Metal localized

The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons impurities in the metal, localized stresses, metal grain size or composition differences, discontinuities on the surface, and differences in the local environment (eg, temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling is usually a more serious problem than equipment failure. 

A) Metal localized scrambling, usually seen in M(CO) or M(CO) L groups. 

Table XI summarizes reports of ligand fluxionality at "very mixed -metal clusters. A number of studies (c.g.. Refs. 28, 8I, 116, 213, 214) have reported that both metal-localized and global carbonyl fluxionality occur but give little mechanistic detail. The following discussion focuses on the examples for which detailed studies have been undertaken and/or those for which mechanistic speculation is available.

Exchange piUhways A. metal localized ligand scrambling B. 

Distinct peaks at the positive (+) and negative (-) regions of the voltammogram (Fig. 17.4), are observed. The cyclic voltammogram of [Re(CO)3Cl]jtpbq is dominated by metal-localized oxidation and ligand-localized reductions, as observed in analogous complexes. Reduction in the complexes may be attributed to the BL/BL... 

These tru 5-dioxoruthenium(VI) complexes have characteristic UV-vis absorption spectra. The fj-saturated nature of the macrocyclic tertiary amine ligands enables the high-energy metal-localized transition to be observed. " The weak vibronic structured band at 370-400nm has been assigned to (0 ) —> Ru charge transfer transition that is vibronically coupled to the... 

The UPS of the bent bis(cyclopentadienyl) metal chlorides (CSH5)2MC12 and (CsH4Me)2MCl2, M = Ti, V have also been measured and interpreted with the aid of Fenske-Hall MO calculations (235). In the Ti complexes the 3a, metal-localized MO is vacant, while in the V complexes this orbital is singly occupied, leading to the additional spectral complexities that are expected for the ionization of an open-shell system comparable studies have been made of analogous Zr and Hf compounds (64a). 

Eig. 23. Ionization energy data for free atoms and for ionization of metal-localized a yg and e2g molecular orbitals. (Data from Refs. 117 and 119.)... 

The hydrides HMn(PF3)s and HMn(CO)s are expected to be similar with respect to their molecular and electronic structures. Since the sequence of the first three valence MOs in HMn(CO)s is ai(oM(,h) < b2(3dxy) < e(3dxy, 3dyz), a comparable ordering is anticipated for HMn(PF3)5. However, in the UPS of the latter (169) only two low IE peaks are apparent, consequently it has been assumed that the e and b2 metal-localized MOs are degenerate (Table XXIX). 

UPS studies of some dithiocarbamates (50) and acetylacetoneiminato (64, 130a) complexes have also been reported. Precise assignments have not been made however, the ionization of the metal-localized MOs appears at higher energy than those of the ligand-type MOs. Comparable studies of diethyldithio-phosphate (212) and difluorodithiophosphate (6) complexes of several metal ions have also been reported. 

The HOMO and LUMO of quadruply bound metal dimers invariably have their provenance in metal-localized orbitals. As a consequence the addition to or removal of electrons from these dimers necessarily alters the metal electron configuration, and investigation of the reduced or oxidized species should provide information about the bonding, nonbonding, or antibonding character of the orbital involved. 

Metal localized ligand scrambling, most commonly seen in the M(CO)3 or M(CO)2L groups. For Pt clusters some examples of PtL2 localized scrambling have also been observed. 

---