Nickel Polymerization Catalysts with Ylide Steering Ligands - Late Transition Metal Polymerization Catalysis - page 9

Tab. 1.3 Bond lengths in (PCCONi) metallocycles

Related (PO) Nickel Complexes d(NiO) (CO) [Å]

[NiPh(Ph 2 PCHCMeO)(i-Pr 3 PCH 2 )] 1b 1.951 1.302

[NiPh(Ph 2 PCHCPhO)(PPh 3 )] 2 1.914 1.313

[Ni(Ph 2 PCHCPhO) 2 ] 3 1.885 1.318

Fig. 1.2 X-ray structure and charge
distribution of the bis(ylide)nickel cata-
lyst [NiPh(Ph 2 PCHCMeO)(i-Pr 3 PCH 2 )].

An X-ray structure determination of [NiPh(Ph 2 PCHCMeO)(i-Pr 3 PCH 2 )]
(Fig. 1.2) confirms the presence of a structurally intact C-coordinated i-Pr 3 PCH 2 -
ylide ligand in the trans position to the diphenylphosphino group of a
Ph 2 PCHCMeO-PO chelate. The most striking finding is a dramatic increase in
the Ni-O bond length to 1.95 Å as a consequence of a structurally intact coordi-
nated ylide ligand (Tab. 1.3). The stepwise weakening of the Ni-O bond correlates
with enhanced catalyst activity.
In summary, prominent features of ylide nickel complexes versus phosphane
complexes have been identified: an electron-rich nickel center, energetically desta-
bilized nickel-localized occupied orbitals, a significant weakening of the Ni-O
bond, the phosphorus moiety being located outside the nickel coordination plane,
thus opening one axial position in the nickel coordination sphere for easy mono-
mer “landing”.