The systematic analysis of the geometry of three-coordinate boron in boronic acid derivatives with a common [CBO2] skeleton is presented. The study is based on the bond-valence vector (BVV) model [Zachara (2007). Inorg. Chem. 46, 9760-9767], a simple tool for the identification and quantitative estimation of both steric and electronic factors causing deformations of the coordination sphere. The empirical bond-valence (BV) parameters in the exponential equation [Brown & Altermatt (1985). Acta Cryst. B41, 244-247] r(ij) and b, for B-O and B-C bonds were determined using data deposited in the Cambridge Structural Database. The values obtained amount to rBO = 1.364 angstrom, bBO = 0.37 angstrom, rBC = 1.569 angstrom, bBC = 0.28 angstrom, and they were further used in the calculation of BVV lengths. The values of the resultant BVV were less than 0.10 v. u. for 95% of the set comprising 897 [CBO2] fragments. Analysis of the distribution of BVV components allowed for the description of subtle in-and out-of plane deviations from the 'ideal' (sp(2)) geometry of boron coordination sphere. The distortions specific for distinct groups of compounds such as boronic acids, cyclic and acyclic esters, benzoxaboroles and hemiesters were revealed. In cyclic esters the direction of strains was found to be controlled by the ring size effect. It was shown that the syn or anti location of substituents on O atoms is decisive for the deformations direction for both acids and acyclic esters. The greatest strains were observed in the case of benzoxaboroles which showed the highest deviation from the zero value of the resultant BVV. The out-of-plane distortions, described by the vz component of the resultant BVV, were ascertained to be useful in the identification of weak secondary interactions on the fourth coordination site of the boron centre.
