Carbonyls, metal bond strengths

The 2e oxidation of the metal-carbonyl anion to the dimer (2M — M2 -1- 2e ) is evaluated as a combination of the oxidation potential of the anion and the metal-metal bond strength [44]. 

Lupinetti, A.J., Jonas, V., Thiel, W., Strauss, S.H. and Frenking, G. (1999) Trends in Molecular Geometries and Bond Strengths of the Homoleptic d Metal Carbonyl Cations [M(CO)J ... 

When propylene chemisorbs to form this symmetric allylic species, the double-bond frequency occurs at 1545 cm-1, a value 107 cm-1 lower than that found for gaseous propylene hence, by the usual criteria, the propylene is 7r-bonded to the surface. For such a surface ir-allyl there should be gross similarities to known ir-allyl complexes of transition metals. Data for allyl complexes of manganese carbonyls (SI) show that for the cr-allyl species the double-bond frequency occurs at about 1620 cm-1 formation of the x-allyl species causes a much larger double-bond frequency shift to 1505 cm-1. The shift observed for adsorbed propylene is far too large to involve a simple o--complex, but is somewhat less than that observed for transition metal r-allyls. Since simple -complexes show a correlation of bond strength to double-bond frequency shift, it seems reasonable to suppose that the smaller shift observed for surface x-allyls implies a weaker bonding than that found for transition metal complexes. 

It is commonly accepted that chemisorption of CO on transition metals takes place in a way that is quite similar to bond formation in metal carbonyls (4). First experimental evidence for this assumption was obtained from a comparison of the C—O stretching frequencies (5) and was later confirmed by data on the bond strength (6) as well as by valence and core level ionization potentials obtained by photoelectron spectroscopy (7). Recent investigations have in fact shown that polynuclear carbonyl compounds with more than about 3-4 metal atoms exhibit electronic properties that are practically identical to those of corresponding CO chemisorption systems (8, 9), thus supporting the idea that the bond is relatively strongly localized to a small number of metal atoms forming the chemisorption site. 

The mass spectra of binuclear cyclopentadienylmetal carbonyl derivatives of chromium and molybdenum provide useful indications of the relative strengths of the metal-metal bonds in various compound types. The mass spectrum of [C5H5Cr(CO)3]2 (S. M = Cr) exhibits no bimetallic ions but only monometallic ions such as CsHsC CO) (n = 3,... 

Bond stretching frequency (measured by vibrational spectroscopy such IR spectroscopy, etc.) is related to stretching force constant and increases with increase in bond strengths. This study is quite useful for transition metal carbonyls. 

For studies of reactivities of such metal-metal-bonded carbonyls to be directly related to the strengths of the metal-metal bonds reactions have to be found for which homolytic fission of the metal-metal bond is the rate-determining step. If the fission of the metal-metal bond is reversible then a reaction scheme as shown in eq 5-7 can be envisaged. The radical species M is... 

---