Some reactions of tetrakis(tert-butylimido)osmium(VIII). X-Ray crystal structures of Os(NBut)O(O2CBut)2(NH2But)2·ButCO2H, (ButN)2 Os(µ-NBut)2Os(µ-NBut)X2(X = Cl or I) and [Os2(NBut)4(µ-NBut)2]I3