Reaction of (CsHs)2Ti(CH3)2 or (CH3).C2(C,H4)2Ti(CH3)2 with pyridine-2,6dicarboxylic acid (dipicolinic acid) yields titanocene dipicolinate derivatives. The molecular structure of (C,H,),Ti dipicolinate is that of an axially symmetric, pentacoordinate titanocene derivative with two carboxyl oxygen atoms and the pyridine nitrogen atom as ligating atoms. Two identical chelate bite angles of only 71 0 make the dipicolinate ligand particularly suited to form a remarkably stable titanocene derivative with unprecedented pentacoordinate geometry. '" To whom correspondence should be addressed. ** Cf. ref. 1 and references therein. At the titanium-(II) and-(III) levels, titanocene derivatives with tricoordinate geometry have been reported in a few instances [2-51. *** Whether complexes with '12-coordinated ligands, such as (CsRshTi(CO)C2(C6Hsh [12] or (CsHshTi(CI)OCCH3 [13] should be considered as examples of pseudotetrahedral geometry or as exceptions to this rule is debatable. In a number of instances, pentacoordinate or hexacoordinate dicyc1opentadienyltitanium(lV) species have been proposed without support from crystal structure determinations [14-19}.
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