A study of structural and bonding variations in the homologous series [Mo2(CN)6(dppm)2]n-(n= 2, 1, 0)

Bera, Jitendra K. ; Szalay, Paul S. ; Dunbar, Kim R. (2002) A study of structural and bonding variations in the homologous series [Mo2(CN)6(dppm)2]n-(n= 2, 1, 0) Inorganic Chemistry, 41 (13). pp. 3429-3436. ISSN 0020-1669

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Official URL: http://pubs.acs.org/doi/abs/10.1021/ic020157n?jour...

Related URL: http://dx.doi.org/10.1021/ic020157n

Abstract

Reaction of Mo2Cl4(dppm)2 (dppm = bis(diphenylphosphino)methane) with 6 equiv of [n-Bu4N][CN] or [Et4N][CN] in dichloromethane yields [n-Bu4N]2[Mo2(CN)6(dppm)2] (1) and [Et4N]2[Mo2(CN)6(dppm)2] (2), respectively. The corresponding one- and two-electron oxidation products [n-Bu4N][Mo2(CN)6(dppm)2] (3) and Mo2(CN)6(dppm)2 (4) were prepared by reactions of 1 with the oxidant NOBF4. Single-crystal X-ray structures of 2•2CH3CN, 3•2CH3CN•2H2O, and 4•2CH3NO2 were performed, and the results confirmed that all three complexes contain identical ligand sets with trans dppm ligands bisecting the Mo2(μ-CN)2(CN)4 equatorial plane. The binding of the bridging cyanide ligands is affected by the oxidation state of the dimolybdenum core as evidenced by an increase in side-on π-bonding overlap of the μ-CN in going from 1 to 4. The greater extent of π-donation into Mo orbitals is accompanied by a lengthening of the Mo−Mo distance (2.736(1) Å in Mo2II,II (2), 2.830(1) Å in Mo2II,III (3), and 2.936(1) Å in Mo2III,III (4)). A computational study of the closed-shell members of this homologous series, [Mo2(CN)6(dppm)2]n (n = 2−, 0), indicates that the more pronounced side-on π-donation evident in the X-ray structure of 4 leads to significant destabilization of the δ orbital and marginal stabilization of the δ* orbitals with respect to nearly degenerate δ and δ* orbitals in the parent compound, 2. The loss of δ contributions combined with the reduced orbital overlap due to higher charges on molybdenum centers in oxidized complexes 3 and 4 is responsible for the observed increase in the length of the Mo−Mo bond.

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