titanium as compared to vanadium. With this in mind, the intrinsic bond strengths of TiV and V2 are expected to be similar, with V, somewhat more strongly bound. This is indeed found to be the case, with the two molecules having intrinsic bond strengths of 3.13 eV (TiV) and 3.25 eV (V?). In this case, the great difference in measured bond strengths for the two molecules (0.68 eV) results primarily from the increased promotion energy which is required to prepare the titanium atom for bonding (0.8 1 eV, as compared to 0.25 eV for vanadium).The remaining species listed in Table I (Ni,, NiPt, and Pt,) are all late-transition-metal diatomics for which the nd orbitals are quite contracted. In Ni, this contraction is so severe that 3d contributions to the chemical bond are essentially absent. In Pt,, however, relativistic contractions of the ns orbitals lead to better shielding of the 5d orbitals from the nuclear charge, causing the 5d orbitals to expand and become more accessible for chemical bonding. As a result, the intrinsic bond strength of Pt, is 0.85 eV greater than that of its coinage group congener, Au,. This implies a very strong interaction between the 5d orbitals on platinum, and suggests s~~du:d?~~d6:d6:~d?r;4 as the primary electronic configuration of fit,, giving a net bond order of 2 for the Pt, molecule. The NiPt molecule falls midway between Ni, and Pt2 in its bond strength, undoubtedly because the combination of a very small 3d orbital on nickel with a large, accessible 5d orbital on Pt gives a 3d-5d bond intermediate in strength between those found in Ni, and Pt,.
V. ConclusionsAbrupt predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiV, V2, TiCo, and VNi, permitting the bond strengths of these molecules to be determined as D,(TiV) = 2.068 f 0.001, D0(V2) = 2.753 f 0.001, Do(TiCo) = 2.401 f 0.001, and D,(VNi) = 2.100 f 0.001 eV. These molecules joinNiPt,18 and Pt2I4 as transition-metal diatomics for which an abrupt predissociation threshold in an extremely congested electronic spectrum has been used to measure bond strengths.An argument for the requirements needed to determine the bond strength by the onset of p r e d i i a t i o n has been presented, yielding two criteria which must be fulfilled for the method to be suocessful. First, the molecule must possess a very large density of electronic states at its lowest dissociation limit. Second, the lowest dissociation limit must generate repulsive electronic states since predissociation in a set of nested potential energy curves may not be efficient.The chemical bonding in TiV, V,, TiCo, VNi, Ni,, NiPt, and Pt, has been discussed in relation to the electronic configurations of the ground-state molecules (in so far as they are known). The contribution of the d orbitals to the measured bond strength has also been considered by taking into account the promotion energy required to prepare the atoms for bonding and by comparison with the filled d-subshell coinage metal diatomics. The d-orbitalcontributions t...