Fac Isomer

Besides the thiocyanates, just mentioned, other 5-donor complexes which are of interest are the dialkyl sulfides, [MCl3(SR2)3], produced by the action of SR2 on ethanolic RhCl3 or on [IrClg] ". Phosphorus and arsenic compounds are obtained in similar fashion, and the best known are the yellow to orange complexes, [ML3X3], (M = Rh, Ir X = Cl, Br, I L = trialkyl or triaryl phosphine or arsine). These compounds may exist as either mer or fac isomers, and these are normally distinguished by their proton nmr spectra (a distinction previously made by the measurement of dipole moments). An especially... 

C20-0090. Draw a ball-and-stick model of the fac isomer of [NiCl3 F3, oriented so that the fluoride ions are in the top and two rear positions. 

A pyridine derivative related to dien with respect to the number and distribution of N-donor atoms, namely bis(2-pyridylmethyl)amine (bpma), also gives comparable complexes with Cd, e.g., [Cd(bpma)2](C104)2 with potentially three isomers (including a pair of enantiomers). As shown by the structure analysis (C2/c, Z = 4), a distorted octahedral fac-isomer with symmetry 2 —C2 has been isolated, necessarily with both enantiomers in the crystal lattice. No significant difference in the two kinds of Cd—N bonds (rav(Cd—N) 235.0 pm) is observed.189 Solid-state 13C NMR spectra of this complex and related Mn and Zn complexes have been discussed. 

The fac- isomer of trice-aminoethanol complex of cobalt (III), with 3 coplanar oxygen atoms, was prepared according to the method [16],... 

In this case, the product is the fac isomer, in which all NH3 ligands are trans to the CO molecules. Ammonia does not form ty bonds to metals because it has no orbitals of suitable energy to accept electron density. Thus, the back donation from Cr in Cr(NH3)3(CO)3 goes to only three CO molecules, and the bond order is reduced even more than it is in Cr(CO)s, where back donation occurs equally to six CO molecules. There is, of course, an increase in Cr-C bond order and stretching frequency in Cr(NH3)3(CO)3 compared to Cr(CO)s. Based on the study of many mixed carbonyl complexes, it is possible to compare the ability of various ligands to accept back donation. When this is done, it is found that the ability to accept back donation decreases in the order... 

Due to the tram effect of Ir-C bonds in the ligand exchange, the product is largely the fac isomer. Compared with the direct ligand reaction with IrCl3H20, this method gives relatively high yields ( 50%) of the desired / c-Ir(ppy)3 or its derivatives. 

Crystals of the iron(III)-acethydroxamate complex [Fe(ahdx)3]-1.5Fl20 contain one molecule of the fac isomer and one of the mer per unit cell, whereas the benzhydroxamate [Fe(bhdx)3]-3Fl20 crystallizes simply as the fac isomer. 

Nitrosyl complexes of formula [Ru(NO)Cl3(diphos)] in which diphos = dppm, dppe, dppp, or (Z)-l,2-bis(diphenylphosphino)ethene have been synthesized. Although P NMR spectroscopic data are consistent with a fac isomer in each case, the solid-state structure of [Ru(NO)Cl3(dppp)]... 

Fig. 8. Schematic representation of the potential surfaces leading to photoisomerisation of (BQA)PtMe2I from mer to fac isomer via a sloped conical intersection at / -like geometries. Shown to the right are the branching space vectors the gradient difference (gd=x1), and the derivative coupling (dc=x2). The primary orbitals involved in the electronic transition are shown to the left [Adapted from Ref. (110) with permission].

With bidentate ligands, (Figure 14) it has been suggested that the fac isomer is preferred.167 However, structure determination of [WSCl3(mte)J (mte = l,2-bis(methylthio)ethane) has shown that it has the mer stereochemistry in the solid state.172 Whether this is indeed the preferred stereochemistry for complexes of the type [WYX3(L-L)] will have to await further studies. 

One of the most extensively studied systems is that of [Co(dien)2]3+ (Table 7.1). The mer- isomer is chiral (C2) because of the two possible orientations of the proton at the secondary nitrogen atomst38,39], the unsym-fac-vsoiasi is also chiral (C2) while the sym-fac-isomer is achiral (Q). For the five-membered chelate rings all possible combinations of S and X conformations (see Fig. 6.5 for the SIX nomenclature) have to be considered, leading to a total of 40 isomers and conformers (some conformers are calculated to be unstable and may therefore be neglected144,451). The calculated distribution is based on partition functions (Eqs. 7.1, 7.2) ... 

The fac isomers were isolated by Schetty [31] by chromatographic separation and confirmed by Jaggi by X-ray crystal determination [32], In solution, each... 

TABLE 75. Selected bond lengths for S11RCI3 Phen complexes (cis-fac isomer)... 

In the case of the mer and s-fac isomers, one conformer is substantially more stable than any other but for u-fac, the difference is smaller. Hereafter, we will focus on only the most stable conformer but you should be aware that this is risky. Changing the force field might reverse the stability order of the conformers or at least change the relative abundances. We leave it as an exercise for you to check the effect of the changes on the most stable conformers and to see whether there is a change in the order. 

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