The basis set superposition error (BSSE) for the Cr(CO) 5 -CO and (CO) 5 Mn-Mn(CO) 5 bonds is calculated for a large variety of STO basis sets. All investigated metal basis sets, being at least TZ for 3d and DZ for 4s are adequate. Ligand basis sets of TZD quality or better are required in order to have a low BSSE (6-8 kJ/mol or less) for both the metal-ligand and the metal-metal bond. When the ligand s, p basis set is of double-quality, the BSSE is significant for the metal-ligand bond (16-22 kJ/mol depending on the polarization functions), but it is partly canceled by a basis set incompleteness error of opposite sign. For the metal-metal bond, the BSSE for DZ s, p ligand bases is unacceptably large (33-57 kJ/mol), leading to much too high bond energies if no correction for BSSE is applied. In general, the bond energies after correction for BSSE are rather stable. It is remarkable that for the metal-metal bond, but not for the metal-ligand bond, there is for all pure s, p ligand bases after correction for the BSSE a discernible basis set incompleteness error (ca. 15 kJ/mol), which only disappears after adding at least one polarization function. Agreement of the converged results for both geometries and bond energies with experiment is excellent.

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The photochemistry of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)PH(3) and ax-Fe(CO)(4)PH(3) is studied with time-dependent DFT theory to explore the propensity of the excited molecules to expel their ligands. The influence of the PH(3) ligand on the properties of these complexes is compared with the photodissociation behavior of the binary carbonyl complexes Cr(CO)(6) and Fe(CO)(5). The lowest excited states of Cr(CO)(5)PH(3) are metal-to-ligand charge transfer (MLCT) states, of which the first three are repulsive for PH(3) but modestly bonding for the axial and equatorial CO ligands. The repulsive nature is due to mixing of the initial MLCT state with a ligand field (LF) state. A barrier is encountered along the dissociation coordinate if the avoided crossing between these states occurs beyond the equilibrium distance. This is the case for expulsion of CO but not for the PH(3) group as the avoided state crossing occurs within the equilibrium Cr-P distance. The lowest excited state of ax-Fe(CO)(4)PH(3) is a LF state that is repulsive for both PH(3) and the axial CO. Excited-state quantum dynamics calculations for this state show a branching ratio of 99 to 1 for expulsion of the axial phosphine ligand over an axial CO ligand. The nature of the phosphorus ligand in these Cr and Fe complexes is only of modest importance. Complexes containing the three-membered phosphirane or unsaturated phosphirene rings have dissociation curves for their lowest excited states that are similar to those having a PH(3) ligand. Analysis of their ground-state Cr-P bond properties in conjunction with frontier orbital arguments indicate these small heterocyclic groups to differ from the PH(3) group mainly by their enhanced sigma-donating ability. All calculations indicate that the excited Cr(CO)(5)L and Fe(CO)(4)L molecules (L = PH(3), PC(2)H(5), and PC(2)H(3)) prefer dissociation of their phosphorus substituent over that of an CO ligand. This suggests that the photochemical approach may be a viable complement to the ligand exchange and redox methods that are currently employed to demetalate transition metal complexed organophosphorus compounds.

Density functional calculations were applied to obtain binding energies for metal cation-oligomer complexes of n-alkanes and poly(ethylene glycol)s (PEG). The B3LYP/6-31G* energies for complexing metal cations (Na+, Li+, Co+, Cu+, Zn+, and Zn2+) with straight chain aliphatics (C n H2 n +2, n = 1−12) are in excellent agreement with the limited available experimental and theoretical data. The strength of the complexes increases with an increasing degree of polymerization and with a decreasing size of the metal ion. The weakest calculated complex is Na+−CH4 (7.8 kcal/mol) and the strongest is Co+−dodecane (52.0 kcal/mol). Smaller sized cations, such as Li+, induce more polarized hydrocarbons. Transition metals give stronger complexes than the main group metals because their d-electrons shield the nuclear charge less effectively. M+−methane binding energies, ranging from 12.6 to 23.1 kcal/mol, are also reported for Sc+, Y+, La+, Cu+, Ag+, and Au+. Doubly charged metal ions give much stronger complexes, that is, 67.1 kcal/mol for Zn2+−CH4. Methane binding energies of ca. 20 kcal/mol are obtained when the Be2+, Mg2+, Fe2+, and Zn2+ dications are ligated with a cyclopentadienyl anion. The poly(ethylene glycol)s (HO−[C2H4O] n −H, n = 1−5) bind significantly stronger to the metal cations (Na+ and Cu+) than the aliphatics. The Na+−monomer (C2H6O2) already has a complex strength of 48.5 kcal/mol, while that of the pentamer (C10H22O6), which is the smallest observable Na+−complex by MALDI-TOF-MS, amounts to 88 kcal/mol. The corresponding Cu+ complexes are even stronger with a value of 79.6 kcal/mol for Cu+−C2H6O2. Binding energies of 53.3 and 64.1 kcal/mol are calculated for the respective K+−tetramer and K+−pentamer of which the pentamer is also observed spectroscopically.