Most known compounds with five‐coordinated Ti4+ are natural and synthetic titanosilicates. The crystal structure of natural fresnoite, Ba2TiSi2O8 [tetragonal, space group P4bm, a = 8.510 (1) Å, c = 5.197 (1) Å, V = 376.4 (1) Å3, Z = 2], has been refined to R = 0.011 on the basis of 807 unique single‐crystal reflections with I > 2σ(I). Titanium has fivefold coordination with one short ('titanyl') bond of 1.692 (5) Å. Bonds in the TiO5 polyhedron are discussed in comparison to analogous coordination polyhedra in other minerals and compounds. A review of all known compounds with Ti4+O5 polyhedra shows that most of them are titanosilicates in which titanium forms a short Ti—O bond (∼1.61 to ∼1.77 Å). Poor Gladstone–Dale compatibility between chemical composition, optical characteristics and density of these compounds is explained by the anomalous contribution of [5]Ti4+ to the optical properties as shown by calculations based on the relationship between electronic polarizabilities and refractive indices. An improved Gladstone–Dale coefficient of 0.29 is suggested for TiO2 with [5]Ti4+. A negative correlation between 'titanyl' bond lengths and wavenumbers of the bands of Ti—O stretching vibrations (in the range of 890–830 cm−1) in infrared and Raman spectra is observed. [ABSTRACT FROM AUTHOR]