A Static Agostic .alpha.-CH.cntdot..cntdot..cntdot.M Interaction Observable by NMR Spectroscopy: Synthesis of the Chromium(II) Alkyl [Cr2(CH2SiMe3)6]2- and Its Conversion to the Unusual "Windowpane" Bis(metallacycle) Complex [Cr(.kappa.2-C,C'-CH2SiMe2CH2)2]2-