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

Transition metal chemistry
(o)
(OC lnM(CO)
-co
+CO
L
M(CO)n --=---+ LM(CO)n
(D mechonism)
18e
e.g. Ni(C0)4
-co
+CO
16e
18e
L
Ni(C0) 3 --=--+ LNi(C0) 3
( b) L + M(CO)n+l- L-----M-----CO - LM(CO)n + CO (Ia)
(CO)n
18e
18e
Tronsition state
( c) L + M(CO)n+ 1 ·-1ţ- LM(CO)n+l (A mechonism)
18e
20e
+
( d) L+ (OC)n M=N=O
N=O
_ (OC) M/ fast
(OCln-l M(NO)L
n \_ L ---=-ce--
18e 18e 18e
(A or
Fig. 5.12 Mechanisms of substitution of CO in metal carbonyls.
initial rate determining dissociation of CO to give a coordinatively unsaturated
intermediate is followed by fast uptake of the attacking ligand L (SN1 or D
mechanism). Alternatively an interchange associative mechanism (b) might
operate (SN2 or la) in which concerted replacement of CO by L occurs. A nonsynchronous
associative mechanism (A) involving addition of L to the metal
centre followed by elimination of CO is precluded as it would entail a 20 electron
intermediate (Fig. 5.12).
The reactions ofNi(C0) 4 with phosphorus donors are first order in Ni(C0) 4 and
essentially independent of the nature and concentration of the attacking
nucleophile. Coordinating solvents such as THF, which can interact with and
stabilize the 16 electron Ni(C0) 3 intermediate, accelerate the first step. The
reactions are retarded by increase in pressure; the volume of activation for attack
of triethylphosphite is positive, + 8 cm 3 mol- 1 at 0°C. AII these observations
point to a dissociative mechanism.
Ni(C0) 4 reacts rapidly at room temperature to give a series of substituted
derivatives Ni(C0) 3 L, Ni(C0) 2 L 2 , Ni(CO)L 3 and NiL 4 • The extent ofreaction with
excess L depends largely on the steric bulk ofL. Tolman has classified this in terms
of the cone angle 0. This is the angle at the apex of a cone (Fig. 5.13) which just
touches the substituents defined by van der Waals radii. It was found that the
degree of substitution of carbonyl groups in Ni( CO ) 4 , on treatment with an eightfold
excess ofL, is proportional to 0 for PR 3 :R=H. 87°; F, 101°; OMe, 107°;
Me, ll8°; OPh, 130°; Ph, 145°; Pri, 160°; Bu\ 182°.
Substitution ofCO in Fe(C0) 5 and in Cr(C0) 6 also follows the D mechanism, but
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