  This paper presents the analysis of a statistically complete sample of 28
serendipitous X-ray sources selected in 82 pointed XMM-Newton fields down to a
count-rate of 0.002 counts s^-1(4.5-7.5 keV energy band). This is the first
sample selected in this energy range to have complete spectroscopic
identifications and redshift determinations for all the objects. Apart from one
Galactic source (an interacting binary), all the objects are AGNs. Their
optical and X-ray properties (derived from the spectral analysis of the
XMM-EPIC data) are compared together. The good correlation between the optical
spectral type and the X-ray absorption properties supports the AGN unified
model. Only one object that does not fit the relation between optical and X-ray
absorption is found, namely a Seyfert 1.9 with no evidence of obscuration in
the X-ray band (NH<1.3x10^20 cm^-2). In total, 7 sources out of 27 are heavily
obscured in the X-ray (NH>10^22 cm^-2), corresponding to a surface density of
0.7 deg^-2 at the flux limit the sample (4-7x10^-14 erg s^-1 cm^-2 in the
4.5-7.5 keV energy band). Among these obscured objects, two sources show a
large (intrinsic) luminosity (L_[2-10keV]>10^44 erg s^-1) and are thus
classified as type 2 QSO. Finally, we have compared the fraction of X-ray
absorbed AGNs (26%) with that predicted by the current XRB synthesis models at
the flux limit of the survey. We find that the models significantly (~90%
confidence level) over predict the fraction of absorbed AGNs thus confirming
also in this hard energy band (4.5-7.5 keV) similar results recently obtained
in the 2-10 keV band.
