Acta Cryst. (1984). B40, 1-6 X-OH and O-H...O Bond Lengths in Protonated Oxoanions BY GIOVANNI FERRARIS AND GABRIELLA IVALDI Istituto di Mineralogia, Cristallografia e Geochimica 'G. Spezia" dell" Universitd, Via S. Massimo 22, 10123 Torino, Italy (Received 27 April 1983; accepted l August 1983) Abstract A survey of the X-OH bonds in protonated oxoanions (X=B, C, N, Si, P, S, As, Se and I; coordination numbers 3 and/or 4) and of the hydro- gen bonds donated by the corresponding OH shows the following trends. (i) For a given oxoanion, X-OH tends to increase in length with the distance O...O of the hydrogen bond; in particular, the overall average value of O-..O for each X is inversely corre- lated with the natural logarithm of Pauling's bond strength p which is donated by X to OH. (ii) The average length of X-OH tends to decrease from mono- to triprotonated anions with the same X. Introduction Variously defined bond strengths (Baur, 1970; Brown & Shannon, 1973; Donnay & Allmann, 1970; Ferraris & Catti, 1973, Pyatenko, 1973), orbitals (Cruickshank, 1961), bond-overlap populations (e.g. Gibbs, Hamil, Louisnathan, Bartell & Yow, 1972), coordination number (Baur, 1978), mean electronegativity of the cations in the structure (Shannon & Cairo, 1973) and distortion of the coordination polyhedron (Baur, 1974) are some of the parameters which, singularly or jointly, have been correlated with the variations of individual or mean bond lengths in oxoanions. Less attention has, instead, been paid to the connec- tions among two or more bonds involving the same atom; exceptions are the hydrogen bond in crystalline hydrates (Chiari & Ferraris, 1982; Ferraris & Fran- chini-Angela, 1972; Ichikawa, 1978), correlations of C-O vs O-H in hydrogen-bonded carboxyls (Ichikawa, 1979) and of S-OH vs O-H-..O in sul- phates (Catti, Ferraris & Ivaldi, 1979). The latter results are here extended to the X-OH bonds of protonated oxoanions where X = B, C, N, Si, P, S, As, Se, and I and the coordination number is 3 and/or 4. important role in inorganic, organic and biological chemistry. Owing to disorder involving the proton, symmetry ambiguities and poor accuracy, about half* the crystal structures retrieved have been excluded from the present research, whose results are, anyway, essentially qualitative. The outcome of the survey is summarized in Tables l and 2. Standard deviations tr of the average values are also given, but their meaning is doubtful because they are obtained with the assumption of a normal distribution of the values. In fact, in addition to the smallness of some of the samples, the distributions (not reported) are usually quite asymmetric and sometimes show more than one maximum, e.g. one for very short hydrogen bonds and another for the other cases. Scatter diagrams of X-OH vs the O...O distance in the donated hydrogen bond are shown in Fig. 1; for phosphates, arsenates and selenites there are enough neutron-diffraction results to justify diagrams of X-OH vs O-H and H...O as well (Fig. 2). In the figures distinction has been made for differently pro- tonated anions. A diagram for nitrates is not reported because only three values are available (Table 1). Fig. 3 shows the variation of the mean overall distance (O'"O)av for the hydrogen bonds donated by each type of oxoanion against the natural logarithm of Pauling's bond strength p (in valence units, v.u.) which each O receives from X. The bridge O of the P207 group has been assumed to receive 2 v.u., one from each P; the remaining 4 v.u. of each P have been distributed among the other oxygens (4 v.u. each). The scattering of the experimental points would justify analytical correlations in only a few cases, as shown by the correlation coefficients R given in Tables 1 and 2. Anyway, in order to stress the trends which will be discussed below, least-squares-fitted straight lines are reported in Figs. l and 2 with the exceptions of borates, selenites and selenates which for the following and, maybe, other reasons show no Results * A list of the 235 structural papers utilized has been deposited Structural data for a large number of compounds with the British Library Lending Division as Supplementary Publi- cation No. SUP 38776 (9 pp.). Copies may be obtained through containing protonated oxoanions have been pub- The Executive Secretary, International Union of Crystallography, lished, particularly for phosphates because of their 5 Abbey Square, Chester CHI 2HU, England. 0108-7681/84/010001-06501.50 O 1984 International Union of Crystallography