The unusual bioctahedral dirhenium(II,III) complex [Re2(µ-Cl)(µ-CNHBut)(µ-dppm)2Cl2(CNBut)2]PF6[dppm = bis(diphenylphosphino)methane]: a product of the non-cleavage reaction between Re2Cl4(dppm)2 and t-butyl isocyanide