A. Foffani

Influence of the ligand donor ability on the ionization potentials and fragmentation patterns of transition-metal nitrosyl complexes

Abstract The molecular ionization potentials (I.P.s) and the fragmentation patterns of the complexes Co(CO) 3 NO, Co(CO) 2 (NO)PCl 3 , Fe(CO) 2 (NO) 2 , Fe(CO)(NO) 2 P(OC 2 H 5 ) 3 and C 5 H 5 NiNO, and the I.P.s of the substituent ligands CO, PCl 3 and P(OC 2 H 5 ) 3 have been determined. These last I.P.s are a measure of the σ-donor ability of the ligands themselves; the importance is illustrated to account for this donor ability on evaluating the influence of the ligands on the molecular I.P.s, on the fragmentation paths and on the carbonyl and nitrosyl vibrational stretching frequencies of the complexes to which they participate. The primary ionization process in these complexes seems to involve an electron pertaining to an orbital of an essentially metallic type and with a partial π character. The fragmentation paths by electron impact of the present nitrosyl complexes are interpreted as involving parallel cascade decay processes. This interpretation considers both the probability of the single fragmentation steps of the cascade and the possible nature of the transitional states involved.

Correlation between the ionization potentials of transition metal complexes and of the corresponding ligands

Abstract The ionization potentials ( IP s) of several monosubstituted carbonylic complexes of the type M(CO) 5 L, Fe(CO) 4 L and Co(CO) 2 NOL (M = Cr, Mo and W; L = PX 3 or CNR) together with the corresponding phosphinic and isonitrilic ligand IP s, were measured. Good linear correlations were found between the complex and ligand IP s for all the examined series. The slopes of the correlation lines for the phosphinic complexes are very near each other, reflecting the fact that what is in common along the five studied series, is a ligand with a phosphor donor atom. The slope of the correlation line for Mo(CO) 5 CNR complexes is much lower and reversed in sign with respect to that for the corresponding phosphinic complexes. The unsubstituted complexes give, in all the cases reported, points well outside the corresponding correlation lines. The results are discussed in terms of the nature of the last occupied molecular orbital in these complexes, which appears to be mainly localized to the metal-ligand ML bond. The influence of the substituent ligands on the charge distribution to this bond is examined.

Kinetics of monosubstitution reactions of phosphane-, arsine- and stibine-ligands with CO(CO)3NO.

Abstract The kinetics of the ligand substitution reactions between Co(CO) 3 NO and P(C 6 H 5 ) 3 As(C 6 H 5 ) 3 ,Sb(C 6 H 5 3 1,2 bis(diphenylphosphino)ethane has been studied. For the first three ligands it was found active a kinetic two term rate law for the monosubstitution reaction of the type v = (k 1 + k′[L]) [Co(CO) 3 NO], coming from a comaparable contribution of a first- and a second-order path. There is evidence, also from activation enthalphy and entropy data, that the first-order path be a dis sociative one; the k 1 values for all three ligands are almost coincident in a reasonably wide range of temperature. The second-order path is assigned as an associative one, the value of its overall rate constant k′ being affected by ligand variation in the order P(C 6 H 5 ) 2 CH 2 CH 2 P(C 6 H 5 ) 2 > P(C 6 H 5 ) 3 »As(C 6 H 5 ) 3 .Sb(C 6 H 5 ) 3 consequently, the relative contribution of the first-order path to the overall reaction rate increases for the last three ligands in the reversed order, mean figures for a ligand concertration of 10 −1 M being 0.8, 36 and 85% resp. with P(C 6 H 5 ) 3 and Sb(C 6 H 5 ) 3 . Such a behaviour has been interpreted as a function of the basicity. π-acceptor ability and polarizability of the ligands concerned, the first two factors appearing in these cases to prevail. Also the first-order chelation process with 1,2-bis(diphenylphosphino)ethane, following monosustitution, appears from thermal data to be an associative one.

Kinetics of exchange reactions of 14CO with monosubstituted derivatives of Co(CO)3NO and with the parent complex

Abstract The 14 CO exchange kinetics with Co(CO) 3 NO and derivatives of the type Co(CO) 2 NOPX 3 (X = C 6 H 5 , n−C 4 H 9 , OC 6 H 5 and OCH 3 ) have been studied in toluene solution. The monosubstituted complexes follow a two-term rate law v = k 1 [C]+k 2 [C][CO]. For the first-order term, the variations of k 1 and of the thermal parameters have been interpreted as arising from the size of the substrate ligand and the coordinating intervention of the solvent in the transition state. For the second order term, the high values of k 2 with respect to the corresponding ligand disubstitution reactions have been mainly attributed to the π-accepting ability of the entering CO ligand. The parent complex follows a first-order rate law, whose rate constants and thermal parameters differ remarkably from those for the corresponding ligand monosubstitution reactions; possible explanations of this behaviour have been presented.

