Hydrotris(pyrazolyl)borate chemistry of uranium(III) and uranium(IV). Synthesis of .sigma.-hydrocarbyl derivatives of uranium(IV) and reactivity of UCl2R[HB(3,5-Me2pz)3] (R = CH2SiMe3, CH(SiMe3)2) and UCl2[HB(3,5-Me2pz)3] toward ketones and aldehydes