Syntheses and structures of [(THF)(n)M{(NSiMe3)(2)PPh2}2] complexes (M=Be, Mg, Ca, Sr, Ba; n=0-2): Deviation of alkaline earth metal cations from the plane of an anionic ligand

The syntheses and solid-state structures of [(THF)(n)M{(NSiMe3)(2)PPh2}(2)] (M = Be, n = 0, 1; M = Mg, n = 0, 2; M = Ca, n = 1, 3; M = Sr, n = 2, 4; M = Ba, n = 2, 5) are presented. Comparison of the geometric parameters within the homologous series and to related systems uncovers the dication-induced alterations of coordination to, as well as bonding within, the anionic fragment. The coordination number increases from 4 (Be, Mg) via 5 (Ca) to 6 (Sr, Ba). Two of the Ph2P(Me3SiN)(2) anions cover the coordination sphere of beryllium and magnesium, while with calcium one single THF molecule and with strontium and barium two additional THF molecules are required to complete the metal coordination sphere. Against steric considerations the two THF molecules in 4 and 5 are coordinated to the same hemisphere of the metal leaving the two anions cisoid. Even against sterical strain the alkaline earth metals leave the plane of one of the Ph2P(Me3SiN)(2) anions with increasing mass demonstrating the preference of Sr and Ba to interact with pi electron density. This effect can also be found in related systems. It might be small and counterbalanced by steric requirements, but it is significant. The metal pi interaction rises continually with the mass of the metal and decreasing bulk of the anion but is independent from cisoid or transoid arrangement of the anions. From the homologous series of complexes presented three structure determining factors can be deduced: (i) dicationic size; (ii) bent X-M-X (M = Sr, Ba) arrangement; (iii) increasing pi interaction with increasing mass of the alkaline earth metal cation.