Facial isomers

The reactions of MeMn(CO)5 with various phosphines and phosphites (L) to give/ac-MeCOMn(CO)3L2 and/or wicr(L srrflMi)-MeCOMn(CO)3L2 have been monitored by NMR spectroscopy 166). Sterically demanding L s promote formation of the meridional over the facial isomer. Interestingly, the meridional acetyls decarbonylate more easily than their facial counterparts, perhaps owing to a weaker Mn—CO bond cis to COMe in the former. 

Preparation of [IrH3(PPh3)3] according to literature procedures (25) gives a mixture of facial and meridional isomers that are separated by recrystallization from benzene-methanol. Irradiation of a degassed benzene solution of the synthetic mixture yields a rapid decrease in intensity of the 1740 cm-1 yIr H of the meridional isomer, a slow decrease of the 2080 cm-1 vIr.H of the facial isomer,... 

Ans. Two isomers, one with each Cl trans to NH3 (facial isomer), one with 1 Cl trans to NH3 (meridional isomer)... 

FIGURE 5.21 Meridional and facial isomers of tris-terdentate structures based on a tricapped trigonal prism. 

The tertiary phosphine Ph2P(CH2COOEt) has been shown by Braunstein and his co-workers to form a rhodium(III) complex when allowed to react with rhodium trihalides. The products are the facial isomers (77). This is in accordance with their NMR spectra which show V(P-P) = 23.5 Hz.956... 

This is a populous class of complex most known tertiary phosphines seem to have been utilized in preparing at least one complex of this type. Meridional isomers are normally the rule, but small tertiary phosphines are also capable of forming facial isomers. 

The facial isomers are obtained only from small tertiary phosphines, usually in poor yield. They are both less soluble and less stable than the meridional isomers. 

Among the important cleavage products of the M2(CO)i0 molecules are the halopentacarbonyls, XM(CO)5 and the M(ju,-X)2(CO)8 molecules that are formed by thermal loss of CO. From either of these halocarbonyls more CO can be displaced by neutral ligands to give tricarbonyls (which are usually the facial isomer) such as the bpy compound,6818-D-XVIII. 

Lifschitz (39) isolated both optical isomers of a-[Co(L-alaninate)3] (meridional), and recently both optical isomers of the P (facial) isomer have been isolated in four laboratories. Bailar (3) isolated both isomers of [Co( —)pn2C03]". He and several of his students have used the selective substitution in such optically active complexes of pn and en by a racemic mixture of a ligand to resolve partially the ligand. Dwyer studied the tris(propylenediamine) complexes of Co(III) (16) and Pt(IV) (17) and found that their formation is not absolutely stereospecific, as previously believed. 

Heptafluoro-tt-propyliron tetracarbonyl iodide reacts with potassium bis- and tris- pyrazolylborate to give complexes [49] and [50], with the parameters shown. The product [49] was shown by its H NMR spectrum to contain about 30% of the facial isomer [49a], but 19F parameters for this isomer are not reported. (42) The shift to higher frequency of the CF2 group adjacent to the metal in these complexes is rather less than that in [51] the parameters for complexes [53, Rf = C2F5, /-C3F7] are similar to those shown for [51] and [52]. (8)... 

The circular dichroism spectrum of the isomer designated as A-fac-Fe(mphen)32+ is shown in Figure 4. The facial designation is based on the sharp XH NMR signal observed for the resolved complex relative to the broader signal observed for the synthetic mixture of fac and mer isomers. The shielding is analogous for all protons, thus only a small difference is observed. Thus the absolute purity of the facial isomer cannot be assured. 

The synthesis of the unsymmetrical 3-methyl-1,10-phenanthro-line (mphen) and the resolution of the facial isomer of Fe(mphen) + has allowed us to investigate the stereochemistry associated with the optical inversion reaction previously observed for the corresponding unmethylated Fe(phen)32+ ion with cyanide. We conclude that the A-fac-Fe(mphen)32++isomer produces an excess of all three A-Fe(mphen)2(CN)(H20)+ isomers when the A-fac isomer reacts with cyanide, even though the final Fe(mphen)2(CN)2 product has only one strongly A isomer. Thus the optical inversion is a chemical inversion as well. 

The facial isomer of the complex [RI1X3P3] has equivalent phosphorus atoms, so the spectrum is a doublet due to rhodium-phosphorus coupling, whereas the meridianal isomer has two types of phosphorus atoms, a pair mutually traws(b) and one irons to X(a) (1). 

Structures for the materials are given in Figure 38. Unless otherwise stated all lr(C N)3 complexes are facial isomers. 

---