Phosphane and related ligands

The series of dirhodium complexes involving P-PR3, p-Sh Pr3 (e.g. 24.18) or p-AsMe3 ligands is steadily growing. 

Infrared spectroscopic data can be used to determine this sequence a ligand trans to a CO affects the M — CO back-donation and, therefore, Fco. e.g. in octahedral Mo(CO)3(PF3)3, Fco = 2090 and 2055 cm , compared with 1937 and 1841 cm in Mo(CO)3(PPh3)3. 

The steric requirements of a PR3 ligand depend on the R groups. Ligands such as PPh3 (Fig. 24.4a) or P Bu3 are 

Ligand Tolman cone ai le/ deg Ligand Tolman cone angle/deg 

The ligands L and L shown below react with PdCl2 to give LjPdClj and (L )ClPd(p-Cl)2Pd(p-Cl)2PdCl(L ), respectively. Suggest why a mononuclear complex is formed in only one case. 

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