M2(hPP)4Cl2 and M2(hpp)4, where M = Mo and W:: Preparations, structure and bonding, and comparisons with C2, C2H2, and C2Cl2 and the hypothetical molecules M2(hpp)4(H)2

The reaction between M2Cl2(NMe2)(4), where M = Mo or W, and Hhpp (8 equiv) in a solid-state melt reaction at 150 degreesC yields the compounds M-2(hPP)(4)Cl-2 1a (M = Mo) and 1 b (M = W), respectively, by the elimination of HNMe2 [hpp is the anion derived from deprotonation of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, Hhpp]. Purification of 1a and 1b is achieved by sublimation of the excess Hhpp and subsequent recrystallization from either CH2Cl2 or CHCl3 (or CDCl3). By single-crystal X-ray crystallography, the structures of 1 a and 1 b are shown to contain a central paddlewheel-like M-2(hPP)(4) core with Mo-Mo = 2.1708(8) Angstrom (from CH2Cl2), 2.1574(5) Angstrom (from CDCl3), W-W = 2.2328(2) Angstrom (from CDCl3), and M-N = 2.09(1) (av) Angstrom. The Cl ligands are axially ligated (linear Cl-M-M-Cl) with abnormally long M-Cl bond distances that, in turn, depend on the presence or absence of hydrogen bonding to chloroform. The quadruply bonded compounds M-2(hPP)(4), 2a (M = Mo), and 2b (M = W), can be prepared from the reactions between 1,2-M2R2(NMe2)(4) compounds, where R = Bu or p-tolyl, and Hhpp (4 equiv) in benzene by ligand replacement and reductive elimination. The compounds 2a and 2b are readily oxidized, and in chloroform they react to form 1 a and 1 b, respectively. The electronic structure and bonding in the compounds 1 a, 1 b, 2a, and 2b have been investigated using gradient corrected density functional theory employing Gaussian 98. The bonding in the M-M quadruply bonded compounds, 2a and 2b, reveals M-M delta(2) HOMOs and extensive mixing of M-M pi and nitrogen ligand lone-pair orbitals in a manner qualitatively similar to that of the M-2-(formamidinates)(4). The calculations indicate that in the chloride compounds, 1 a and 1b, the HOMO is strongly M-Cl sigma antibonding and weakly M-M sigma bonding in character. Formally there is a M-M triple bond of configuration pi(4)sigma(2), and the LUMO is the M-M delta orbital. An interesting mixing of M-M and M-Cl pi interactions occurs, and an enlightening analogy emerges between these d(4)-d(4) and d(3)-d(3) dinuclear compounds and the bonding in C-2, C2H2, and C2Cl2, which is interrogated herein by simple theoretical calculations together with the potential bonding in axially ligated compounds where strongly covalent M-X bonds are present. The latter were represented by the model compounds M-2(hPP)(4)(H)(2)- On the basis of calculations, we estimate the reactions M-2(hpp)(4) + X-2 to give M-2(hpp)(4)X-2 to be enthalpically favorable for X = Cl but not for X = H. These results are discussed in terms of the recent work of Cotton and Murillo and our attempts to prepare parallel-linked oligomers of the type [[bridge] - [M-2]-](n).