Structural criteria for the mode of bonding of organoactinides and -lanthanides and related compounds

Following the structure analysis of uranocene, U(C{sub 8}H{sub 8}){sub 2}, we found to our surprise that almost nothing was known about the structure and bonding of organoactinides and lanthanides - a situation we have endeavored to change during the intervening years. The reported synthesis of uranocene in 1968 by Mueller-Westerhoff and Streitwieser sparked renewed interest in a field that had remained dormant since the synthesis of the cyclopentadienide compounds in the early 1950's. Two recent Accounts articles have described some of the subsequent developments in this field. A question which frequently occurs in discussions of this chemistry is: 'How covalent is the bonding in these compounds?' Within a careful and limited structural definition of covalent and ionic bonding, this question can be examined in some detail. This Account, while summarizing much of the structural information accumulated during the last ten years, will focus on the unifying structural trends in these compounds and the question of the mode of bonding. Complete reviews of the structural chemistry and further discussion of the structural aspects of the bonding can be found elsewhere. Our discussion will consider scandium and yttrium as lanthanides, based on their general chemical similarity to the true lanthanides.