SUBVALENT GROUP-4B METAL ALKYLS AND AMIDES .9. GERMANIUM AND TIN ALKENE ANALOGS, THE DIMETALLENES M2R4 [M = GE OR SN, R = CH(SIME3)2] - X-RAY STRUCTURES, MOLECULAR-ORBITAL CALCULATIONS FOR M2H4, AND TRENDS IN THE SERIES M2R'4 [M = C, SI, GE, OR SN - R' = R, PH, C6H2ME3-2,4,6, OR C6H3ET2-2,6]
SUBVALENT GROUP-4B METAL ALKYLS AND AMIDES .9. GERMANIUM AND TIN ALKENE ANALOGS, THE DIMETALLENES M2R4 [M = GE OR SN, R = CH(SIME3)2] - X-RAY STRUCTURES, MOLECULAR-ORBITAL CALCULATIONS FOR M2H4, AND TRENDS IN THE SERIES M2R'4 [M = C, SI, GE, OR SN - R' = R, PH, C6H2ME3-2,4,6, OR C6H3ET2-2,6]
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DOI：
10.1039/dt9860002387
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发表时间：
1986-11-01
期刊：
影响因子：
--
通讯作者：
SCHILLING, BER
中科院分区：
文献类型：
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作者：
GOLDBERG, DE;HITCHCOCK, PB;SCHILLING, BER
同晶中心对称晶体 Ge2R4 和 Sn2R4[R = CH(SiMe3)2] 的 X 射线结构揭示了反式折叠的 C2h M2C4 框架，M = Ge 的折叠角 θ 为 32°，M = Sn 的折叠角 θ 为 41°，但 MC2 平面没有绕 M–M 轴扭曲。 M-M 距离 [Ge 为 2.347(2)Å，Sn 为 2.768(1)Å] 比四面体单元 M∞ 稍短（M = Ge 为 4%，M = Sn 为 1.5%）。 M2R4 中每个 MR2 部分的构象近似于平面 syn,anti（参见气态 MR2 中的 Ca. syn,syn），并且四个配体 R– 以“桨轮”方式定向。因此，M2R4 中的 M-C 键合存在不对称性，如图所示（方括号中的 M = Sn 的数据），通过 M-C 和 M-C′(Å)、1.979(9) 和 2.042(8)[2.207(5) 和 2.225(6)] 的变化； M′MC和M′MC′(°)，113.7(3)和122.3(2)[112.0(1)和119.4(1)°]；各个MCSi角度(°)、110.0(4)[110.2(3)]、113.9(4)[109.3(3)]、119.1(4)[119.1(2)]和121.8(4)[118.9(2)]。平均 M-C 键长相当，但 M2R4 中的 CMC 角比之前在气态 MR2 中发现的更宽。对模型化合物 M2H4 进行优于双 zeta 基础的从头算分子轨道计算表明，(i)反式折叠平衡结构比平面结构更稳定，M = Ge 时为 13 kJ mol–1(θ= 40°)，Sn 时为 26 kJ mol–1(θ= 46°)； (ii) Ge 的 M-M 键距为 2.30 Å，Sn 的 M-M 键距为 2.71 Å； (iii) Ge 的 M-M 解离能为 130 kJ mol-1，Sn 的 M-M 解离能为 90 kJ mol-1。这些能量大约是 H3GeGeH3 或 Me3MMMe3 实验 M-M 单键解离能的一半。 M2R′4系列中M-M键的强度递减[M = C、Si、Ge或Sn； R′= R, Ph, C6H2Me3-2,4,6, 或 C6H3Et2-2,6] 或者更一般地，M2X4(X = R′ 或 H) 随着 M 原子序数的增加，以及反式折叠相对于平面结构的稳定性增加，归因于 MX2(X = R' 或 H) 单体中电子孤对的惰性增加，这反过来又反映在单→三重态的增加激发能。
X-Ray structures of the isomorphous centrosymmetric crystalline Ge2R4 and Sn2R4[R = CH(SiMe3)2] reveal a trans-folded C2h M2C4 framework, with a fold angle θ of 32° for M = Ge and 41° for M = Sn, but no twist of the MC2 planes about the M–M axis. The M–M distance [2.347(2)Å for Ge and 2.768(1)Å for Sn] is slightly shorter (4% for M = Ge, 1.5% for M = Sn) than in the tetrahedral element, M∞. The conformation of each MR2 moiety in M2R4 approximates to planar syn,anti(cf. Ca. syn,syn in gaseous MR2), and the four ligands R– are oriented in a ‘paddle-wheel’ fashion. There is consequently an asymmetry in the M–C bonding in M2R4 as shown (data for M = Sn in square brackets) by variations in M–C and M–C′(Å), 1.979(9) and 2.042(8)[2.207(5) and 2.225(6)]; M′MC and M′MC′(°),113.7(3) and 122.3(2)[112.0(1) and 119.4(1)°]; individual MCSi angles (°), 110.0(4)[110.2(3)], 113.9(4)[109.3(3)], 119.1(4)[119.1(2)], and 121.8(4)[118.9(2)]. The average M–C bond lengths are comparable but the CMC angles are wider in M2R4 than those previously found for gaseous MR2. Ab initio molecular orbital calculations with better than double zeta basis on the model compounds M2H4 show that (i)trans-folded equilibrium structures are more stable than planar by 13 kJ mol–1(θ= 40°) for M = Ge and 26 kJ mol–1(θ= 46°) for Sn; (ii) the M–M bond distance is 2.30 Å for Ge and 2.71 Å for Sn; and (iii) the M–M dissociation energy is 130 kJ mol–1 for Ge and 90 kJ mol–1 for Sn. These energies are about half the experimental M–M single-bond dissociation energies of H3GeGeH3 or Me3MMMe3. The decreasing strength of M–M bonding in the series M2R′4[M = C, Si, Ge, or Sn; R′= R, Ph, C6H2Me3-2,4,6, or C6H3Et2-2,6] or, more generally, M2X4(X = R′ or H) with increasing atomic number of M, as well as the increasing stability of the trans-folded relative to planar structures, is attributed to the increasing inertness of the electron lone pair in the MX2(X = R′ or H) monomer, which in turn is reflected in an increasing singles → triplet excitation energy.