isolobal-analogy in organometallic chemistry.pptx

Isolobal Analogy

1. PRESENTATION ON ISOLOBAL ANALOGY PRESENTED BY KUNDAN KUMAR OJHA ROLL NO. – 204120011 M.Sc. 2 ND SEM. 1

2. INTRODUCTION The problem we have in organometallic chemistry or even inorganic chemistry is the fact that we do not have the same systemisation, that is available in organic chemistry. So, a concept of isolobal analogy was introduced by Nobel lauerate Roald Hoffmann in 1981. The isolobal analogy allows us to relate and compare organic, inorganic & organometallic compounds on one uniform basis. Hoffmann described molecular fragments as isolobal , if the number, symmetry properties, approximate energy and shape of the frontier orbitals and the number of electrons in them are similar. 2

3. Considering fragments of methane and an octahedral complex. The parent compounds have filled valence-shell electron configurations, eight electrons (octet) for CH 4 and 18 electrons for ML 6 . Methane is considered to use sp 3 hybrid orbitals in bonding, with eight electrons occupying bonding orbitals formed from interactions between the hybrid orbital and 1s orbitals on the hydrogens. The metal in ML 6 uses d 2 sp 3 hybrids to bond to the ligands, with 12 electrons occupying bonding orbitals and six nonbonding electrons occupying dxy , dxz , and dyz orbitals. ISOLOBAL ANALOGY 3

4. Orbitals of Parent Structures CH 4 and ML 6 ISOLOBAL a NALOGY . 4

5. For the purpose of the analogy, these fragments are assumed to preserve the original geometry of the remaining ligands. Tetrahedral CH 4 and octahedral ML 6 can form 7- and 17- electron fragment. To form the fragments, the C–H and M–L bonds are assumed to cleave homolytically . ISOLOBAL ANALOGY 5

6. Each of these fragments has a single electron in a hybrid orbital at the vacant site of the parent polyhedron. These orbitals are sufficiently similar to meet Hoffmann’s isolobal definition. Using Hoffmann’s symbol to designate groups as isolobal , we may therefore write the following. CH 3 Mn(CO) 5 ISOLOBAL ANALOGY 6

7. Similarly, 6-electron CH 2 and 16-electron ML 4 are isolobal . Each of these fragments represents the parent polyhedron, with single electrons occupying two hybrid orbitals at otherwise vacant sites. Each fragment also has two electrons fewer than the filled shell octet or 18-electron configurations. ISOLOBAL ANALOGY 7

8. ISOLOBAL FRAGMNENTS S.No . Transition element/ fragmnent Main group element/ fragmnent 1 18 8 2 17 7 3 16 6 4 15 5 5 14 4 Correlation between transition element & main group fragments according to electron count 8

9. Isolobal fragments can be formally combined into molecules. For example, two CH 3 fragments, when linked, form ethane, and two Mn(CO) 5 fragments form the dimeric (OC) 5 Mn–Mn(CO) 5 . Similarities between fragmnets 9

10. Furthermore, organic and organometallic fragments can also be intermixed. E.g. H 3 C–Mn(CO) 5 is also a known compound. Similarities between fragmnets Organic and organometallic parallels are not always this complete. For example, whereas two 6-electron CH 2 fragments form ethylene, H 2 C=CH 2 , the dimer of the isolobal Fe(CO) 4 is not nearly as stable. 10

11. Extension of Analogy 11

12. Analogies are not only limited to organometallic fragments of octahedra. Similar arguments can be used to derive fragments of different polyhedra . For example, Co(CO) 4 , a 17-electron fragment of a trigonal bipyramid, is isolobal with Mn(CO) 5 , a 17-electron fragment of an octahedron. The compound composed of these two fragments, (CO) 5 Mn—Co(CO) 4 , is known. Extension of Analogy 12

13. Isolobal Relationships for Fragments of Polyhedra 13

14. APPLICATION A practical use of isolobal analogies is in suggesting syntheses of new compounds. For example, CH 2 is isolobal with 16-electron Cu(η5 –Cp) and with 14-electron PtL 2 (L = PR 3 , CO). All three of these fragments are two ligands and two electrons short of their parent polyhedra . Recognition that these fragments are isolobal has been exploited in preparing new organometallic compounds composed of fragments isolobal with fragments of known compounds 14

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