Solvent effects on the kinetics of the monosubstitution reaction of Co(CO)3NO with triphenylarsine and triphenylphosphine

The kinetics of monosubstitution reactions of Co(CO)3NO with As(C6H5)3 and P(C6H5)3 in cyclohexane, toluene, nitromethane, tetrahydrofuran, acetonitrile and dimethylsulfoxide fit the two-term equation v = (k1 + k′·[L])·[S], where L and S refer to the ligand and the substrate. The rate constant, k′ for the second-order process (associative with the ligand) varies over a 5-fold range with the change in the dielectric constant of the solvent. The rate constant, k1, for the first-order process changes by factor of less than 2 with the dielectric constant in the first three, non-coordinating, solvents, but in the latter three the variation is 103 times greater. This large increase is accounted for in terms of an associative intervention of the solvent in the first-order process. This interpretation is supported by the activation parameters, which are typical of a dissociative mechanism for the first three non coordinating solvents, while for the latter three they tend towards values typical of an associative mechanism.

Dipole moments and molecular conformations of (C6H5)3SnFe(CO)2NOL and related complexes

Abstract Dipole moment measurements on (C 6 H 5 ) 3 SnFe(CO) 2 NOL complexes[LCO, P(C 6 H 5 ) 3 , As(C 6 H 5 ) 3 , P(OC 6 H 5 ) 3 ] indicate that the ligand L is in a cis position relative to the axial (C 6 H 5 ) 3 Sn grouping, in contrast to the situation with analogous Co and Mn complexes, for which both the ligands are in the axial positions of the trigonal bipyramidal structure. Similarly, it is found that the two P(OC 6 H 5 ) 3 ligands in the trimetallic linear complex Hg[Fe(CO) 2 NOP(OC 6 H 5 ) 3 ] 2 do not both occupy axial positions and probably both occupy equatorial positions. There is evidence that in these compounds the metal-L-group dipole moments are more dependent on the electron density at the central metal, as determined by the presence of σ or π acceptor groupings, than on the nature of the central metals and the structures of the complexes.

Kinetics and equilibria in the replacement by carbon monoxide of the ligands of Co(CO)2(NO)L complexes

Abstract Equilibrium and rate data have been estimated for the reverse ligand substitution reaction Co(CO)2(NO)L + CO ⇄ Co(CO)3(NO) + L with L=Sb(C6H5)3, As(C6H5)3 and P(C6H5)3 in toluene solution. The kinetics follow a two-term rate law v=k−1·[C] + k−2·[C]·[CO]. Variations in the equilibrium and rate constants appear to be mainly associated with the σ-donor abilities of the substrate ligands. The results are discussed in terms of the possible reaction mechanism.

Kinetics of disubstitution reactions of 1,2-bis(diphenylphosphino) ethane with monosubstituted derivatives of Co(CO)3NO

Abstract The kinetics of reaction between a series of monosubstituted cobalt carbonyls Co(CO) 2 NOL (with L = Sb(C 6 H 5 ) 3 , As(C 6 H 5 ) 3 , P(C 6 H 5 ) 3 , P(OC 6 H 5 ) 3 , P(OCH 3 ) 3 , and P(−C 4 H 9 ) 3 ) and the same bidentate ligand 1,2-bis(diphenylphosphino)ethane, have been studied. In all cases the end product is the disubstituted chelate complex Co(CO)NOP(C 6 H 5 ) 2 CH 2 CH 2 P(C 6 H 5 ) 2 . For L = P(C 6 H 5 ) 3 and P(OCH 3 ) 3 there are formed to an appreciable extent, at relatively low ligand-to-complex conc. ratios, also the disubstituted complexes Co(CO)NOL 2 . There is good evidence that the associative reaction mechanism goes through a path involving a primary step of substrates ligand displacement, followed by chelation. Only for Co(CO) 2 NOP(OCH 3 ) 3 , a primary step of substrate ligand displacement, follow-ligand again followed by chelation, is competitive with the above one. The order of reactivity for primary ligand displacement is the following: Sb(C 6 H 5 ) 3 >CO> As(C 6 H 5 ) 3 >P(OC 6 H 5 ) 3 >P(C 6 H 5 ) 3 >P(n−C 4 H 9 ) 3 . It is interpreted on the basis of the varied basicity and polarizability of the substrate ligand, and in specific cases of their additional π-bonding ability. Steric factors do not seem to play a relevant role.

Mass spectrometric study of paramagnetic and diamagnetic transition metal complexes

Abstract The mass spectra and the ionization potentials of several paramagnetic and diamagnetic transition metal complexes of the type M(NO)2HaIPR3 [M = Co, Fe; Hal = Br, I; PR3 = P(nC4H9)3, P(iOC3H7)3, P(C6H5)3] have been measured. Both the mass spectra and the ionization potentials of these two classes of complexes are very similar to each other. The mass spectral results are interpreted in terms of the driving forces of the ionization-fragmentation processes. From the ionization potential values, information on the nature of the last occupied orbitals of these complexes are obtained.