Reactions of Ti((OPr)-Pr-i)(4) with different phosphonic acids RPO3H2 (R = Ph, 4-CNPh, Me, Bu-t) in organic solvents have been investigated. In the presence of small amounts of water, the new molecular titanium oxide alkoxide phosphonates [Ti-4(mu(3)-O)((OPr)-Pr-i)(5)(mu-(OPr)-Pr-i)(3)(RPO3)(3)].DMSO [R = Ph (1), Me (2),Bu-t (3), 4-CNPh (4)] were isolated. The single-crystal X-ray structure analyses of 1 and 2 revealed hexacoordinated titanium atoms and a connectivity of (111) for each phosphonate. Under rigorous exclusion of water, the reaction of Ti((OPr)-Pr-i)(4) with tert-butylphosphonic acid in toluene gave the titanium phosphonate tetramer [Ti((OPr)-Pr-i)(2)((BuPO3)-Bu-t)](4) (5). A single-crystal X-ray structure analysis of 5 revealed a 5 + 1 coordination of the titanium atoms as a result of the (112) connectivity of each phosphonate; such a coordination mode has never been reported for a titanium phosphate, phosphonate, or phosphinate. Compounds 1-5 were characterized by FT-IR, P-31 MAS NMR, and solution multinuclear NMR (H-1,C-13{1H}, (31)p{H-1}) spectroscopies. C-13 CP MAS NMR experiments were carried out on arylphosphonates 1 and 4. Solution NMR experiments were also used to investigate the exchange reaction between 1 and 2 and the conversion of 5 to [Ti-4(mu(3)-O)((OPr)-Pr-i)(5)(mu-(OPT)-P-i)(3)((BuPO3)-Bu-t)(3)].(PrOH)-Pr-i by partial hydrolysis in the presence of Ti((OPr)-Pr-i)(4) The phosphonate clusters 1-5 are soluble in organic solvents and are likely intermediates in the sol-gel processing of inorganic-organic hybrids based on titanium oxide and phosphonate groups that we are currently developing.