Single-crystal polarized electronic spectra of the compounds dichlorotetrakis ([mu]-pivalato)dirhenium(III),tetra-n-butylammonium octachlorodirhenate(III), tetra-n-butylammonium octabromodirhenate(III), dimolybdenum(II)tetrapivalate, and the X-ray diffraction cyrstal structures of tetra-n-butylammonium octobromodirhenate(III) and two polymorphs of dimolybdenum(II) tetrapivalate