Ab initio calculations have been carried out on the first‐ and second‐row transition‐metal mono‐ and dicarbonyl positive ions. The bonding in these systems is discussed in detail. Trends in the series of mono‐ and dicarbonyl ions and between the first‐ and second‐row transition metals are explained in terms of a dominantly electrostatic bonding interaction and differences in metal ion state separations, ionization potentials, and s and d orbital sizes. Dissociation energies are presented and a detailed comparison is made with experimental data. Where reliable experimental data exists, agreement with the theoretical results is generally good. An exception is Mo(CO)⁺_1,2, where the computed binding energies are much smaller than the experimental values.

The bonding of water to the transition metal positive ions is electrostatic in origin. The electrostatic bonding is enhanced by a variety of mechanisms: mixing in 4p character, 4s–3d hybridization, and 4s promotion into the compact 3d orbital. The importance of these effects varies between the different metal ions due to changes in the separation of the metal ion atomic states. Furthermore the changes in the metal–water repulsion when a second water is added also changes the relative importance of the different metal asymptotes. The second water binding energy varies from being 11 kcal/mol smaller than the first for Mn⁺ to 3 kcal/mol larger for V⁺ and Fe⁺.

Ab initio calculations have been performed for Cr(CO)₆ and Fe(CO)_n, n=1,5. Basis sets of better than double zeta quality are used and correlation is included using the modified coupled‐pair functional method. The computed geometries and force constants are in reasonable agreement with experiment. The sequential bond dissociation energies of CO from Fe(CO)₅ are estimated to be: 39, 31, 25, 22, and ≳5 kcal/mol. We note that the first bond dissociation energy is relative to the singlet ground state of Fe(CO)₅ and the lowest singlet state of Fe(CO)₄, whereas the second is relative to the ground triplet states of Fe(CO)₄ and Fe(CO)₃. In addition, the binding energy for Fe–CO would be modified to 18 kcal/mol if dissociation occurred to the Fe(⁵F) excited state asymptote. The CO binding energies for Fe and Cr are found to be in poorer agreement with experiment than those found in a previous study on Ni(CO)₄. The origins of this difference are discussed.

The binding energy of H2O to transition-metal ions is determined not only by the electrostatic interaction, but by 4s4p and 3d4s hybridization and by 4s to 3d promotion on the metal. All of the M(H2O)+2 systems are linear except for Mn(H2O)+2 and Zn(H2O)+2, which we show to be bent with 7A1 and 2A1 ground states, respectively. The O–Mn–O bond angle is 94° and the O–Zn–O bond angle is 99°. These systems bend to allow 4s4p hybridization, which reduces the repulsion. For Mn+, the 3dn4s1–3dn+1 separation is too large to allow 3d4s hybridization or 4s to 3d promotion. For Zn+ the ground-state 3d104s1 occupation precludes 4s to 3d promotion or 3d4s hybridization.