Fac-isomerism

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

In our final case study we shall focus on the photoinduced mer to fac isomerization reaction recently observed by Harkins and Peters (109). We have investigated this system in detail (110), and it provides a nice example of a general conical intersection in an inorganic photochemical problem, i.e., one that is not imposed by symmetry via a Jahn-Teller degeneracy. 

The infra-red spectrum of Mo(CO)3[P(OCH3)3]3 (VIII) shows three absorption bands at 1993,1919, and 1890 cm-1 in the region in which CO stretching frequencies usually appear. But Cr(CO)3(CNCH)3 (IX) has two absorption bands in the C—O stretch region at 1942 and 1860 cm-1. Octahedral ML3(CO)3 complexes can exist in either the mer or fac isomeric forms (Figure 9.2). Assign the structures of the above two molecules. How many bands would you expect to see in the vibrational Raman spectra of these two molecules, and for which of these bands would the scattered Raman radiation be polarized ... 

The application of 59Co n.m.r. spectroscopy to the study of cobalt(m) complexes is beginning to expand.285,286 An n.m.r. study of the stereoisomers of propane-1,2-diamine (pn), en, acac, CF3acac, etc., showed that the chemical shifts do not appear to be related to the absolute configuration, but that larger linewidths are observed with lower symmetry complexes 286 The chemical shifts are larger for ob-lel than for mer-fac isomerism in Co(pn)3 +. 285... 

Octahedral complexes with the general formula MA3B3 can exhibit another type of geometric isomerism, called mer-fac isomerism. This can be illustrated with the complex ion [Pt(NH3)3Cl3]+ (see Table lS-1). In one isomer the three similar ligands (e.g., the CC ligands) lie at the corners of a triangular face of the octahedron this is called the r isomer (for facial). 

Comparisons of results from gas-phase and solution photochemical studies may prove valuable in elucidating mechanistic details, but unfortunately very few inorganic complexes are sufficiently volatile for gas-phase studies. One conveniently volatile complex is [Rh(tfac)3], which undergoes mer - fac isomerization on irradiation in the gas phase, but not in benzene or mesitylene. ... 

Aluminium. Adduct formation between [(Bu 2A10Ph)2] and Me0CH2CH20Me has been investigated. Mer-fac Isomerism of [A1(NHC5H4N)3] has been investigated using variable temperature NMR spectroscopy. ... 

Racemization of some macrocyclic Sehiff-base complexes of nickel(n) is reported to involve inversion of both asymmetric secondary nitrogen atoms activation parameters are reported. The mer- fac isomerization of [Ni(oxine)3] is facile in methanol-chloroform solvent mixtures. ... 

In contrast, chromium (ITT) and cobalt(III) form 2 1 dye metal complexes that have nonplanar stmctures. Geometrical isomerism exists. The (9,(9 -dihydroxyazo dyes (22) form the Drew-Pfitzner or y rtype (23) (A = C = O) whereas o-hydroxy—o -carboxyazo dyes (24) form the Pfeiffer-Schetty or fac type (25), where A = CO 2 and C = O. 

The structure of [IrCl2(edmp)2]PF6, (193), edmp = (2-aminoethyl)dimethyl phosphine, confirms that the coordination geometry around the Ir center is approximately octahedral. If the edmp ligand is replaced by edpp, edpp = (2-aminoethyl)diphenylphosphine, then the isomeric form (194) may be isolated and structurally characterized. The X-ray crystal structure of fac-[Ir(edmp)3]Cl3 5H20 has also been reported.338... 

As far as the fac/mer isomerization is concerned, the typical redox-induced conversion is that illustrated in Scheme 3 for complex [MnI(CO)3(dppm)Cl] (dppm = Ph2PCH2PPh2).10... 

In this group, Lj and L2 represent symmetrical terdentate ligands, which may be identical. The main product from the reaction of [Co(NH3)5Cl]Cl2 with dien is the s-fac Co(dien) + isomer 5 a. On heating solutions of this isomer for long times, small quantities of the other isomeric forms, u-fac and mer, result, 5b, and c (R, =R2=H). Separation of the three forms is carried out by using Sephadex cation exchange columns. Characterization of the isomers is possible by nmr which together with absorption spectroscopy, is used to study the interconversions. 

The same reactions were followed by HP NMR using selective CO and C-Mel labeling. The HPNMR results taken with the IR data suggest that it is the fac, cis isomer of [Ir(C(0)Me)(C0)2l3] which forms initially at 25 °C in CH2Cl2/MeOH and that this partially isomerizes to the mer,trans isomer on heating. Both isomers are formed in broadly equal amounts in the higher temperature reaction in PhCl... 

The Pickett s model and extensions thereof have the potential to deal with such situations, namely involving cis/trans or mer/fac octahedral complexes, provided the Eg and p parameters are known for the corresponding isomeric metal centers. Although this application still remains virtually unexplored, the following case is illustrative. The trans- and cis-(ReCl(dppe)2 centers display quite different values of those parameters Eg = 0.68 V, p = 3.4 [21] for the former, and Eg = 0.41 V, P = 1.88 [24] for the latter. Hence, for example, for the carbonyl (Pi = 0) complexes [ReCl(CO)(dppe)2], the cis isomer is oxidized at a lower potential than the trans (0.41 vs. 0.68 V), whereas for the nitrile [ReCl(NCR)(dppe)2] complexes, with Pi (NCR) in the range from —0.23 to —0.33 V [24] (Table 4), the reverse occurs. 

Lever has successfully predicted Mn"/ potentials of 24 Mn-carbonyl complexes containing halide, pseudohalide, isonitrile, and phosphine co-ligands, with additivity parameters derived from the potentials of Ru "/" couples [39]. An important consideration for heteroleptic complexes is the influence of isomerism on redox thermodynamics. For Mn(CO) (CNR)6- complexes, with n = 2 or 3, the Mn"/ potentials for cis/trans and fac/mer pairs differ by as much as 0.2 V [40]. The effect arises from the different a-donor and 7r-acceptor abilities of carbonyl (CO) and isocyanide and their influence on the energy of the highest energy occupied molecular orbital (HOMO). 

CpMnfCO jfCNR) (x = 1-3) (180) have been completed for alkyl and aryl isocyanides. The product Re(CO)2(CNMe)3Br was isolated in three isomeric forms, fac, mer-trans, and trans, an important precedent in substitutions of M(CO)sBr (184). 

---