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

Alkyls and aryls
now believed that the main reason that ligands such as IJ 5 -C 5 H 5 , phosphines and
CO often stabilize alkyls and aryls in 18-electron (and sometimes 16-electron)
complexes is that they firmly occupy available coordination sites.
Many such sites, however, are present in binary compounds such as TiR 4 •
Having realized that /J-hydrogen transfer is an important pathway for decomposition,
people set out to prepare compounds in which the alkyl groups lack P­
hydrogen atoms. Methyls, although stiU unstable, are generally more robust than
ethyls, but benzyls are more inert than either. Thus Ti(CH 2 Ph) 4 decomposes only
at its melting point (91 °C), but Ti(CH 2 CH 3 ) 4 has never been prepared at ali. The
TiCC bond angles at the a-carbon of the benzyl groups in Ti(CH 2 Ph) 2 are only
about 90°, whereas in Sn(CH 2 Ph) 4 normal tetrahedral angles are adopted. The
tight angles in the former molecule bring the benzene rings within about 2.8 A of
the metal atom. Some interaction between the n-system and titanium is therefore
indicated and this may help to stabilize the compound.
The neopentyl (Me,CCH 2 -) and trimethylsilylmethyl (Me,SiCH 2 -) groups
have proved particularly useful in blocking /J-hydrogen transfer. Not only do they
lack /J-hydrogen atoms, but their bulk probably also restricts sterically both intraaud
intermolecular decomposition.
Et,o
MC1 4 + 4LiCH 2EMe, ------> M(CH 2 EMe,) 4
-78°C
(M =Ti, V: E = C, Si)
THF
0 2 (air)
CrCl, + 4LiCH 2 EMe, ------> Li[Cr(CH 2 EMe,) 4 ] -----+ Cr(CH EMe )
Na/Hg 2 3 4
Ti(CH 2 CMe 3 ) 4 is yellow solid, m.p. 105°C with decomposition, which sublimes at
40°C/10- 3 mm Hg. It is monomeric in solution in benzene. It is thermally quite
stable decomposing in solutionat 60°C to CMe 4 and a black solid with a halflife of
14h. The deep green paramagnetic liquid V(CH 2 CMe,) 4 is spontaneously
inflammable in air and must be stored under nitrogen or argon at - 30°C. The
initial product from chromium(III) ehi oride is a dark blue solution which contains
the anion [Cr(CH 2 CMe,l.r. This is oxidized in air to purple Cr(CH 2 CMe 3 ) 4 (Jlerr
= 2.9 B.M.), in which chromium has two unpaired electrons. The unusual
tetravalent state is paralleled in the alkoxides Cr(OR) 4 and amino compounds
Cr(NR 2 ) 4 •
While the tetrakis compounds MR 4 are obtained from Me 3 ECH 2 Li, tris
complexes MR~ (M =Ti, V, Cr; R' = CH(SiMe,) 2 ) result from reactions between
MC1 3 and the very bulky (Me,Si) 2 CHLi. Here steric effects apparently determine
the valency (cf. Sn, p. 12 3 ). Norbornyl compounds MR 4 (M =Ti, V,
Cr, Fe, Co) are even more inert to thermal decomposition and to oxidation than
the neopentyls. {3-Hydrogen transfer from the norbornyl group to the metal is
strongly disfavoured as this would result in the formation of a strained alkene
with a double bond ata bridgehead carbon atom, contrary to Bredt's rule. (There
is also no a-hydrogen). The bulk ofthe group is probably responsible for the great
resistance to oxidation observed for these complexes.
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