A very short Re26+ quadruple bond:: first DFT calculations on a paddlewheel complex with an element of the third transition series

The first complexes containing third row transition elements with bridging hpp (the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) are presented. Re-2(hpp)(4)Cl-2, 1, is a typical paddlewheel complex with two axial chloride ions. The Re-Re distance in 1 of 2.191(1) Angstrom is one of the shortest known for a quadruply-bonded Re-2(6+) complex and the Re-Cl distance of 2.749(5) Angstrom is the longest one reported for this type of complex. The second compound, Re-2(hpp)(3)Cl-3, 2, has three bridging hpp ligands, two equatorial chloride ions and one axial chloride ion. The Re-Re distance is 2.189(2) Angstrom. The electronic structure of 1 has been determined by DFT calculations, the first time this has been done on a paddlewheel complex containing a third row transition element.