Parallels between main group and organometallic …ipc.iisc.ernet.in/~jagirdar/docs/Isolobal_analogy...RoaldHoffmanninhis1981Nobellecture; Hoffmann defined molecular fragments tobeisolobal

Parallels between main group and organometallic chemistry

7 electrons 17 electrons

Electronically equivalent

Parallels between Cl and Co(CO)4

Parallels between S and Fe(CO)4

Parallels between P and carbonyl complexes

Tetrahedral tetramers

Exceptions: IF7, XeF4 � no organometallic analogues;

ligands weaker than CO in the spectrochemical series

Roald Hoffmann in his 1981 Nobel lecture;

Hoffmann defined molecular fragments

to be isolobal

if the number, symmetry properties,

Isolobal Analogy

if the number, symmetry properties,

approximate energy and shape of the

frontier orbitals and the number of electrons

in them are similar-not identical, but similar.

Hoffmann, R. Angew. Chem. Int. Ed. 1982, 21, 711

We symbolize this relationship by

Two “fragments” are called isolobal if their “working”

frontier orbitals, that is HOMO and LUMO:

i) are the same in number,

ii) have the same number of electrons on them;

iii) have similar shape and symmetry,iii) have similar shape and symmetry,

iv) are of approximately the same energy.

Isolobal fragments can replace each other in morecomplex structures and have similar bondingpatterns

Fragments with one working singly occupiedorbital:

Fragments with one working empty orbital:

All Lewis acids above can produce an adduct with aLewis base pyridine (py):

Orbitals of octahedral and tetrahedral fragments

Cyclic trimers

Note: dimer of Fe(CO)4 – not stable

The isolobal species Ir(CO)3, Co(CO)3, CR, and P

Structures resulting from combinations of Co(CO)3 and CR

2. Gain or loss of electrons from two isolobal fragments yields isolobal fragments

1. The isolobal definition may be extended to isoelectronic fragments having the same coordination number

Mn(CO)5 CH3, Re(CO)5 CH3[Fe(CO)5]

+

[Cr(CO)5]-

Extensions of the analogy

17 electron 7 e 17 e 7 e

isolobal fragments

Mn(CO)5 CH3, Cr(CO)5 CH3+

Mo(CO)5W(CO)5

Fe(CO)5 CH3-

Ru(CO)5Os(CO)5

Fe(CO)5 and CH4 are not isolobal – reason it out

17 e 7 e 16 e 6 e

18 e 8 e

3. Other 2-electron donors are treated similarly to CO

4. Ligands η5-C5H5 and η6-C6H6 are considered to occupy

three coordination sites and to be 6-electron donors

Mn(CO)5 Mn(PR3)5 MnCl55– Mn(NCR)5 CH3

(η6-C6H6)Mn(CO)2 Mn(CO)5 CH3

(η5-C5H5)Fe(CO)2 Fe(CO)5+ CH3

(η5-C5H5)Mn(CO)2, (η6-C6H6)Cr(CO)2, [Mn(CO)5]

+, Cr(CO)5

5. Octahedral fragments of formula MLn (where M has a dx configuration)are isolobal with square-planar fragments of formula MLn-2 (where M hasa dx+2 configuration and L is a 2-electron donor).

Octahedral Fragments Square-planar FragmentsMLn MLn-2Cr(CO)5 PtCl3

-

d6 d8

Fe(CO) Pt(PR )Fe(CO)4 Pt(PR3)2d8 d10

5. Octahedral fragments of formula MLn (where M has a dx configuration)are isolobal with square-planar fragments of formula MLn-2 (where M hasa dx+2 configuration and L is a 2-electron donor). Comparison of square-planar fragments with octahedral fragments

Square planar d8

ML3 has an emptylobe of anonbonding hybridorbital as a LUMO

Comparison of square planar fragments with octahedral fragments

orbital as a LUMO[PtCl3]

–; similarly,octahedral d6 ML5fragment [Cr(CO)5],thus the isolobalanalogy between thetwo

d10 ML2 fragment [Pt(PR3)2] would have 2 valence electrons

more than PtCl2

These electrons occupy two nonbonding hybrid orbitals

This situation is comparable to Fe(CO)4 fragmet; each

complex has two singly occupied lobes

Examples of isolobal fragments

The 5-electron fragment CH is isolobal with P and other Group 15atoms. A potential application of this relationship is to seekphosphorus-containing analogues to organometallic complexescontaining cyclic ligands such as C5H5 and C6H6

Examples of applications of the analogy

Angew. Chem. Int. Ed. 1987, 26, 59 Science 2002, 295, 832