if the mutual replacement of σ-‐donor ligands and metal electron pairs is introduced:
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– Isolobal Analogy
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– Isolobal Analogy
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– Isolobal Analogy
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– Isolobal Analogy
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– Isolobal Analogy
Examples of isolobal fragments:
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