Lecture 33 1) h 3 -Allyl complexes. h 3 -Allyl ligand C 3 H 5 - is anionic, donates 4 electrons from its p -system to form one s - and one p -bond with the metal and forms complexes with many transition metals such as Ni( h 3 -C 3 H 5 ) 2 .
Lecture 331) h3-Allyl complexes • h3-Allyl ligand C3H5- is anionic, donates 4 electrons from its p-system to form one s- and one p-bond with the metal and forms complexes with many transition metals such as Ni(h3-C3H5)2. • Allyl is capable of p-backbonding but it is not so important as in the case of olefins or CO. • In symmetrical h3-allyl complexes the M-C1 and M-C3 distances are identical and slightly longer than M-C2. In addition, Hmeso and Hsyn are bent 7-130 towards the metal while Hanti are bent 300 away. • h3-allyl can easily convert into h1-allyl:
2) Coordination can make olefins and allyl electron-poor • Alkenes typically considered in organic chemistry as electron-rich compounds can become electrophilic if coordinated to an electron-poor metal: • Even anionic allyl can behave as an electrophile if it is attached to PdII:
3) p-MO’s of cyclic conjugated ligands • For qualitative analysis of bonding between transition metals and p-electron donors such as organic compounds with C=C bonds it is necessary to know how the ligand p-MO’s look like. • Frost diagrams allow establish the shape, degeneracy and the energy sequence of the ligand p-MO’s for the case of cyclic conjugated species. • The diagrams below describe qualitatively the p-MO’s in the n-membered cyclic systems. The number of the nodal surfaces (shown with dashed lines) of the MO’s increases as you move up along the diagram.
4) h5-Cyclopentadienyl complexes • p-MOs’ of cyclopentadienyl anion: • And bonding in h5-Cp complexes:
5) h6-Arene complexes h6-Bonding in transition metal – arene complexes (5 MO’s only): h6-, h4- and h2-arene complexes are known: