Inorg. Chem. 2003, 42, 2833−2835
Structural Measure of Metal−Ligand Covalency from the Bonding in Carboxylate Ligands Rosalie K. Hocking and Trevor W. Hambley* Centre for HeaVy Metals Research, School of Chemistry, Sydney UniVersity, Australia 2006 Received February 22, 2003
The data set of over 40 000 crystal structures containing the carboxylate group that has been reported to the CSD has been used to extract structural changes to the carboxylate group upon binding to different elemental centers. We find quantifiable structural changes to the carboxylate group depending on the elemental center it is interacting with. The trends follow those traditionally associated with covalency; elements exhibiting electronegativity closest to that of oxygen exhibit the largest structural change. In addition, we find the measure is extendable to transition metal systems where we observe the trends of Pauling neutrality not only are maintained but also are quantifiable; i.e., the structural change increases with oxidation state, i.e., II < III < IV, and decreases with an increase in coordination number, 4c > 5c > 6c. Further, the measure gives us a quantifiable measure of the difference between the covalencies of the long and short bonds of Cu(II) complexes. From the bond lengths of the bound carboxylate arm, we are able to derive bond orders and hence calculate the covalent character in the adjoining metal−carboxylate bonds. As such, we have a structurally derived quantification of metal−ligand covalency.
The distinction between ionic and covalent bonding is one of the most fundamental concepts in chemistry.1,2 Despite the fact that these concepts are well accepted, they have proven difficult to quantify experimentally. Covalency has been derived from experimental data ranging from melting points to compound reactivity.3-5 Arguably, the best uniform quantification of covalency is that deriving from the electronegativity difference of two bonding atoms, as proposed by Pauling in the 1940s.2 Despite its reasonable accuracy at * To whom correspondence should be addressed. E-mail: t.hambley@ chem.usyd.edu.au. (1) Smith, R. Conquering Chemistry; McGraw-Hill: Sydney, Australia, 1987. (2) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1960. (3) Huheey, J. E. Inorganic Chemistry Principles of Structure and ReactiVity; Harper & Row: New York, 1978. (4) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry; John Wiley & Sons: New York, 1995. (5) Mingos, D. M. P. Essential Trends in Inorganic Chemistry; Oxford University Press: Oxford, U.K., 1998.
10.1021/ic034198a CCC: $25.00 Published on Web 04/03/2003
© 2003 American Chemical Society
Figure 1. Covalent and ionic bonds to the carboxylate ligand.10,11
determining the relative covalency of main group elements, electronegativity differences provide significantly less insight into transition metal bonding. There have been many subsequent attempts to quantify metal-ligand covalency ranging from the nephelauxetic series developed in the 1950s and 1960s6 to the combinations of DFT and ligand K-edge XAS applied today.7 Arguably, no method to date provides a uniform experimental quantification of covalency in transition metal systems, and this is something we have sought to address. Many chemical species are stabilized by electron delocalization or resonance stabilization. For example, in the case of carboxylate anions8,9 the two C-O bonds will be degenerate unless there is an external interaction that can overcome the resonance stabilization energy. Covalent bonds will exert the greatest effect on the delocalization between the two bonds. If one arm of the carboxylate group is covalently bound to another element, the delocalization in the system will be lost resulting in distinct single and double bonds, as illustrated in Figure 1b. Thus, if metal-carboxylate interactions could be determined to a sufficient resolution, say 5c > 6c. This is illustrated for ZnII in Figure 4, and we also observe it for SnIV, PbII, CoII, and CuII (Figure S15). In addition, the trends of other measures of covalency are followed, including those of the nephelauxetic series,6 XAS experiments,19 and NQR spectroscopy.20 This analysis also allows us to compare the character of the long and short bonds in CuII, an issue that has been debated for many years.21,22 The carboxylate groups bound via long CuII-O bonds exhibit an OA-C bond length of 1.252(0.003) Å, which is within the error of the ionic limit derived from carboxylate anions indicating that the bonding is almost purely electrostatic in origin. This is in contrast to carboxylate groups bound via the short CuII-O bond lengths, which exhibit a significantly longer average OA-C bond length of 1.272(0.002) Å, indicating 22(2)% covalent character in these bonds. From this data analysis, we are able to derive bond orders, in the same manner as Pauling in the 1940s.2,23 However, the data set analysis gives us approximately 100-fold better resolution than the structures of 1950, and still 40-fold better (16) Martin, A.; Orpen, A. G. J. Am. Chem. Soc. 1996, 118, 1464-1470. (17) Orpen, A. G.; Quayle, M. J. J. Chem. Soc., Dalton Trans. 2001, 16011610. (18) Hocking, R. K.; Hambley, T. W. Inorg. Chem. 2002, 21, 2660-2666. (19) Shadel, S. E.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1995, 117, 2259-2272. (20) Nakamura, D.; Ito, K.; Kubo, M. Inorg. Chem. 1963, 2, 61-64. (21) Pasquarello, A.; Petri, I.; Salmon, P. S.; Parisel, O.; Car, R.; Toth, E.; Powell, D. H.; Fischer, H. E.; Helm, L.; Merbach, A. E. Science 2001, 291, 856-859. (22) Solomon, E. I. Curr. Opin. Chem. Biol. 2002, 6, 250-258. (23) Pauling, L.; Sherman, J. J. Chem. Phys. 1933, 1, 679.
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Figure 3. Bond length of the bound arm of the carboxylate group (OA-C) (Å) vs periodic group number, for main group elements. Data represented in this chart are derived from 16 859 observations of element-carboxylate bonds reported to the CSD: 10 000 of these observations represent esters (C), 5247 carboxylic acids (H), 22 group 1, 320 group 2, 300 group 13, 320 group 14 (excluding C), 111 group 15, 459 group 16, and 35 group 17. The data are tabulated by element in the Supporting Information (Tables S2-S5): Na+ xc, Li+ xc, Be2+ 4c, Mg2+ 6c, Ca2+ 8c, Sr2+ 8c, B 4c, Al3+ 6c, Ga3+ 6c, In3+ xc, Tl3+ xc, C 4c, Si 4c, Ge 4c, Sn xc (see Figure S15 for different coordination numbers), Pb xc (see Figure S15), N xc, P xc, As xc, Sb xc, Bi xc, O xc, Te xc, and I 3c. Abbreviations are as follows: xc any coordination number, 1c a coordination number of 1, etc. Asterisk (*) indicates that the data set fails the adjoining single C-C bond criteria, Figures S9-S10. The relationship between the C-OA and C-OB bond lengths and bond order is derived in the Supporting Information. A cubic estimation of the relationship between bond order and C-OA bond length is as follows: bond order(C-OA) ) -114.9764(C-OA)3 + 466.8704(C-OA)2 - 635.1154(C-OA) + 290.4723. Percentage covalent character and bond order are related by the following expression: % covalent character ) {100[(1.5-bond order)2]}. This measure is only valid when applied to data sets that obey the relationship shown in Figure 2; it should only be applied to individual structures with an appropriate consideration of both crystallographic error and structural variation, refs 16 and 17.
adjoining metal-ligand bonds. As such, we have a quantification of metal-ligand covalency, derived from structural data. Spreadsheets containing complete sets of data are available by writing to the authors.
Figure 4. Bond length of the bound arm of the carboxylate group (OAC) (Å) vs periodic group number for the transition metals. The data represented in this figure are derived from 6163 separate observations of carboxylate-metal bonds. The data are tabulated by element in the Supporting Information with mean, median, standard deviation, and confidence intervals (Tables S6-S9). Complexes are six-coordinate unless otherwise stated. Abbreviations: hs high spin, ls low spin, sq pl square planar. Sufficient data do not exist for all transition metals.
resolution than examination of any one crystal structure would today. From the bond order of the OA-C bond, we are able to calculate the amount of unassigned oxygen density and hence calculate the percent covalent character in the
Acknowledgment. We would like to thank all the researchers who contributed structures to the crystal structure database, particularly those who had the forethought to create the database in 1965. In addition, we would like to acknowledge conversations concerning this topic with Dr. Robert J. Deeth, of Warwick University, and Professors Leonard F. Lindoy, Peter A. Lay, and James Beattie of Sydney University and thank the Australian government for a Ph.D. scholarship to R.K.H. Finally, we thank Ms. Genevieve Bulluss for proofreading the manuscript. Supporting Information Available: Tables detailing the data represented here including mean and median standard deviation, several further plots of the data, and a detailed description of the statistical theories applied herein. This material is available free of charge via the Internet at http://pubs.acs.org. IC034198A
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