METAL-LIGAND BOND DISTANCES IN FIRST-ROW TRANSITION-METAL COORDINATION-COMPOUNDS - COORDINATION-NUMBER, OXIDATION-STATE, AND SPECIFIC LIGAND EFFECTS

Mean metal-ligand bond distances for the coordination ligands isothioc yanate, pyridine, imidazole, water, and chloride, bound to the transit ion metals Mn, Fe, Co, Ni, Cu, and Zn in their 2+ oxidation states, we re 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 coo rdination number or geometry and to be strongly ligand-dependent; the values (by ligand) are as follows: isothiocyanate = 2.56 +/- 0.13; imi dazole = 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 chl oride was found to be 3.09, approximately one bond order unit larger t han 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 Angstrom, regardless of the spec ific coordination geometry. The physical basis for the ligand-specific variation in bond order sum is also discussed.