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

Five electron ligands
---v~···
Fe,PFs
Cp
HPF6
(R=Me
R'=Hl
The stability of a carbonium ion can be expressed in terms of a pKR+ value,
which relates to the equilibrium
[R +]
R + + 2H 2 0:;:=ROH + H,o+: pKR+ = log 10 --+ HR
[ROH]
HR is an 'acidity function' which provides a reference point for the pKR+ scale. The
more positive the pKR+ value, the more stable is the carbonium ion (Table 9.2).
The property of stabilizing a-carbonium ions is also shared by similar organometallic
groups. Their effectiveness lies in the order
~ <•h< 9-<~<~
Co
Cp
Cr
(C0) 3
Mn
(C0) 3
The molecular structure of [(C 5 H 4 CPh,)FeCp]+PF;. determined by X-ray
diffraction, shows that the CPh 2 group is bent down through 20° out of the plane
of the ring towards the iron atom. It is suggested that the vacant p-orbital of the
carbonium ion interacts not only with the delocalized n-system of the ring but
also with the metal centre. Although the barrier to rotation about the Cring -CR 2
bond in this and similar compounds is only 80 ± 1 O kJ moi- 1 , this is sufficiently
high to prevent racemization during substitution.
9.2.4 Some characteristics of the chemistry of other metallocenes
(a) VANADOCENE AND CHROMOCENE. Vanadocene is a 15-electron complex,
so it is not surprising that most ofits reactions involve addition to the metal centre
to give products in which the electron count has increased to 17 or 18 (Fig. 9.5).
In the adducts Cp 2 VX or Cp 2 V(X)Y, the cyclopentadienyl rings are no longer
parallel. Structures of related compounds are described on p. 290.
Chromocene is very easily hydrolysed and oxidized with disruption of the
complex. Solutions of a cation Cp 2 Cr+pp .;-, however, can be prepared by
Fe
Cp
Table 9.2 Relative stabilities of carbonium ions in aqueous acid
+ + +
Ion (C.H,CPh,)CoCp Ph 2 CH (C,H. CPh 2 )Mn(CO),
pKR+ -15.2 -13.3 -6.5
+
FcCH 2
-1.2
Fc,(H Fc,C
4.1 5.8
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