_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Organometallic compounds of the transition elements
Frontier orbitals
Metal
Ligand
Symmetric (a')
hy, 'xz'
(nl),, (n2)s
Antisymmetric (a")
The main orbita! interactions between C 5 H 5 and Mn(C0) 3 are between (n 1 ),
and 'hy', (nJ, and 'xz' and (n 2 )a and 'yz'. A partial energy level diagram
{Fig. 6.14) illustrates this. Involvement ofthe frontier orbitals only gives a rather
oversimplified picture; it is usual to extend it somewhat by including overlap of
'xy' and 'x 2 - y 2 ' with the n 3 m.o.s of C 5 H 5 as shown. A substantial energy gap
between the highest occupied and lowest unoccupied m.o.s is predicted. This is
consistent with the lack of reactivity towards substitution of CO by other ligands
which is a property of this molecule. A similar energy diagram for CpCoL 2 ,
however, {Fig. 6.15) shows that the metal atom possesses a non-bonding pair of
electrons. The consequence of this in the chemistry of CpCoL 2 is referred to below
(p. 295).
Calculations ha ve shown that the number, symmetry properties, spatial extent
and energies of the frontier orbitals of the fragments M{C0) 3 , M(C 6 H 6 ) and
M' ( C 5 H 5 ) are very similar. (If electron occupancy is considered M' has to provide
one more electron than M for the groups M(C 6 H 6 ) and M'(C 5 H 5 ) to be
isoelectronic, e.g. if M = Cr, M' = Mn.) Such fragments are termed 'isolobal'. In
fact all hydrocarbon metal fragments M{C"H")(n = 4, 5, 6, 7) are essentially
isolobal.
Fragment analysis is particularly useful in predicting the favoured conformations
of molecules. In cyclohexadienyltricarbonylmanganese there are two
extreme orientations of the ligand with respect to the Mn(C0) 3 fragment.
Molecular orbita! calculations as well as n.m.r. measurements on derivatives over
+5e(Cp)
CpCo CpCoL2 L2
Fig. 6.15 Schematic interaction diagram showing the bonding in CpCoL 2 • Here Lis a a­
donor only; no n-bonding to L is included. C 6 H 6 FeL 2 or C 6 H 6 RuL 2 resemble
CpCoL 2 •
210