The crystal structures of the terdentate ligands 2,2' : 6',2"-terpyridine (terpy) and 2,6-bis(pyrazol-l -yl)pyridine (bppy) were determined by single-crystal diffraction studies. The compound terpy crystallizes in the non-centrosymmetric orthorhombic space group P2,2,2, with a = 3.947(1), b = 16.577(7) and c = 17.840(6) A; the structure was refined to R = 0.0745 for all 1087 independent data and R = 0.0470 for those 609 data with F > 60-(F). The compound bppy crystallizes in the centrosymmetric orthorhombic space group Pnma with a = 11.929(3), b = 21.320(3) and c = 3.889(0) A; this structure was refined to R = 0.0409 for all 896 independent data and R = 0.0300 for those 723 reflections with F > 60(F). The structures of the free terpy and bppy ligands were compared directly with the structures of the co-ordinated terpy and bppy ligands in the [RuL(NO,)(PMe,),] [CIO,] complexes (1 = terpy 1 or bppy 2) in order to determine if any ligand structural changes occur upon co-ordination to ruthenium. To act as terdentate ligands, it was observed that both terpy and bppy must adopt the cis,cis ligand configuration a s opposed to the trans,trans configuration found in the solid state and as the equilibrium configuration in solution. Both terpy and bppy distort upon co-ordination t o ruthenium. The greatest distortions for terpy occur primarily at the central pyridine ring. Large distortions were observed for bppy at both in the central pyridine ring and in the terminal pyrazole rings.

Mean metal−ligand bond distances for the coordination ligands isothiocyanate, pyridine, imidazole, water, and chloride, bound to the transition metals Mn, Fe, Co, Ni, Cu, and Zn in their 2+ oxidation states, were collected from searches the Cambridge Structure Database. The metal−ligand bond distances were converted to bond orders through the bond distance-bond order technique, as suggested by Pauling. The mean bond order sums at the 2+ metal centers were found to be independent of coordination number or geometry and to be strongly ligand-dependent; the values (by ligand) are as follows:  isothiocyanate = 2.56 ± 0.13; imidazole = 2.13 ± 0.04; chloride = 2.12 ± 0.07; pyridine 1.95 ± 0.10; water = 1.88 ± 0.10. The bond order sum for Fe(III) bound to chloride was found to be 3.09, approximately one bond order unit larger than for the 2+ metal centers bound to chloride. Division of the ligand-specific bond order sums by coordination number allows prediction of the M−L bond distance to within 0.017 Å, regardless of the specific coordination geometry. The physical basis for the ligand-specific variation in bond order sum is also discussed.

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The forces responsible for the observed geometries of the YX(3) (Y = N or P; X = H, F, or Cl) molecules were studied through ab initio computations at the HF-SCF/6-31G level. The calculated molecular orbitals were grouped as contributing primarily to (a) the covalent bonds, (b) the terminal atom nonbonding electrons (for X = F or Cl), and (c) the central atom nonbonding electrons. This grouping was accomplished through 3-D plotting and an atomic population analysis of the molecular orbitals. The molecules were then moved through a X-Y-X angular range from 90 degrees to 119 degrees, in four or five degree increments. Single-point calculations were done at each increment, so as to quantify the energy changes in the molecular orbital groups as a function of geometry. These calculations show that the nonbonding electrons are much more sensitive to geometry change than are the bonding orbitals, particularly in the trihalide compounds. The molecular orbitals representing the nonbonding electrons on the terminal atoms (both valence and core electrons) contribute to the spreading forces, as they favor a wider X-Y-X angle. The contracting forces, which favor a smaller X-Y-X angle, consist of the orbitals comprising the nonbonding electrons on the central atom (again, both valence and core electrons). The observed geometry is seen as the balance point between these two sets of forces. A simple interaction-distance model of spreading and contracting forces supports this hypothesis. Highly linear trends are obtained for both the nitrogen trihalides (R(2) = 0.981) and phosphorus trihalides (R(2) = 0.992) when the opposing forces are plotted against each other. These results suggest that a revision of the popular conceptual models (hybridization and VSEPR) of molecular geometry might be appropriate.