James F. O'Brien

A Mössbauer effect study of the bonding in several organoiron carbonyl clusters

After a brief review of the applications of the Mössbauer effect to cyclopentadienyl containing compounds, the chemistry and spectral properties of the various iron carbonyl complexes are described. The electronic properties of a series of trinuclear and tetranuclear organoiron clusters have been investigated through Fenske-Hall self-consistent field molecular orbital calculations, and the results are compared with the Mössbauer effect isomer shifts. A linear correlation is found between the Slater effective nuclear charge, as calculated from the Fenske-Hall partial orbital occupancy factors, and the isomer shift. In these compounds the 4s orbital populations are rather constant. However, thecis andtrans isomers of [CpFe(CO)2]2 have a significantly lower 4s orbital populations. In this case, the reduced 4s population must be accounted for by adding it to the effective nuclear charge to obtain a good correlation with the isomer shift.

Theoretical Comparison of the Bonding in CpCr(CO)2(NX) [X = O, S, Se, Te]

Molecular orbital calculations employing the PM3 model have been used to examine the bonding in the complexes CpCr(CO)2(NX) (X = O, S, Se, Te). The previously established trend of increasing Cr-N interaction as X changes from O to S is demonstrated by these calculations, and found to extend to Se and Te. Bond lengths, bond orders, vibrational frequencies, and heats of reaction are used to support the conclusion that metal to ligand π-backbonding increases down the periodic chart from NO to NTe.

THEORETICAL COMPARISON OF THE BONDING OF CHALCOCARBONYL LIGANDS

Abstract Fenske-Hall molecular orbital calculations on the complexes CpFe(CO)2(CX)+ (X = O, S, Se, and Te) have been used to quantify the nature of bonding between the CX ligands and the metal atom. In addition, conclusions have been reached about the reactivity of the complexes under both nucleophilic and electrophilic attack. The previously established trend of increasing metal—ligand bond strength as X changes from O to S to Se is demonstrated by our molecular orbital calculations, and found to extend to Te. The mechanism for nucleophilic attack, variously explained in the past by either charge control or orbital control, is quantitatively ascribed to orbital control only. The nature of electrophilic attack on these complexes is also found to begin with orbital control.

Bonding in VClx(CH3CN)3−x6−x

Abstract Solutions of vanadium(III) chloride in acetonitrile contain the solvate VCl3(CH3CN)3. NMR spectra indicate that bound solvent molecules exchange positions with bulk solvent molecules in two distinct processes. One proposal claims that these two processes are due to separate exchange rates for the facial and meridional isomers of VCl3(CH3CN)3. Another suggestion that has been made is that the two different types of acetonitrile molecules in meridional VCl3(CH3CN)3 undergo chemical exchange at different rates, and that no facial VCl3(CH3CN)3 is involved. Extended Huckel calculations have been done in order to examine the bonding in facial and meridional VCl3(CH3CN)3. The results suggest that acetonitrile molecules trans to Cl atoms have one rate of exchange, while those trans to another acetonitrile have a different rate, regardless of which isomer is present. Calculations have been done on other vanadium, chlorine, acetonitrile species in order to clarify the effect of changing the trans or the cis neighbor.

Investigation of Alkali Metal-Water Interactions in Acetonitrile

Abstract The exchange of acetonitrile into and out of the coordination sphere of alkali metal ions is fast on the NMR time scale and slow on the vibrational time scale. This results in separate infrared and Raman spectra for coordinated and non-coordinated solvents in solutions of alkali metal salts in acetonitrile. The 1H NMR spectra, on the other hand, are averages of coordinated and non-coordinated environoments. Addition of water to solutions of alkali metal salts in acetonitrile indicates that the cation is again preferentially solvated by water. The water completely replaces acetonitrile in the lithium coordination sphere until four waters have been added. Water also displaces the first acetonitrile from the coordination sphere of sodium and potassium ions. However, when additional water is added there is an equilibrium mixture of various species. Spectroscopic study of these solutions allows one to gain insights about the energetics and geometry of the cation. Experimental results from NMR, Raman and IR measurements along with extended Huckel molecular orbital calculations are used to discuss the alkali metal cation- water-acetonitrile system.