A New Bonding Mode for the Bicyclic Guanidinate, [hpp]-, in the Tetrametallic Yttrium Oxide Cluster, Y4(hpp)8Cl2(m4-O)

Oxygen incorporation in the product of the reaction between YCl3 and the lithium salt of 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidine (hppH) afforded the tetrametallic cluster Y4(hpp)8Cl2(¿4-O), in which four of the guanidinate anions are involved in a previously unreported ¿,¿2,¿2-bridging mode between two metal centres.